Experiments on a $50 DIY air purifier (2020) | Hacker News Reader

Experiments on a $50 DIY air purifier (2020)

dynomight.net

409 pointsZainRiz267 comments

When the Caldor fire (now 210 thousand acres, 328 mi^2!) was smoking up Placerville, the downtown Home Depot had, at the main entrance, a big stack of 20x24x1-inch #9 furnace filters, box fans, and rolls of tape, with one all constructed sitting on top, running, black with trapped smoke.

Kudos to HD for not jacking up prices.

It is best to make them with a four inch thick filter, which HD does not stock, and higher than a #9, likewise, bought online. You can stack a #8 in front of a #17, to make the ~$100 #17 last longer, and stack two box fans (one intake, one exhaust) to get more suction. Maybe stack #8/fan/#17/fan, to keep the fans clean.

But the single one-inch #9 filter cleared the smoke from my whole house in no time flat! Don't let "best" stop you from doing "good".

An advantage you have from high airflow is that particles that make it through the filter once are likely to get trapped the next time around, or the 20th. Atomizing some water ought to make filters momentarily stickier, until it dries.

Fan blades getting dusty can cut airflow in half! (Same on laptop cooling fans; use a vacuum cleaner on your laptop air path.)

Kudos to HD for not jacking up prices.

Not sure all the credit goes to them since if they jacked up the price during the fire, they'd be violating price gouging laws.

But it is nice that they put the components for a DIY filter all in once place

https://www.oag.ca.gov/consumers/pricegougingduringdisasters

That is news to me, and welcome. By the FAQ, it appears furnace filters must be covered. Box fans might be borderline, but even being seen publicly accused of violating gouging laws would be bad for HD.

The statute is broad enough (it includes "emergency supplies", and the CDC has a how-to guide on creating a box-fan filter) that I'm sure they'd be running afoul of the law if they tried to jack up prices for box fans.

But yeah, any large retailer would avoid any appearance of price gouging even if they could get away with it legally just to avoid the bad publicity.

I guess that explains why McMaster's price for N95 masks never really changed, they were just unavailable.

Although on Amazon it was pretty wild to watch -- I guess if they're not technically N95 masks they can take the price as high as they want since they aren't actually recommended for protecting against viruses. But actual certified N95 masks just became unavailable instead of having high prices.

>But actual certified N95 masks just became unavailable instead of having high prices.

Part of that is because 3M has been incredibly good about not raising their prices, and has been very clear to its distributors that they're not ok with them raising the prices. Of course this does nothing to stop the hoarders/private sales jacking up prices on eBay...

Home Depot has 3M N95 masks from time to time both in-store and for delivery, I picked up a 10 pack for a home improvement project a month or so ago. I only needed a few, so sent the rest to my immune compromised sibling.

Just FYI! You want P100, not N95. People in ERs use P100 with a surgical mask ontop. ALSO! 3M is the only trusted supplier (it is hard to make P100s). Why did n95 get all the hype??

https://www.3m.com/3M/en_US/p/d/v000057510/

P100's are significantly harder to breath in, and I'd question whether the typical user has a good enough fit to actually get the full filtering performance.

I can't speak for ER healthcare workers, but I know from experience that hospital surgical nurses wear N95 masks. I'm not sure why anyone in healthcare would need a P* mask, since they aren't generally exposed to oil.

If I really thought I needed a P100 mask, I'd get a rubber respirator with screw on filters -- they are easier to fit, and the dual respirators have more surface area for easier breathing.

The number in a rating tells you the minimum amount of airborne challenge particles the mask protects against: an N95 mask keeps out at least 95% of particles but isn’t oil resistant, and a P100 mask is oil proof while protecting the wearer from at least 99.8% of particles. (AKA just a better mask).

It's a real bummer to hear such comment - why spread this?

P100 Approved, NIOSH's highest rated filtration efficiency in a filtering facepiece respirator. It is just a higher rating. "P100's are significantly harder to breath in" <- not at all! You should try one - they breath great. They are meant for workers to wear all day. They even have a nice leather like seal making it easier to have proper seal. Also! They are perfect for a quick trip to the grocery store or standing on a bus.

ALSO! The screw on versions are great but they are MUCH heavier and impossible to put in a pocket so they are not practical at all. Nothing about your comment was accurate :(

It's a real bummer to hear such comment - why spread this?

Why spread what? You're the one that claimed that medical workers all use oil resistant P100 masks, while what they overwhelmingly use is N95's (the ones that are medical grade are ASTM certified to be water/fluid resistant, not oil resistant).

In the hospital I worked at, if staff needed better filtration than that N95 could provide, they used full face, powered respirators. (positive pressure means a perfect seal is less critical, and it provides eye protection).

Here's 3M's healthcare line of healthcare masks, they are all N95:

https://www.3m.com/3M/en_US/p/c/ppe/respiratory-protection/h...

I don't know what a P100 is but I don't find a N99 more difficult to breathe than a N95 (but I do feel that I have to exert more force/pressure when breathing).

I use it to cycle and it's ignorable when cycling casually but discomfortable when going for performance. Anedoctally I don't find the extra effort to breathe to be a problem but just the saturated & hot air. So if I were to exhale each time with the mask of and inhale if it on it wouldn't be a problem. PS. I use masks w/ valves and those without one worsen this issue.

That's a good demonstration of the results that come from price controls.

> McMaster's price for N95 masks never really changed, they were just unavailable.

Of course. The choices are:

1. high price, and availability

2. same price, and none available

Americans seem to always choose #2.

High price in these kind of events rarely increases availability - after all, it takes time to increase production or ship more. If high price appears to increase availability, I suspect that's largely because you're pricing people out of the product. (A slight price increase to counterbalance the overheads of realigning supply lines might be reasonable, but whether that's significant compared to the normal profit margin - probably situational?)

As such, consumers are right to feel vindictive when companies do try and engage in that kind of profiteering; and that social dynamic is one of the ways to keep the market healthy when there are many more consumers than producers - effectively a kind of collective bargaining - at least, if the producer is sensitive to that kind of thing. Sometimes the profit will be worth it and they'll just shrug it off, of course.

But in particular, we should be careful not to support a narrative that empathizes with profiteering, because such empathy (absurd as it is anyhow for a corporation) undermines the shared social frustration needed for the implicit collective bargaining that's happening with these kind of social events. If everybody believes that it's just natural for a company to seek profit above all else, and crucially thus accepting that it's OK for them to screw over the consumer, then it's going to be hard to build momentum for any backlash, and thus hard to improve the negotiating position of the consumer.

Even from a radically capitalist standpoint that's bad, because capitalism works best (is most efficient) in transparent markets where all participants have equal power - and since that's never the reality, it's necessary to counterbalances against larger market participants, or those with more information.

>High price in these kind of events rarely increases availability - after all, it takes time to increase production or ship more.

If something is an issue for just 1-2 days maybe. If it's more then it seems that should be enough to get at least some express shipments if there was profit to be made.

It's possible; I'm sure the details matter.

However, if you can expedite shipping like this, can you do it in bulk? If so, you probably don't need to raise prices by much at all. But it's also quite likely not simply a matter of pressing the conceptual "expedite" button, because the stuff needs to arrive from somewhere and if it's an unexpected spike in demand, finding a bunch of supply that's ready to ship that quickly strikes me as being unreasonably lucky. More likely, it'd take time to find alternate suppliers, and shipping sufficient quantities to make a dent will take a while.

Of course, if you're doing it for one or two items - individuals can do that themselves, and yes, it's going to be expensive, and no, it's not likely to really increase supply a lot overall.

My assumption here is that shipping in bulk is much more efficient than small-scale shipping, but takes longer to arrange and prepare. But because the costs are spread out over so many items, even if shipping costs extra, that rarely translates to significant price hikes; bulk shipping, even expensive bulk shipping, is assumed to be a fairly small slice of the overall purchase price.

> High price in these kind of events rarely increases availability

Sure it does. For example, with gasoline, it incentivized people outside the disaster zone to load up jerry cans with gas, and drive into the zone to sell it. Anti-gouging laws sure put a stop to that! Now no gas is available.

> consumers are right to feel vindictive when companies do try and engage in that kind of profiteering

Exactly what I said - people prefer no availability.

> not to support a narrative that empathizes with profiteering

Then you'll need to be happy with shortages, long lines, and misallocation.

> capitalism works best (is most efficient) in transparent markets where all participants have equal power

This is not true at all. Lack of information is accounted for by being pricing in as "risk". I do not really understand where the notion that free markets require perfect information comes from. We price in risk all the time. People pay less for items that are risky (like knock-offs) and will demand higher investment returns for risky investments.

Anti-gouging laws also cause hoarding:

1. stocking up whenever possible because of anticipated shortages

2. hoarding in order to sell on the black market

3. hoarding by people who don't need it "just in case"

4. runs on items, so the people who get it were the ones who got their first, either by being lucky or by getting tipped off. (How is this fair?)

We saw this play out magnificently in the Great TP Shortage of 2020.

100% caused by anti-gouging laws.

The idea that anti-gouging laws (and rationing) produce "fair" results should have been amply proven in that shortage, but people can't get past "but it isn't fair!"

I do wish anti-gouging laws would only apply to regular gasoline. Allow premium to float to whatever the market will demand. Midgrade should have a more relaxed anti-gouging rule.

What do you think the benefit of this plan would be exactly?

Happy medium between not gouging desperate people and still ensuring there is some gas available to those that truly need it.

