Hacker News Reader

Comment by colechristensen | Hacker News Reader

I have an AirVisual Pro, which is quite expensive, but my half finished air quality monitor project had been on the bench for much too long.

I have windows open all the time, air conditioning on too often, and dehumidifiers and HEPA filters as well. It is not efficient or particularly environmentally friendly... but I want a CO2 level < 800 ppm and a PM2.5 level under 10... so here we are.

My next half baked idea is figuring out a way to have a sealed home and an appliance (or messy bench-top device) to remove CO2 from the room continuously and exhale outside -- current ideas revolve around using a carbonate (lithium? sodium?) in water in an electrolysis cell and an aquarium bubbler to collect CO2 and expel it outside. The best I've been able to accomplish so far is baking CO2 out of baking soda (sodium bicarbonate) and sealing it in about a cubic foot container and achieving ~300 ppm CO2 (lower than outside which is ~400-450).

View on HN

> and an appliance (or messy bench-top device) to remove CO2 from the room continuously and exhale outside

There are these things called "plants"...

https://en.wikipedia.org/wiki/NASA_Clean_Air_Study

The study you linked did not measure CO2.

From my armchair-research, there are scale challenges with plants.

Algae is commonly the go-to for CO2 scrubbing for it's relative efficiencies. Algae might scrub 1-3 grams of CO2 per liter per day (https://www.osti.gov/servlets/purl/1485133).

Humans exhale approximately 1kg CO2 per day.

I consider 300-1000 L of algae scrubber tanks per person a lot of volume.

Bubbling air through quicklime slush is an easy way of removing CO2. You can turn the calcium carbonate that forms back into quicklime by applying heat.

But it's probably better if you invested into a counterflow heat exchanger for ventilating your home. You can recover a large part of the heating/cooling energy.

The question is how much CO2 can I fix with a reasonably sized apparatus and really I would prefer a continuous system (which is why electrolysis) rather than a batch system.

Also I would prefer levels lower than outside, obviously air exchangers will necessarily be worse than just being outside.

Really prefer co2 levels lower than outside? What's your outside environment?

Before the industrial revolution the CO2 levels outside were around 250ppm or at least less than 300.

Current levels outside as measured by my device just now are about 450ppm.

My measurement is also about 450ppm and it's fine for me. How do you discovered lower level is better for you?

I don't know for sure, but there are certain things I notice which I think may have been improved when I started keeping track of CO2 and keeping my windows open. (I don't spend most of my time outside and I certainly don't sleep outside; outside levels might be 450 but inside levels even with my mitigations are usually around 700)

Like so many things I think it is a small effect; it's not like I can go anywhere for a week to experience 300ppm air.

I haven't gone to the levels of designing experiments and data tracking on myself. The fact that I've lived with allergies and a broken nose most of my life probably hasn't helped my personal respiration characteristics.

Here is a nice historical chart: https://www.climate.gov/sites/default/files/CO2_emissions_vs...

I'd assume that some chemist or other has already published a paper about that. But you could probably figure it out yourself experimentally by weighing quicklime, letting it react and weighing it again. Start a youtube channel to finance the project :)

Well I've gotten as far as purchasing 20 pounds of baking soda... my experiments thus far haven't been able to remove a detectable amount of CO2 from a room.

My literature searches haven't had much success thus far, lacking a chemistry degree hasn't helped. I have found some things but not enough to come to any sort of engineering designs. I'm assuming it's one of those things that's almost too simple for anyone to have bothered writing a paper about unless they had the exact sort of ideas I did. (OR they're using terms that I haven't been able to come up with yet)

Try buying 20 lbs of calcium hydroxide, it is sometimes used for soap making iirc, so it should be available on amazon. Bubbling air through a concentrated solution should do something. At least at the CO2 concentrations of exhaled air you get visible amounts of insoluble carbonate: https://www.youtube.com/watch?v=Vl9A8Iyc_LY

You might try using oxygen conentrator with high throughput to concentrate oxygen from outside and pump it into your home.

With small device I could drop CO2 levels to zero in volume of a gargabe bag in few minutes.

But that would give you oxygen rich atmosphere. You'd probably want to add some inert gas to that. Nitrogen preferably. Are there nitrogen concentrators?

Also all of that is probably really dangerous because human body doesn't detect oxygen level, just CO2 level. So you might easily pass out and suffocate without any warning if your oxygen supply unit get damaged but your nitrogen supply unit doesn't.

We are still talking about home... on planet Earth, right?

Of course. :-) The whole idea is based onthat ouside of the fact that there's plenty of oxygen for the concentrator to concentrate.

> by applying heat

More heat than is common for home appliances I know! A commonly referenced stat is 825 C. There's really a gradient: https://en.m.wikipedia.org/wiki/Calcium_carbonate#Calcinatio...

Ah yes, with "heat" I meant a nice fire.

You are describing an anaesthesia circle breathing system, otherwise known as a rebreather scuba set. :-)

You would need to consider the occupant oxygen consumption. A typical human respiratory quotient is 0.8 (0.8 mole of CO2 produced for every 1 mole of O2 consumed). You’ll get a net inward movement of gas into house all things being considered.

Anyway, don’t do if. If you do it ‘right’ the argon will slowly build up and you’ll asphyxiate. Thankfully achieving that level of seal is very difficult.

If you are really worried about PM2.5, just wear a PAPR.

Oh I'm not interested in hermetically sealing my apartment, there are no worries of normally constructed buildings being able to withstand a meaningful difference in oxygen pressure between inside and outside. CO2 pressure though can easily be increased by a large factor (5x is easy) and maintained with humans inside and closed doors and windows.

PM2.5 is easy to maintain with HEPA filters.

CO2 is not easy to maintain without forced ventilation, and really I would like to try inside levels lower than outside levels. (say 300 ppm)

I’m intrigued by the idea of chemically scrubbing co2 at home as well. Hepa filters are easier to use than I’d expected. The next big step is co2 reduction.

Maybe get in touch

Don’t those requirements conflict? Maintaining a partial pressure difference between inside and out for CO2 but simultaneously preventing a partial pressure difference in O2? I guess with a large enough scrubber running constantly it might work but it will be highly inefficient