If gas is $3 a gallon, someone with 1/2 tank may stop and fill up. If gas is $30 a gallon, he will be more likely to drive on until he can get far enough away that gas prices would not be high. This leaves gas available for those that are running close to empty and will not make it to the cheaper gas.

But what does that have to do with regular vs. premium gas?

Its designating some product to be a reserve (market price) and others to be anti-gouging. Making regular be anti-gouging would be easier to enforce than other hybrid methods.

Well, you have a natural experiment right here. N95 masks were out of stock from the beginning, while KN95 never were, and prices were higher, but still affordable for the vast majority of the time since the beginning of the pandemic. It varies between suppliers, but I (in an eastern european country) bought my parents a box-of-boxes of high quality masks when I found a decent price. I think it was about $200, and there's still enough left for a couple of years. Still no shortage.

I would skip the misting. I haven't done the experiment, but a lot of modern filter materials get some of their filtering power from a built-in electrostatic charge, and that may not work if it's wet.

Thank you, good point I had not thought of. Anyway, they might be permanently degraded by too much moisture.

This is one of the reasons why N95 masks are hard to clean/sterilize AND return them to the original effectiveness. You have to remove the water that has condensed on the fibers. Vacuum pump appears to be an effective method.

That said, if you don't have a filter, a wet cloth would work as a filtration medium.

wouldn't you want the cheap filter first to pre-filter the air somewhat to make the more expensive one last longer?

Yes, pre-filtering out the big stuff makes finer and more expensive filters last longer.

Interestingly, initially the captured particles actually make the filter capture finer particles in the future. The captured particles plug up the larger holes and themselves become part of the filter. Of course that doesn't last =)

My understanding (based on my not so great memory of an EPA air filter report) is that there are two basic types of this general type of filter and the electrostatic ones do best near the beginning and get worse over time while the others get better over time but with flow rate suffering. There are a few consumer air filter makers that do much better than most of them, particularly when it comes to the smallest particles, and I think they rarely if ever use the electrostatic ones for the inner filter. Those filters also cost quite a bit; I think the electrostatic ones can do a better job at low cost. For smoke in particular filters can easily aquire a smoke smell that makes them unusable well before they otherwise would be, unless there is a large carbon filter before it, so that favors low cost filters.

https://www.amazon.com/Lasko-FF305-20-inch-Purifier-Purifyin...

I bought one of these, and have been happy with mine. I use a #10 filter in it. I have not needed mine for forest fire smoke (I know these higher rated ones are rated for smoke), but it has been amazing with pulling fine particulate matter from collecting (baking soda, mica, etc).

This simplifies the taping/untaping, but before this purchase I did the tape method too. I was about to look for some aluminium channel to build a frame for the filter, but this product save me the time and hassle of that. Plus, this fan comes with a stronger motor to compensate for the filter.

> It is best to make them with a four inch thick filter, which HD does not stock

I bought a 20x24x4 #10 pleated wool air filter at HD yesterday.

Heres my set up:

- 20 inch box fan

- 20x20x4 MERV12 filter. The 4-inch pleating is key here, to reduce the air resistance on the box fan.

- 1 inch dust pre-filter. This is course, low air resistance, and is for increasing the life of the more expensive MERV12/HEPA filter (so it doesn't get clogged with easy to filter dust).

- Both filters are on the intake side of the box fan. This means you don't need a bungee cord because the intake has negative pressure, the filters just "stick". It also means you keep your box fan flowing with only cleaned air.

Heres a pic of my setup: https://imgur.com/a/O4I6Udx

And here's a graph this morning from my air quality sensor, after turning on the fan/filter: https://imgur.com/a/c5GMNcC

The peak at ~30ug/m^3 PM2.5 is approximately 100 AQI. The fan was on the other side of my 900 sqft house from the sensor.

Thanks for sharing, that's neat how simple it is. What air quality sensor setup do you have?

HM3301: https://wiki.seeedstudio.com/Grove-Laser_PM2.5_Sensor-HM3301...

Attached over I2C to an ESP32 running ESPHome: https://esphome.io/#air-quality Which in turn reports to a raspberry pi running Home Assistant: https://www.home-assistant.io/installation/ Took about 20 mins to set everything up.

> Took about 20 mins to set everything up.

Given you have Home Assistant installed and have previously used ESPHome. And attached wires.

Otherwise, you may be up to a journey that takes many hours/days. That's the difference experience makes :)

The only thing set up was the raspberry pi having an OS installed. Had never used HA or ESPHome before. I was shocked how easy it was if you're comfortable with a terminal. I'd never even used docker before.

My steps were:

1. Install docker

2. Install/run HA through docker

3. Install/run ESPHome through docker (there's a config wizard)

4. Edit the ESPHome .yaml from (3) to my preferences

5. Connect 4 wires from the ESP32 to the air quality sensor

6. Plug ESP32 USB to RPi

7. Run docker esphome on the above yaml. This compiles, flashes, and boots your ESP32, auto-connecting to your network

8. Add the ESP air quality sensor to HA through the web GUI

That's (almost) exactly what I did and without prior docker experience. I must admin it was less hassle with docker, but still docs for particular containers had to be read to understand configuration options.

+ setting up for external access, letsencrypt and tying that everything within docker-compose. I didn't manage to do it in 20minutes, but more like full working day maybe?

Anyway, HA is recommended. Haven't tried ESPHome, but Tasmota is an alternative that also does the job, if you someone ever wants to research the options.

Turnkey microcontroller projects can be really quick. Adafruit and Sparkfun sell many of their breakout boards with special cables for I2C now, so you might not even have to solder anything. From there, you can drag a Python script to the microcontroller (which appears on your PC as a flash drive).

Building enclosures for these projects used to be an annoyance, but everyone provides CAD models of their stuff now, so you just kind of lay it out in your CAD program, extrude a box around it, and print it out.

Never been a better time than now to be building sensors and that sort of thing. (Even with the STM32 shortage!)

As for the Raspberry Pi -- my rule is to treat embedded systems like hardware. Download a Linux image with the software you want on it, copy it to an SD card, plug that in, and go. I use this model for Octopi and while I might have changed the password and uploaded a profile for my printer file, that's it. If the RPI blows itself up, $35 and 5 minute fix. What version of systemd is running? Don't care. How close is the clock to Coordinated Universal Time? Don't care. It's an appliance and the time on the front is blinking :P

(I spent most of my free time this week unbreaking my Beaglebone that I ostensibly don't treat that way -- I log in and fuck with shit. Though when debugging failed I just blew everything away and copied over what I saved in the project's Git repo. It was enough to be back up and running with no loss of functionality. I guess the key is: don't let it run away from you -- it can be an endless time and energy sink.)

A 3D printer alone is a many-weeks learning experience, not to mention learning CAD if you've never done it before.

BTW, a 3D printer running is a potent source of VOC pollution and particulates, meriting an activated-carbon filtration system for wherever it is.

What air quality sensor are you using?

You seem to have relatively clean air already. I probably wouldn't even bother with air purifiers in such a situation.

Where did you buy air filters? I live in a third-world country and couldn't find them anywhere, so I had to buy a factory-built purifier. It really struggles to keep up when it's 1000+ µg/m³ outside. I'd like to add a second one (preferably DIY).

Sure, air quality is relative, but 100 AQI is the point where I start getting headaches. Obviously it can get much worse.

I got the air filters on Amazon, which can be US centric, but usually you would get them at a home/hardware store. They are made for slotting into a furnace/HVAC set up. For places where that is less common, they may be harder to find.

> You seem to have relatively clean air already. I probably wouldn't even bother with air purifiers in such a situation.

By my estimates, a lifelong exposure to 33 µg/m³ rather than 0 µg/m³ would on average result in a loss of around 1 disability adjusted life year.

https://dynomight.net/air/#a-heuristic-to-quantify-harms

Obviously this is very hard to estimate and you shouldn't trust this as anything more than at most a rough order of magnitude estimate. But it does suggest that the ambient levels they are experiencing are worth reducing if you can do it easily.

Might those numbers might be exaggerated a bit due to confounders? Specifically, the high density and traffic causing stress.

It's entirely possible! But my perspective is that there's a very one-sided risk. It's extremely unlikely that particles are beneficial. Thus, given the uncertainty I think it's good decision theory to run a purifier, especially given how easy it is relative to other lifestyle changes that could plausibly have a benefit of the same magnitude.

Running a purifier does cause noise however, which isn't healthy; assuming your purifier is close by. (No idea at what point those two effects are of similar impact.)

> It really struggles to keep up when it's 1000+ µg/m³ outside

Also worth noting that 1000ug/m³ is not accurate, maybe you are thinking of some other measurement? In the literal middle of a forest fire, you may see readings of ~300 ug/m³. No where on earth sees readings of 1000+ µg/m³.

It is very much accurate. This is a pretty typical PM2.5 level at winter time where I live. Put a few thousand houses heated with coal close together, add −30-40 °C winters, a particularly bad location where wind almost never blows (maybe ~30 days per year total), and voila.

I think you have us all curious now - where do you live? Sounds like Siberia, except I'd expect there to be more wind.

Relatively close, but no cigar. I'll quickly deanon myself if I describe the exact place, there's not many HNers here.

In California, there is the Caldor Fire raging right now in South Lake Tahoe. Heres what it looks like on the ground: https://www.reddit.com/r/weather/comments/pfczpl/caldor_fire...

The highest purple air sensor, near/on the fire front, in raw PM2.5 ug/m³ is at 368 when I checked: https://www.purpleair.com/map?opt=1/mPM25/a10/cC0&select=735...

I expect there are a lot of particles bigger than 2.5 um?

I don't think the purple air sensors (laser scattering plantower sensors) are very accurate at such extreme measurements. There are numerous laboratory permanent sensors deployed by various state, federal and local authorities that you can compare. I've seen purple air at much more extreme values. Still terrible.

Well, if you find it hard to believe, I welcome you to visit us and live here for a year or two. Don't forget to bring a good supply of spare lungs.

These measurements are supported by the official (state-sponsored) air quality monitoring.

I am not saying we have 1000+ micrograms 24/7. The daily average in winter time is closer to 300-500, depending on your location. However, I see such extreme measurements pretty much every single evening, with some going above 1500 µg/m³.

Back in the days of the great London smog particulate concentrations were going up to 5 _milli_grams per cubic meter:

https://sci-hub.se/https://pubmed.ncbi.nlm.nih.gov/29596258/...

It's not impossible or even particularly unusual.

Do you live in a place where forced-air heating is common? If so, you should be able to buy filters wherever people go to buy miscellaneous hardware for houses. In the US, they are rated on a scale called MERV. In other countries, other rating systems are used. You want something at or near the high end of the scale — getting near 100% removal with, for example, a real HEPA filter is unnecessary and potentially counterproductive, but getting to at least, say, 30% reduction in fine particles per pass is important. In the US, this is around MERV 13.

> You seem to have relatively clean air already. I probably wouldn't even bother with air purifiers in such a situation.

Everyone should have filters. Filters are for more than dust, they have also been shown to greatly reduce the amount of COVID in the room air. (i would assume flu and other viruses as well). Cheap and easy, with no stupid controversy like masks or a vaccine (even though both are better than an air filter, every little bit helps)

'Masks', as worn by most, are assuredly not.

Edit: I'm not anti-mask. I would love it if you explained how ill-fitting cloth in a single pass is better than multi-pass HEPA or equivalent filtration rather than (or even as well as) down-voting, though.

Sure, yeah.

A simple cloth mask doesn't really protect the wearer; it protects others from the wearer. It slows droplets down as you exhale them. So they travel X feet, instead of X+Y feet. Depending on air currents and other variables the level of protection this offers others will range from "nothing at all" to "some".

Obviously, a HEPA filter is orders of magnitude better. But, is there a HEPA filter directly between you and the person next to you in line at the supermarket?

> A simple cloth mask doesn't really protect the wearer; it protects others from the wearer.

Sure. I'd call that 'different' rather than 'better', but I suppose it's subjective.

> Obviously, a HEPA filter is orders of magnitude better.

You would think, and yet my comment saying so, objecting to 'masks are better than an air filter' (when qualified to ill-fitting cloth as worn by most) is apparently highly objectionable.

> But, is there a HEPA filter directly between you and the person next to you in line at the supermarket?

Not a comparison I made, but yeah, if I went to a supermarket (I'm not, because I almost entirely had groceries delivered pre-pandemic, so now easily enough entirely) I would wear an FFP3 dust mask rather than ill-fitting cloth, which is two nines to HEPA's 3.5.

(More than that is probably not justified since fit won't be perfect, some leakage. Industrially etc. where there's a need for greater filtration I expect that's when you have to step up to fully enclosed hoods. I digress..)

   > Not a comparison I made

Sure, but it's the relevant one.

   > if I went to a supermarket [...] I would wear an FFP3 dust mask 
   rather than ill-fitting cloth, which is two nines to HEPA's 3.5.

Good. That would be very safe. Ideally everybody would do that. I'll let you think about how feasible that is.

I mean, your entire argument seems to be that cloth masks aren't perfect, I guess?

They sure aren't. But that's not the claim anybody is making. The evidence-backed claim is that they are better than nothing -- and "doing absolutely nothing" is apparently the hill that hundreds of millions of Americans have chosen to (literally) die upon We are trying to take a step forward from that, via a simple harm-reduction method that almost anybody can afford.

Let me ask you something. How effective would a public health measure need to be before you'd endorse it?

1%? 10%? 99%? 100%?

There have been 40 million confirmed cases in America, and likely several times that amount in reality. Over half a million deaths. Each single percentage point improvement in prevention represents > 400,000K cases and > 6,000 deaths.

Anything is better than nothing. Even if half the air isn't filtered because of bad fit, that is still half the air that is filtered.

I live in an area that had over about 6 weeks with us pm2.5 aqi above 300 almost every day and many days exceeding 400 and it shocks me how many people still don't have filters of any kind and see this kind of DIY fan as excessive and silly. Ok, more filters for me I suppose.

Not sure how this helps. As soon as your kid sneezes near you, you're gonna catch covid. Air quality seems like a solid usage but in your house vs covid/flu? Your fan is gonna lose. Covid from outside your home? It's a non-issue and doesn't happen.

> As soon as your kid sneezes near you, you're gonna catch covid

That isn't for sure. Last I checked (over a year ago, so different variants) 50% of spouses sleeping together didn't catch COVID.

We are reasonably sure that amount of virus particles you are exposed to is a factor on if you will get COVID. So the more you can eliminate the better your odds are.

Delta is at least twice as virulent as the old strains.

I haven't been able to prove this well yet but I got the dyson air purifier and it seems to have done really well at reducing my hey fever. Even when leaving home it seems to be not much of an issue. Possibly just reducing the allergens in the air for most of the time makes it not an issue when I experience them outside for short periods.

Hay fever that’s caused by particulates in your house sounds weird. Maybe get a check for mold?

The pollen comes in through the windows and gets me hard. While I'm at home I have no issues, when I go outside I start to get hit with them but by the time it's really kicking in I'm usually on my way home again anyway.

It's hard to actually prove the air filter did anything since a lot of factors for me have changed since last season but this year I am affected far less than I was last and not feeling any effects right now while some coworkers are.

During fire season this year, living in one of the cities with the worst air quality in Canada this year, I had to double up my fans to keep up when it reaches 400+ US pm2.5 aqi in my home office that is about 300 sq ft. I am using filtrete 1900 20x20x1 from amazon (Canada)

I also have a Dyson air cleaner and it works for moderate to light smoke but it's just not enough for heavy smoke.

I've been doing this wrong all along then? I've got my filter on the outtake side. What is the difference I wonder.

From my understanding the biggest difference is the blades not getting covered in grime, because you're pre-filtering the air.

I've also heard airflow dynamics arguments, but I feel like I've heard them argued in both directions.

You can use two fans, one on intake, one on exhaust. Or three, if you have a particularly fine filter. But building another whole unit to run in another room might be better.

With a #9, one-inch filter, a single fan does fine.

The difference is your "fan/blower" will not function as well as a "compressor". You want to vent towards the largest, least constrained volume available, from higher constraints towards lower constraints.

Outtake muffles agitation from the airflow.

Intake still allows the air coming out to be more turbulent, mixing with the existing air better and giving it more "throw" in larger spaces.

You should be fine, here is the comparison

https://www.youtube.com/watch?v=zE4qmN_EOXw

edit1: exact time of the result https://youtu.be/zE4qmN_EOXw?t=250

> Both filters are on the intake side of the box fan. This means you don't need a bungee cord because the intake has negative pressure, the filters just "stick"

I agree with putting them on the intake side, but I still want them to stay put when the fan is turned off (with the switch or to move it to another outlet or in a power outage).

There are large rubber bands you can buy specifically to hold the filter in place on the box fan. I've seen them on Amazon.

Is such device useful to minimise dusting of objects in the room? Like plants, cupboards etc. ? Do you still have to clean dust in the room?

After using a purifier for two years pretty much non-stop (close to 24/7) I am pretty certain they don't help with dust at all. That's not why we install them, though.

Re: home dust filtration

How can you even search for this? Google results are not relevant to dust at all:

https://www.google.com/search?q=diy+air+dust+filter

This is a really strong example of how bad goog sucks for long-tail queries.

SEO spammed: https://www.google.com/search?q=home+dust+filtration+old+hou...

I've found that tacking 'pdf' onto search queries gets through some of the SEO garbage. For instance I did 'home dust filtration pdf' and found this pdf:

https://www.epa.gov/sites/default/files/2018-07/documents/re...

which cites this paper about deposition rates of different particle sizes:

https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=86094...

> How can you even search for this? Google results are not relevant to dust at all:

Try this: https://www.google.com/search?q=diy+air+%22dust+filter%22

I have a couple of blu air purifiers. They really cut down on the amount of dust in the room. We used to have to dust weekly, in the rooms where we have the purifiers we dust maybe every other month now and you can't tell a difference.

Since this describes such a different experience from my sibling comment, I'll have to add that my purifier cleans particulates just fine (close to 0 µg/m³ in summer and 20-40 µg/m³ in winter), which is supported by three separate PMS5003 and a couple of years of data piped into Grafana.

I don't know why it's so useless against dust. Maybe we have more coarse-grained dust which quickly falls down on the ground.

The blu air filter has a prefilter specifically for dust and it gets filthy. That has to get washed in the laundry regularly. Maybe some combination of that, it's cfm's and placement might make a difference?

I have 2 coway units in a ~800sqft apartment and still have tons of dust but I open the windows a lot when its not smoky and I have a dog so who knows. The pre-filter does trap a lot of dust so there is obviously some (maybe minor) reduction.

There is still apparent dust after years (have cats) but OMG the filters get gross.

How strong is the air flow with the filter attached? Is it a slow trickle or can you really feel a decent amount coming out?

Also, 4 inches is quite a lot of depth for a MERV filter, could you elaborate on this point?

4 inches is deep for a standard dimensioned MERV-rated filter. It’s normal for a high quality but sadly usually proprietary fancy HVAC filter (Aprilaire, Lennox, etc).

That's true, but I want to know what they meant when they said the 4-inch pleating is designed to reduce air resistance. Since it's an unusually large amount of depth for a MERV filter, I would expect the opposite to happen?

Air resistance goes down with thicker filters because filter area goes up.

Ok, yes. Do we know how the air resistance/air flow varies as a function of surface area? Is it almost linear? I'm wondering how important it is.

Feel the air coming out, vs without a filter. The slower the air is, the fewer opportunities you get to trap each dust grain. Overall miss rate improves exponentially with airflow speed.

That doesn’t make sense, the higher the flow rate the higher the energy each particle has so more chance of it making it through the filter.

You might imagine so, but that turns out not to be how it works. Each time through there is a chance the particle will be blocked. It doesn't matter how fast in went in, because the mv^2 of a dust mote is tiny at any plausible speed. What matters is that N trips through gives you (1-p)^N probability of not getting caught, which falls off very fast... in fact, exponentially.

A deeper-pleated filter is the way to reduce velocity in traversal.

I've used both. I don't have hard numbers since they are different filters, but I replaced a 1" filter with a 4" filter and I can run my fan at a lower speed.

Note, the actual filter material is not "thicker", but is just folded or "pleated".

The 4-inch pleating translates into a larger surface area of filter material, which in turn means the flow rate per area is reduced. So you have the same amount of air going through 4x the area of filter material compared to a 1" filter. Its analogous to using a thicker wire with the same current.

thanks, do you have a website to put updates ? I'd love to know how it goes long term wise (filter changes, average air quality, your health)

How do you measure your air quality? What sensor do you use? I have a similar setup and I'd like to measure the results.

I know its working because I don't smell smoke, but I'm curious to see a measurement

Nice setup! Could you also share the names (brands/model, e.g. 3M Filtrete 20x20x4) of the three components? I see a few options out there but rather go with something that is vouched for here.

What sensor do you use?

Thank you for sharing !

I built a "four filter fan box" this year when the smoke started rolling in (off MERV 13 filters, I think?), and have also gone about getting the measurement devices to verify that, yes, it really does work! I used a round fan, and a lot of tape to seal things to the fan, though it appears this might not matter as much as I thought. The filter area is sufficient that the fan doesn't really struggle - fan motors burning out from a single filter (as someone else reported) is a well known failure mode, and a good reason not to just strap filters flat to a fan. They rely on the airflow to cool, and it's easy to screw that up.

Air quality (in terms of PM2.5 and PM10, the stuff my testers measure) coming out of the fan is very good - far lower than the room, and the filter will start pulling down air particulates in a room fairly quickly. If the room is around 5ug/m^3 PM2.5, the output from the fan is sub-1, and it moves enough air to do quite a few air exchanges per day. It really does make a difference.

I'm using a couple Temtop meters to measure air quality, and while they're Chinesium, they do seem to work fairly well.

Also, if you ever wondered: Use your stove's vent fans! The air quality difference in PM2.5/PM10 between cooking without those running, vs cooking with them on, is HUGE! I'll see spikes in the kitchen up well past 100ug/m^3 PM2.5 after browning meat, which is worse than it is outside in heavy forest fire smoke!

https://www.sevarg.net/2021/08/28/temtop-air-quality-sensors...

Filter's airflow restriction is primarily a function of surface area and filtration medium layers. More surface area results in a better airflow and more filtration medium layers results in a worse airflow.

To maximize surface area of a filter, you can get a thicker accordion filter. This will generally provide good airflow and filtration characreristics.

Sorry about video links below, I liked them at chart times, so you don't have to watch the video to check out the summary.

Here is an example of airflow restriction vs filter. Note that 4 inch Honeywell filter only provides small static pressure rise over no filter option: https://youtu.be/RkjRKIRva58?t=456

At the same time, same filter is 5th overall in large particle removal: https://youtu.be/RkjRKIRva58?t=661

Thicker filters cost more, but also last longer. General rule seems to be that when you double the filter thickness, time between replacements doubles as well.

It’s more a function of surface area to volume ratio of the filter. More surface area to volume means more energy is lost due to flow friction between the air and filter walls. However the trade-off is that a higher surface area to volume ratio results in higher filtration efficiency so you have to optimize between flow rate and filter efficiency to get a overall optimum filtration performance.

As part of your testing, did you notice a big difference in the amount of airflow when you attached one versus four filters onto the fan? I wish to know how important that surface area is for air flow.

I never tried just one filter, so... no. Sorry. Too many stories of fan motors burning up with a single filter attached.

What would you estimate is the percentage reduction in air flow when comparing the fan by itself to the fan with the filters attached?

> Use your stove's vent fans!

We saw a huge improvement in air quality (less smoke, less steam, less grease) when I installed a vent to the outside so the microwave fan over the stove could vent outside instead of just circulating the smoke through some charcoal filters.

That makes sense during normal times, but when the air quality outside is hazardous, venting the air outside means it's pulling unfiltered air inside your house. So use sparingly.

The last time I lived in a place with an unvented stovetop microwave fan, the previous owner never bothered to install the filters, so it was just recirculating unfiltered air.

It pull in outside air, yes, but things are more dynamic than that. Most likely you don't have perfect seals on all your windows, walls, doors, pipes, fans etc.

Secondly, the air from cooking is generally very bad, especially if you're using nonstick surfaces or using much of anything with oil. It's probably better to trade it for smokey air outside.

Also, please make sure to clean them regularly. Depending on the type of cooking, I've found them dripping grease before.

Last year when the west coast was blanketed in smoke, I setup a box fan with a HEPA filter. It probably helped, but we also found that the fan's motor was working really hard. The cord's connection point to the fan started melting (unbeknownst to us), causing the fan to intermittently cut out. I only noticed the problem when, long after the forest fires were gone, we noticed a strong smell of melted plastic coming from the fan itself.

All in all, probably a good tradeoff to clean our air during the dangerous smoke levels, but there's the risk of burning out the motor.

Be careful out there.

You just need to increase the total area of the filter material that the fan is pushing air through. I've seen people shape them into a triangle or box with plastic/tape over seams or openings.

Do we know how the air flow improves in proportion to surface area? I assume it's not quite linear. It would be good to know whether it's worth the effort to do what you're describing.

I'm sure there is an answer to that but this is DIY, so maybe just touch the fan while its been operating for a while, say "damn that sucker's getting hot!" and add more surface area until satisfied you won't burn your house down.

That's because box fan are not designed as static pressure fan. They are a airflow fan for the purpose of moving much air as possible with minimal resistance. Static pressure designed as forced pull/push air, it have slightly thick blade and have a different motor for it. The air purifier uses the static pressure fan because of the filter media.

Take computer fans for example. There are two common case fan type, the airflow and the static pressure. The airflow fan is for the tower case, the static pressure one is for the radiator (water-cooling kit).

Try putting the filter on the suction side of the fan. This will put less load than normal on the fan and there should still be enough airflow to cool the motor.

Technically, since you're sending the air through the filter multiple times, you don't need a HEPA filter. HEPA filters are optimized for single-pass airflows.

And there are thicker filters which give you more pleats, and thus less air impedance, in addition to the "triangle" method suggested by a sibling comment.

If you notice your fan motor or cord getting hot you can reduce the pressure differential (and therefore torque/load on the motor) by allowing some airflow to get around the filter- i.e. push the filter a bit to the side or add a tiny gap between the filter and the fan by sticking in a little block of wood.

This will come at a cost of filtration effectiveness (less airflow going through the filter) but will save your fan in the long run. Make sure not to let your gap or hole get too big or your filter will stop flowing enough air to work.

And of course, air will circulate through the fan multiple times over time so it doesn't have to catch everything in one go.

Or burning your house down!

I'm glad you caught that before it caught fire.

The ~$25 box fan I got for a DIY air filter was simply failing to push much air through the 3M filter, but I didn't think to check whether the cord/plug or motor was getting hot in addition.

The author later made a more involved DIY filter using a duct fan

https://dynomight.net/better-DIY-air-purifier.html

I keep seeing articles about these and I have to ask: are people actually building them? Sure they work but they’re so ugly. Is it really worth saving $40 for your bedroom to look like a garage?

I certainly built an ugly filter box this year.

What's better? My bedroom having some home built filtration art in it, or the inside of my lungs looking like they've sucked down a lot of forest fire?

If you care that deeply about what your room looks like, great. There are plenty of good looking air filters you can pay lots of money for. I'll spend far less to have a far better filter out of the deal.

OP's comparison wasn't a homemade filter versus no filter, it was versus commercial options.

I've done box fan filters before, and they certainly help if they're what you can afford or can find in stores, but after enough years of dealing with wildfire smoke this year I invested in commercial units and I am significantly happier.

After doing a fair amount of research, I ended up with a series of Coway units. They're more expensive than box fan filters, but they're smaller and significantly quieter, especially with a sensor-driven auto mode that adjusts fan speed based on detected pollution levels. This means I'm far more willing to leave them on all the time and get the benefits 24/7. The fact that they're multi-stage is a big plus too, particularly with cat hair around. It means that I have to clean the outer charcoal filters more often but the inner HEPA filters only 1-2 times a year. As an added bonus, power consumption is significantly lower than my box fans.

Which Cowan units do you use? The only concern I have with smaller air filter devices is they sometimes can have significantly less surface area than the furnace filters typically used with box fans.

The Airmega 400 for the main living room and a pair of AW-1512/Airmega 200 (they're the exact same unit with cosmetic differences, get whichever you can find in stock unless you strongly prefer the look of one). Surface area wasn't my main focus. I'd rather change filters more often (not that that's a huge issue outside of wildfire season) and deal with a quieter unit.

These articles made sense 5 years ago when there were only a few high priced air filters widely available online, but now the chinese copies have proliferated amazon and other sites and the market is pretty saturated imo. My filter was $30, about the size and look of a larger bluetooth speaker, it is dead silent on the lowest setting, replacement filters another $15 or so, and its good to filter my bedroom pretty heavily in terms of square footage. Can't beat that. At that price we have one of these in a couple different rooms.

Ditto. Mine is really quiet too on low settings, which is great if you want to leave it on while you sleep.

They also have a carbon layer that's supposed to help with odours and that pollution that can't meaningfully be caught by a simple HEPA filter, although I don't know how effective it is.

Have you actually measured particulates to see if it is doing its job? Currently running a box fan plus filter but might switch if these are actually good.

I am using a cheap Xiaomi (there's not much choice where I live), and a few PMS5003 sensors scattered around the apartment. It works fine for 25 m²-something room when it's relatively clean outside (around 100 µg/m³ or less), but you really have to push it (and suffer the resulting noise) when pollution outside goes above 300 µg/m³ or so. It's been able to keep indoor air around 20 µg/m³ with outside levels going above 800 µg/m³, but I have to use ear plugs because the noise is impossible to tolerate for more than half an hour (and it's pretty difficult to sleep next to).

If you let the heavily polluted air in (a few hundred µg/m³), then close the window and set the purifier to maximum power, it cleans the room in ~20-25 minutes.

I have built two. $40 for the both of them (cheap box fans), and a 20x20x1 filter fits the back quite nicely. They look good enough, and they work fantastically.

In one month of north-west smoke, they got nasty as hell, which is that much less crud aggravating my lungs.

I have seen variations of this used to build laboratory flowhoods. People have been building variations of these in amateur bio labs for the past 20 years and probably longer.

There was a particularly nasty American west coast wildfire a couple years ago, skies red like a science fiction movie. Air purifiers were sold out everywhere, but you could still get a box fan and a couple furnace filters.

> but you could still get a box fan and a couple furnace filters.

That wasn't my experience. The day the smoke moved in, I ran to the home store to grab a couple extra furnace filters, and there was a steady stream of people grabbing them, and heading to the box fan section. The secret seems to be out.

Yeah, the filters sold out everywhere here in Washington State in Sept 2020. My daughter was in Yellowstone at the time and, on her drive back, stopped at multiple home and hardware stores along the way and found none available.

It's incredible how much some people simply do not care about the aesthetics of their living areas.

Some people like that workshop-esque aesthetic. I find it nicer than the clinical Ikea aesthetic.

Ah yes, the two aesthetics.

Even when you tell yourself that you do not care, it still has an impact on your mental state. I used to have a dump of a room with junk like this stacked up and after getting rid of it all, I just felt so much better while being in the space.

In my experience, no it's not worth it indoors in a relatively clean environment. I built 3 of these for myself with dust, carbon, & hepa filters, and I used them for over a year.

They are louder, use more energy, are less efficient, uglier & cost more money than several store bought alternatives which often include several carbon & hepa filter replacements which is the main expense in building these fans.

They work well in the garage though, and if you don't plan on changing the filters frequently they are the cheapest build option.

for 2020 I made one, it was well worth it for having indoor air that wasn't AQI ~300.

It was a stop gap until I could actually get my hands on a large commercial air purifier, those were unavailable at the time.

> Is it really worth saving $40 for your bedroom to look like a garage?

What is more important to you: superficial nonsense such as looks or your health?

Go ahead and spend the money so your crush pad can retain its splended wonder while the rest of us breath happily along with some extra dough in our pockets.

Or I can just spend $40 more and get a nice looking air purifier? You’re making a false dichotomy. That’s like asking if I would rather wear plastic bags or be naked. There’s other options

> Or I can just spend $40 more and get a nice looking air purifier?

Then YOU go ahead and do that. My point of arguing your elitist criticism is that some people either don't have or need that extra $40.

How many purifiers have you bought?

The custom purifiers always have proprietary filters. That’s where they make their money. It isn’t $40 difference, it’s $40 per year. Often more. And once they stop selling that model, they’ll discontinue the filters and so you’re buying another one. And another. And another. Each with its own problems that you have to live with or shop around for, which means more purifiers purchased while sorting it out.

To each their own, but I agree, I like having a tidy living space and an ugly box fan contraption isn't too high on my list, just to save a few bucks.

Even then, I just keep a big stack of furnace filters handy and swap them regularly. When we got hit with the wildfire smoke last year I put in a new filter every day. More practical than putting box fans around the house.

I have a regular air filter and it’s fine for maintaining air quality.

However, The box fan setup uses a much larger filter and pushes way more air. I only take it out if the outdoor quality is abnormally poor.

Yes it’s ugly and noisy but it’s for emergencies.

Also, you’re saving way more than $40. Wirecutter’s recommended one costs $200.

Now that regular cost-effective home HEPA filters are widely available I mostly rely on those. However, I find that the box fan + furnace filter works really well in the window to bring in fresh air without letting the pm2.5 in. This is nice because otherwise CO2 levels build up and it gets stuffy inside. This works on all but the worst days when you can really smell the fire/smoke outside, AQI > 200.

Overall, its not the prettiest and I'd prefer a HEPA filtered energy recovery ventilation but I'm not going to install that in my rental for a couple weeks a year. Usually in Oakland climate the windows are open.

They aren't hard to store. If you already have a box fan keeping a few extra filters ready it's a good temp solution.

For those that don't want to build their own, I bought one of these about 5 years ago:

https://www.whirlpoolairpurifiers.com/product/whirlpool-wp50...

I measured it last year during bad air quality days, on high, it would take the bedroom from ~150 AQI to less than 10 in about 15 minutes. On low it's quiet enough to be unobtrusive while sleeping.

Filters cost around $100 and are rated to last a year (and even after a heavy fire season, I've found that to be true)

More expensive than a box fan + filter, but easier to use and a little more attractive.

Though this year, I upgraded my furnace with an AprilAire filter box and an MERV13 filter, which has worked surprisingly well, I thought I'd still want to use the standalone HEPA filter in the bedroom, but the furnace filter keeps the entire house below AQI 10 even with outside AQI at 250 (haven't had air worse than that yet this year)

Great study & article. Perhaps incense is not the most scientific test, but it's effective enough for this setup.

Does anyone know if there is raspberry pi add-on or device that acts as an "air quality" sensor, or similar? It would be interesting to monitor this a bit more scientifically in real-time in a variety of environments.

Lastly, I think I've seen this here before, or on reddit. I was inspired to do something similar myself - I use a 19" box fan and a purple 3M MERV furnace filter, 20" square. Except the rounded corners they are almost exactly the same size. Some white duct tape around the edges to seal/hold in place. Total cost is about $40 usd for the fan & filter. I would anticipate a similar performance to the leaky MERV's, as mine is filtering more air as a ratio to total air flowed by the fan.

You can buy air quality sensor boards that should be pretty easy to integrate with arduino or an rpi. I have one from sparkfun.

Yes, just set up a sensor with home assistant (HA). Incredibly easy. Raspberry pi runs the HA server. The air quality sensor is attached to an ESP32 running ESPHome: https://esphome.io/#air-quality

Almost plug and play. Compiles the FW, flashes it, and the device auto connects to the network and is ready for integration into HA. I used the HM3301 air quality sensor FWIW. You can see my filter here: https://news.ycombinator.com/item?id=28395232

>Does anyone know if there is raspberry pi add-on or device that acts as an "air quality" sensor, or similar?

Yes. They contain an annoying tiny fan though that gets irritating fast due to high pitch

I've made one of these, and they do work extremely well, but they're missing one key thing that I now consider vital: Activated Carbon.

A HEPA filter is doing the heavy lifting within any air purifier, it is therefore the most important ingredient. Activated Carbon is well known for removing smells/cents, but it does more than just that: It also neutralizes VOCs and some other gaseous pollutants which can travel through a HEPA filter.

This is why I currently have a commercial air filtering solution instead of my box fan + 3M filter, it has suspended Activated Carbon around the HEPA filter fabric. I don't know how I'd achieve that with my box fan solution, and I've not seen anyone else try to address it.

Other air purification technologies have safety issues (e.g. ionizers, ozone) or just outright don't work (e.g. UV light inside a channel). HEPA and Activated Carbon have measurable upsides with no downsides aside from their cost. They're the best we have today in spaces occupied by people.

A DIY solution would be to get activated carbon that's intended for water filtration (fish tanks, etc). Then add a step that moves the air through the media (put it into a mesh box, or heck even just blow the air down onto the carbon that's sitting in a "dish" or some kind.

Certainly not as efficient as a commercial all-in-one solution, but it that's not available to you, it can work.

You can even make your own "activated charcoal" but it's a pretty messy and time consuming process.

Would activated charcoal for aquarium work thought for gas? We're talking about a significant difference in porosity here. Even going with something like ROX (https://www.bulkreefsupply.com/bulk-premium-rox-0-8-aquarium...) it's going to have much larger pores and much smaller surface area than a carbon coating on a filter

Good question. I'm imagining that with a small enough space / large enough quantity of charcoal you'd still get benefits, but it's not like I sat down and tried to do all the math.

You can find activated carbon filters online (and in many stores). Maybe that would work for a DIY solution? https://www.amazon.com/s?k=activated+carbon+air+filter

Yeah the activated carbon filter in my commercial purifier is just a sheet that sits in front of the filter. You can buy one and duct tape it to the DIY solution. It has to be vacuumed regularly and eventually replaced.

Also there doesn't seem to be any need to worry about sealing the filters as the author's experiments showed, and because the stuff you're filtering is already floating around. It would matter in a vacuum cleaner or some such pollutant-kicking stream.

These filters have activated carbon in them, though probably not very much of it: https://www.amazon.com/Nordic-Pure-20x20x4-Pleated-Furnace/d...

FWIW a 24" wheeled shop fan from Harbor Freight, Tractor Supply, Rural King, etc etc (sometimes HD/Lowes have them) move way more air with more authority than a box fan. They will run $150 but they can be very useful to have around even when you don't need to filter the air. (Note the motors pull more current so you still have to be careful not to overheat them)

Some examples:

https://www.harborfreight.com/merchandising-promotions/hot-b...

https://www.tractorsupply.com/tsc/product/countyline-24-in-d...

https://www.homedepot.com/p/Commercial-Electric-24-in-Heavy-...

Axial fans are still better for this application, but these fans are great to have around in summer evenings when the air outside is cooler than it is inside. Put it in one window of the house sucking the air outside, then open windows on the other end of the house and let the cool night air stream in. It's fantastic.

Wirecutter performed a similar experiment a few years back. Their results were somewhat different, showing that the DIY version helped significantly, but not nearly as well as the best of the commerical ones they tested:

https://www.nytimes.com/wirecutter/reviews/best-air-purifier...

I wonder what accounts for the difference in results.

Wirecutter pretty consistently produces spurious results, and bad recommendations.

Ben Kuhn and Dan Luu have written about this, Cf.

The wirecutter DIY test used a filter with a rating of FPR 9. Since FPR isn't standardized like MERV or HEPA, it's hard to tell what this means, but likely to be < MERV 13. The OP used a HEPA filter on their testing which is > MERV 16.

Wonder if any PC gaming case manufacturers will ever think to add some HEPA filters to the unit - be good for the PC to have cleaner air going in and that airflow out would be put to good use to cool the PC.

I recall seeing a video (BigClive on YT) in which he gets a fan and used a peace of kitchen roll as a filter and found it fairly effective and just dug that out - https://youtu.be/kK7sJq2E0bE

But then you’d have to change the filters and if you didn’t possibly cause airflow problems and then overheating. It would be a nice feature as long as you keep up with it though.

Probably cheaper MERV 8 filter would make more sense, since for this application you're mostly concerned about those larger dust particles. Also it seems like this would be a job for the fan manufacturers rather than the PC case manufacturers. The fan just needs a little slot where you can slide in a thin replaceable filter.

you don't really want something that extreme for case fan intake. you end up "paying" for the cleaner air with higher temps or faster (louder) fans.

there are some newer cases with a fine mesh screen over the intake fans. I have these in my fractal meshify 2, and they seem to work pretty well so far.

I used to have a PC case with a cooling fan filter -- by the time I retired that PC, the dust was nearly an inch thick. I had forgotten that it had a filter, and "out of sight, out of mind". It wasn't even a HEPA filter, just a fine mesh.

I've been doing this for years using off the shelf filters you can buy at most any hardware store and a 20" box fan. I use the 2" blue masking tape to attach the filters to the fan and seal them so all the air that flows through them is filtered.

The blue tape comes off easier than cheap masking tape and without leaving any sticky residue on the fan.

I leave the fan running 24/7 and I've bought two of them in the past 15 (or more) years for my home office. I replace the old one earlier this year.

It's pretty amazing how much they grab out the air. I live in a pretty rural area and the road to our house is gravel and I'm surrounded by a forest so there's dust and pollen pretty much constantly in the air. And we have cats. Lots of cats. It's also amazing how much cat hair and dander they collect.

I can sure tell the difference when they're not on, and when I replace the filters after waiting longer than I probably should.

How often (if at all) do the filters require washing to keep them going for that long?

Sorry about the slow reply. It depends on how much dust is in the air. I live in an older home on a gravel road surrounded by forest so there's a lot of dust and pollen. It only takes about a week to start seeing some color in the filters.

I use the paper disposable filters so washing them isn't an option.

Interesting thanks! So I guess you change them weekly then?

I upgraded my heat pump and furnace system last year and spent the extra money to put in a filtration system. We had a two week stretch with AQI of 500+ outside from the fires and never even got a hint of smoke in the house. Great investment.

I have one "built" in my house right now. I say "built" as I have simply bungie-tied an air filter to a box fan. I have also hooked it up to one of those analog outlet timers so it's not running the whole day. I'm sure folks on HN could do a much better and more interesting job:

- Rasberry Pi Zero runs PM2.5 sensor.

- Fan is controlled by Raspberry Pi Zero.

- Fan only turns on when PM2.5 levels are over threshold for n seconds.

- Fan only turns off when PM2.5 levels are below threshold for n seconds.

- Add some collection and graphing of PM2.5 levels over time for fun.

What PM2.5 Sensor are you using? I'm about to start a build to monitor indoor AQI/CO2.

So currently I'm not using any. (the list above is what I am NOT doing)

Previously, I had sensor from this family hooked up to a Raspberry pi doing measurement only:

https://www.amazon.com/AITRIP-Precision-Quality-Detection-Co...

Oh, ha! So much for my reading comprehension. Thanks for the link. I think that should be compatible with a Pi. I can try it out in a couple weeks when I get some free time to tinker.

I have a PM2.5 sensor hooked to a Raspberry Pi Zero W, I use the PMS5003 ($40 at Adafruit). I have a script running in a screen session that posts its measurements once a minute to a Google Cloud Function which then inserts it into BigQuery, and then a DataStudio dashboard to visualize the data.

The cloud cost is $0, it all falls in the free tier. The Raspberry Pi Zero W is just $10.

I have a separate station with an SCD-41 CO2 sensor (+ temp/humidity/pressure with an MCP9808) all hooked up to a TinyPico board with Stemma QT that also post metrics to BigQuery in a similar way. And yes I'm planning to combine both into a single device very soon :-)

One thing I've found trying to find HEPA filters is that many reviews complain of the filters themselves having some kind of stinky off-gassing. I don't know what the reason for this is, or if it's a problem, but it is worrying.

Reviews on most air purifiers seem to have examples of exploding fans and melting components almost burning houses down.

It really seems like there's room in the air purification market for a company that is really trustworthy, effective, and affordable. Apple could do it, for example.

I'm pretty happy with my Coway air purifier, and that seems like the consensus based on the reviews I've read. The purifier was pretty affordable, as are the filters (keep in mind that filter costs and electricity consumption can impact cost of ownership much more than the original purchase price!).

It isn't Apple, but one of their models does have a striking resemblance to an iPod shuffle: https://m.media-amazon.com/images/I/61716qA3rEL.jpg

> Reviews on most air purifiers seem to have examples of exploding fans and melting components almost burning houses down.

I've read the reviews on my air purifier. These sorts of problems were prevalent among the negative reviews.

Upon further inspection, the cause of most of these problems was the air purifier being shipped with a shrink-wrapped filter already installed. Unsurprisingly, turning it on without removing the shrink-wrap results a loud and non-working purifier at best - and fire, death, and bedlam at worst.

One of the key features of the expensive purifiers is reduced noise. Box fans are rather loud.

I have one that cost me $30 all in and you can't hear it at all on the lowest setting (but still decent airflow when you put your hand down). Honestly the biggest benefit in my home environment is how much it has killed the fungus gnats living in my plants. I checked the filter after running it for a few weeks and it still looked relatively clean, but probably a thousand dead flies were in the bottom of it too. I have taken other measures against these flies with not much success (vinegar traps, sticky paper, watering from the bottom, mosquito bits designed to kill larvae, I tried everything I could find one the internet to no help).

On the other hand a loud fan is great to help shut out the neighbors annoying dog barking.

I have box fan filters, and a small room Honeywell filter. On the lowest settings, they're about the same, on the highest, the box fan is definitely a little louder (but moves a lot more air).

Turn them down when you're in a room with them - best advice I ever read.

Yeah, I think these DIY filter plans are great and I've built them myself, but I switched to commercial purifiers because of the sound. I don't have measurements but the box fan is substantially louder than either the Blue Air 211 or the Levoit Core 200 that I have.

I had considered something similar, but recently I found that Ikea has started selling air purifiers and sensors for very reasonable money. They even sell an activated carbon filter that you can add.

Ikea FÖRNUFTIG: https://www.ikea.com/us/en/p/foernuftig-air-purifier-white-5...

For years I had planned to make a DIY one, but never got round to it. But a week ago bought one of the IKEA ones, plus a carbon filter, and I've been very happy. The air feels cleaner and somehow just nicer.

Lots of reviews say it's underpowered for a normal sized room compared to alternative products. But for me, the alternative is a DIY one that I still wouldn't have got round to making.

I'd recommend it.

I've been running one of these for a few months. The filter has slowly turned darker brown - which suggests it's working (not anywhere near forest fires here!).

It has a trickle mode which is completely silent and uses about 1W of power. Something you won't get on a DIY fan.

I read a wired review [0] that says the ikea purifier "blocks 99.95 percent of particles 2.5 microns or larger".

Could you confirm the technical spec of the filters you are using? Actual rating? Is it actually HEPA?

The ikea website seems to just be pretty vague marketing stuff with no details.

[0] https://www.wired.com/story/ikea-fornuftig-air-purifier/

You can read more details after you click "Product details". It says that the filter corresponds to EPA12 class, so no HEPA.

> The particle filter is tested according to EN 1822-1 and ISO 29463-3 which corresponds to class EPA12.

> I borrowed a cheap-ish ($100) air quality monitor from a friend. I think it’s made by some company in China and then re-sold by various white-label brands. I can’t figure out who the original manufacturer is. Based on data I’ve seen for the reliability of other air quality monitors, I wouldn’t trust the absolute numbers, but the I think the relative measurements should still be OK.

That seems like a pretty big assumption.

Does anybody from Europe has a similar DIY setup? I cannot seem to find box fans here. And in my country particular - no rectangular HEPA filters.

Product engineer here from a major filter manufacturing company. I've done a ton of tests this summer evaluating DIY box fan filters and I have a couple insights for people building box fan filters this year.

1) Box fans (and other axial fans like your ceiling fan) are terrible at pulling air and an HVAC filter will significantly reduce fan speed due to the added pressure differential across the filter. As a reference, a new MERV 13 filter can reduce fan speed by ~33% when mounted to the intake side, and ~66% if mounted to the front of a box fan. The motor used in your box fan is cooled by the air passing around it, which is why choking off the air flow through your fan can lead to overheating, damaging your fan and creating a fire hazard.

If you're going to make a DIY air purifier, mount the filter to the intake side. You'll get much better performance, your fan will stay cleaner, and there's less risk of damage.

2) Some analysis of the test data from the posted article: CADR stands for Clean Air Delivery Rate, measured in cfm. It's basically a measure of how efficiently ambient air in a confined space is purified. A higher CADR means particles are pulled out of the air quicker. For now, just treat this as a relative quality value. Using the PM 2.5 graph for the tiny room experiment in the article, I'd estimate that the DIY filter the author created has a CADR value of 45-50. This is a pretty low value. The room air purifier (RAP) he used is even worse. Then, from the large room graph, I'd estimate the DIY purifier as a CADR of 30-35. This is a rough estimate for two reasons. One, room size is an important variable in this calculation and I don't have an exact value. Two, when I test a purifier, the test chamber starts at a PM 2.5 around 10^5. At the extremely low starting concentrations used in the author's experiments, the percentage of particles that are removed due to natural decay is much more significant. Side note, cigarette smoke is the standard for room air purifier testing. Incense sticks are used less commonly due to their slower particle generation.

3) For wildfire season, I recommend a MERV 13 filter for overall performance, cost effectiveness, and for smoke particle capture (PM 2.5). There is a clear trend of diminishing returns for a DIY box fan filter beyond MERV 12-13 filters, peaking at a CADR value of ~150 for a MERV 13 filter for box fans I've tested at their highest fan speeds. A MERV 13 filter is about 50% better than a MERV 10 in a box fan configuration, while a MERV 14 is actually slightly worse. This will vary based on filter brands, too. Rule of thumb, quality matters. We actually rate our filters a bit lower than their actual performance for a number of reasons.

So, my company offers a standing room air purifier with a HEPA filter for about $200. It has a CADR value of 158. A box fan and a MERV 13 filter cost about $40 and has a CADR around 150 - pretty good for a DIY substitution. We even offer cheaper room air purifiers with even lower CADR values. So why buy an expensive room air purifier?

First, room air purifiers use a radial fan rather than an axial fan. Axial fans create a low pressure area on the exhaust side, drawing air through the fan. Radial fans draw in lower pressure air near the axle of the fan and push out higher pressure air at an exhaust port at the radius of the fan. Room air purifiers use a radial fan to push air through the high pressure drop HEPA filters they are designed to use. HEPA filters are qualified to remove +99.9% of tiny particles (PM 0.3) in one pass. The thick filtration media requires a high pressure differential to pull air through it. A HEPA filter on an axial box fan is going to kill the motor. If you care about PM 0.3 particles, only a HEPA filter will do the job.

Also, longevity. A room air purifier and HEPA filter should run for a year or longer without needing to change filters. An HVAC filter is meant to last 3 months in your home air system under a normal particle load. This lifespan can be much shorter due to poor conditions, such as smoke particles during wildfire season or drywall dust from a renovation project. (Side note - seriously, replace your filter after doing any drywall work. Anything better than a fiberglass filter can clog in just a day or two of dusty drywalling. Also also, fiberglass filters do absolutely nothing, don't buy them.) The room air purifier is designed to run for years with a high pressure differential filter. Your box fan is not. A cheap 20" box fan with an HVAC filter is a good temporary solution, but will not last nearly as long as a room air purifier used daily.

Last, noise and style. RAP units are very quiet and blend into your living room space. A large box fan with a strapped on furnace filter makes for an interesting conversation piece, if you could hear your guests.

4) Don't worry about hermetically sealing your 20x20 filter to the box fan. From my experience, a filter with tape sealing every side to the fan is no more efficient than a filter held to the fan at a couple of contact points. Once the fan is on, the intake air will hold the filter close to the fan. Even with a fully sealed filter, remember that axial fans suck at sucking. Your axial box fan will actually draw air around the front corners and into the fan, even without a filter on the back. And with how easy it is to reduce the fan speed of a box fan, you're better off allowing some air through so that the fan runs at a more efficient and safer speed.

5) A filter on a box fan is definitely better than nothing and a good, cheap short term solution. RAP units are great for long term use and capturing all sizes of particulate. I'm not going to try to sell you my brand, but there is one product I advise you avoid. Recently, Lasko has released a box fan with a filter slot as a 2 in 1 product. I've tested this thing with the provided filter and it does not perform nearly as well as a DIY filter you could make. Also, the filter slot does not fit standard 20x20x1 filters. The slot is designed for a slightly smaller, 19x19x1 sized filter, meaning that if you want to buy a replacement filter or a higher quality one than provided, you won't be able to. A normal 20x20 filter (which is actually slightly smaller in W x L nominally) can barely be squeezed into this fan's slot. Just buy a better filter and some tape instead.

6) If you look for DIY box fan filters, you will find examples with 4-5 filters in a cube shape. 5 filters improves filtration efficiency by ~50%, but costs x5 as much. That's $100-$150 in filters that will last just about as long as one filter while only cleaning the air 50% better. Just invest in a room air purifier unit of the same cost instead and enjoy much cleaner air over an entire year.

Thanks for such detailed info!

Who would you recommend as quality filter brands?

Thanks, this is all helpful.

Why is there any value in trapping 99.9% of particles on the first pass? There will be plenty more opportunities as the air circulates. Five passes at 75% each gets 99.9%, too.

I understand the need in a vacuum cleaner, or in a window fan, where the filter is only traversed once.

>6) If you look for DIY box fan filters, you will find examples with 4-5 filters in a cube shape. 5 filters improves filtration efficiency by ~50%, but costs x5 as much. That's $100-$150 in filters that will last just about as long as one filter while only cleaning the air 50% better. Just invest in a room air purifier unit of the same cost instead and enjoy much cleaner air over an entire year.

The cube box fan configurations with 5 filters cost 5x as much, but how can they not last longer than one filter? Surely each filter must get used up at about 20% of the rate that it would have it if were the only filter attached the fan? So a fan with 5x filters should last about 5x as long. Or is something I'm missing? (Time decay?)

previously - https://news.ycombinator.com/item?id=25442097

Posted before: https://news.ycombinator.com/item?id=26106542

Warning: The fan motor gets cooled from the airflow. This is drastically altered to something it is not designed for, and can result in fire.

So you mount another, smaller fan inside of the box fan to cool the motor ... I wonder how far this can regress.

So, put the filter on the intake side, and prefer a 4-inch thick filter, if you can find one.

Absolutely shameless plug: If you'd like a cleaner looking, 3d printed solution for attaching a filter to a pretty standard box fan, in this fashion, we sell some over on Etsy (use the coupon code HACKERNEWS, 20% off) [1]

Does it work better than duct tape? Probably not. Duct tape does a great job of creating an airtight seal, whereas with these clamps there is a small gap between the fan and the filter. But, it does make it easier to replace the filter, looks clean, and we've ran tests with an AQI meter; definitely has a positive impact.

[1] https://www.etsy.com/listing/980605509/box-fan-filter-holder...

About taping the fan so all the air goes thru the filter:

How does a filter work? Air that goes thru it, gets particles removed. We want to maximize air that goes thru it, and it should be air that has particles in it.

To make more air go thru the filter, increase the pressure difference between the dirty side and the clean side. Fan speed is how we control this. And getting clean air away from the fan and dirty air to come close to the fan also seems directly related to fan speed - how fast and how far it ejects clean air.

It's all about fan speed, baby.

Plugging up parts of the fan unrelated to the filter cross section, can only slow the fan down. I see no reason why it would help in any way.

Yikes. Putting high pressure drop genuine HEPA filters on a fan like that where most of the air will bypass them defeats the purpose and is a waste of energy and money. Just get a bigger MERV 13 or better filter and cover the whole fan.

> defeats the purpose

Based on his testing, it looks like it achieves purpose just fine, and slightly better than the more expensive option.

> and is a waste of energy and money

His setup has over 5 times the filter area (1,400 cm² at 5 cm thick vs. 300 cm² at 4 cm thick), for $10 more on the filter replacement. Energy use is higher, but it's not clear that it exceeds the savings from cheaper filter replacements.

And by having those gaps in the filters, the fan motor is probably being saved from overheating. Plenty of air still gets entrained through the filters, even with some bypass.

It's kind of like premature optimization when writing a program. Where you think improvements are needed isn't necessarily where they should be. Test your code, find the hot spots, and improve those. With a filter setup, it might see obvious that a good seal is needed between filter and fan. Turns out, that's not always true.

This is awesome. I love DIY stuff and love the idea of an affordable clean air solution. For me, the aesthetics, noise, and issues like overworking the fan motor, make the extra $50 for a purpose-built filter worth it.

Only if it actually works better.

But many "purpose-built" units just cost more, but work less well. How do you tell if it will work well, before you spend?

For the one I purchased, I researched them on Consumer Reports, which independently assessed their efficacy. How do you tell they work less well before you make that assertion?

It was sub $50 a few weeks ago and comes with a Merv 10 filter. You can always buy the heavy duty Lasko 20 x 20 for sub $30 and stick a MERV 13 Filter behind it when it's on. The suction keeps the filter pinned to the back and you get HEPA with VOC removal.

https://www.amazon.com/Lasko-FF305-20-inch-Purifier-Purifyin...

The MERV 13 is neither a HEPA nor VOC filter. It is almost as good as a HEPA filter (it performs the same function albeit it's very slightly less effective), but it doesn't filter out VOCs. For VOCs you need an activated carbon layer.

I do recallusing a 3M filter that removed VOC's but perhaps it was a higher MERV Rating.

Here is a substitute.

FPR 12 HomeDepot filter.

https://www.homedepot.com/p/Honeywell-16-x-25-x-1-FPR-12-Air...

The Honeywell Ultimate Allergen filter is made of a reinforced pleated media designed to capture particles down to 0.3 microns, 300x smaller than the diameter of a human hair. This filter incorporates the power of Arm and Hammer odor reduction technology into the media in the form of performance enhancing dots. Its large surface area improves the efficiency, providing a greater dust-holding capacity and lower energy costs. The filters electrostatic charge helps attract particles that would otherwise pass through. The Honeywell Ultimate Allergen Filter is effective against the following contaminants: dust/lint, airborne dust mite debris, pollen, pet dander, mold spores, bacteria, microscopic allergens, virus carriers, odor, most smoke and smog particles, as well as VOCs.

Even a MERV 17 will not clear VOCs. You need a chemical reaction, and MERV describes a purely mechanical property.

It is smart to combine an activated-carbon filter with a particulate filter, particularly (heh) if you are operating a 3d printer. Those really foul the air, in ways hard to detect without instruments.

Same site, different setup: https://dynomight.net/better-DIY-air-purifier.html

I've been keeping an eye on the so-called Corsi-Rosenthal box. I'm probably going to build one or more at some point.

There's a bunch of folks that have been optimizing this design on Twitter. The initial results look quite good and properly put together might last an entire school year.

Link here: https://www.texairfilters.com/iaq-research-practice-in-actio...

You could also go with smaller filters, like 14x20 to reduce the stack size, but the prices don’t scale exactly with surface area, while the pressure does. So I’m not sure how much that really buys you.

IIRC, increased surface area reduces pressure. From what I remember from reading the thread, it appeared to have pretty decent CFM through the use of more and larger filters.

EDIT: I found the thread. See https://twitter.com/davidelfstrom/status/1429526273009889286...

Thread shows the difference between using 1 and 2 filters. I think he has a separate thread in his Twitter feed measuring a 4/5 filter setup too.

Increased surface area alone results in higher pressure losses / flow energy losses due to increased frictional losses from the higher surface area.

It is amusing how this person has the temerity to name this configuration (partially) after themselves, while people have been making and documenting these configurations for at least a decade and surely longer.

Small aside, but if you're looking for a cheap, decent fan, I recommend this one. Bonus: this Lasko fan is made (or should I say "assembled") in the USA: https://www.homedepot.com/p/Lasko-20-in-Power-Plus-Box-Fan-B...

My covid project was building a gym in my unfinished basement, and this is the box fan + furnace filter I use to keep the dust levels down.

Many, perhaps most things with a "Made in USA" sticker are done with prison labor, particularly if made in Louisiana, Mississippi, or Alabama.

States have contracts with such for-profit prisons requiring they be kept fully "staffed", railroading as many victims as needed not to pay contract penalties. Police departments are generally agreeable about such policies; everyone they arrest makes them look more important.

So, to first order, "Made in USA" implies slave labor.

After reading this page, I think Lasko perhaps is not using "slave labour": https://www.lasko.com/social-responsibility/

It is always hard to find out whether such declarations are wholly composed by the PR department, with no commitment from management.

The page cited does not mention prison labor.

The California fire crews do make quite heavy use of prison labor, which seems less problematic than corporate use, except for the safety angle.

It does mention it. Just search for "prison labor" :)

OK, I see that now, thank you.

It would be nice to see this test done with other particulate sizes. I'd wonder how effective this is for smaller particle sizes compared to competitors.

I attached a graph from this morning with raw particulate counts here: https://news.ycombinator.com/item?id=28395232

> This was unexpected. I thought the tape would help, but I wouldn’t have been surprised if it hurt instead. Instead, there’s basically no difference at all. I don’t know enough about fluid dynamics to even speculate about what’s happening here, so I won’t try.

The fan pumps as much air through the filters as it will, regardless of whether the areas not covered by fans are taped off or not.

Here in the toxic air of California, I decided to go with an industrial level "DIY" solution: activated charcoal air scrubber with a high CFM fan. Just hopped down to my local indoor grow shop (4 in a 10 mile radius!) and had all manner of choices. Yes, I spent about $150, but I'm quite sure that I am getting much much more than 3x value out of it.

In Colorado I have been using a few different Coway air purifers and they have been able to keep the air quality in my house good, and quickly can recover that good air quality after running my swamp cooler in the morning/evening. Been overall happy with them but perhaps this box fan/merv filter is a good backup to have in case it gets really bad.

For my spouse's classroom, I made a 4-filter-cube and fan filter setup using this video as the basis.

https://youtu.be/Y7eL2OAnqc8

I added a fan shroud on the exhaust side of the fan using a piece of cardboard with a circular cutout matching the diameter of the fan blades.

Good ideas. Several months ago we had bad fires in the mountains of Central Arizona where my wife and I live. I bought a HEPA filter, the complete kit with enclosure, electronic controls, etc. With an extra filter, I spent about $80 and I feel it was money well spent.

Making your own for less money is even better.

I've built a few of these and compared with higher end air purifiers like Alen. The only downside I could find with the DIY box fans was the power consumption is much higher.. about 60W vs 6W for the commercial purifier.

That is a $30/yr increased cost in electricity based on CA rates.

Surely you can use less powerful fans?

It's about power efficiency, not total power draw. Use less powerful fans with an inefficient setup and you're not cleaning your air as well. Engineers put thought into chassis design and motor/fan specs.

For this same reason I decided to get a used box from a reputable, long lasting company with top of the line CADR, and I can find plenty of aftermarket HEPA filters that are compatible.

That was a comparison was roughly for the same air cleaning rate. I think it has to do with the fan architecture. With purpose built purifiers using static pressure fans instead of air fans.

Some lab tests conducted by a TV show gave the Box Fan With HEPA Filter the highest marks over commercial air purifiers:

https://news.ycombinator.com/item?id=26101153

They didn't give the box fan the highest marks, but they found that it outperformed some commercial products, including the $800 Dyson (which has a very bad price-performance ratio)

there is a company that's selling DIY air purifiers in India (and probably its neighbouring countries).. https://smartairfilters.com/in/en/product/diy-1-1-air-purifi...

they put the filter in front of the fan, not the air intake aka backside.. they have detailed info on how their purifiers are effective than other costly brands.. probably putting the filter on the front side is more effective?

They've confirmed that they tested it. https://twitter.com/herpiko/status/1388621526757298178

I'm using this kind of DIY filter. After of putting the filter in the front of the fan for months, now I modified it into intake mode because of uncomfortable smell occurs whenever I turned the fan on. Some dusts that are already trapped here may have escaped again by sudden push of air.

Don’t most home HVAC systems already filter the air blown through them? I know I replace mine every so often. If they do, what does a separate “air purifier” buy you?

Not sure how much you'd gain, but one thing to note here is that I believe not all HVAC systems have an option of running just the blower w/o also heating the air.

The only furnace I've seen without that option was from the 1950s (probably replaced by now for old age), but I'm sure more exist. It wouldn't be hard from someone who knows power to wire up the fan to run all the time though.

Furnace is one thing, the wiring in place is another I guess. My house is from the '50s, and the furnace was already replaced, but we have not laid new wiring. I think I need to look into that before the next wildfire season starts...

If nothing else it should be easy to put a fan switch right on the furnace.

For central hvac systems, standard systems typically have a filter grille for each return (where the air goes in).

You can also add on a higher performance filter on the return side. Something like a Trion Air Bear.

Function >= Form, at equality life is good.

Can anyone provide insight as to why not taping/sealing up the uncovered portion of the fan may improve performance?

What about no filters? Could it be just pushing air around into things in the room is the most effective?

Prefer the commercial ones with built in particule sensor because they can ramp up & down dynamically

No brainer. Basically all commercial purifiers suck (air), the DIY blow it. Big difference!

He mentions cooking: do the recently-introduced air fryers create significant emissions?

They are basically just a little oven. They don't seem to create anywhere near the smoke level of frying on a pan.

if you watch any of the woodworker youtube channels, this is basically what they do in their shops. a little more elegant, with wooden enclosures (of course), but the idea is the same -- regular house filter on a fan.

That looks pretty similar to the Alton Brown way for making beef jerky at home.

It will be good to see the same tests for PM1.0, TVOC and HCHO. But great job

Needs a (2020) tag. This article already hit the front page when it was new.

While I'm cool with this for allergy sufferers or "smokey" conditions I think the modern world tries a bit too hard to purify and sanitize everything. Eventually you do have to step outside your home and you can't take your filtered air with you.

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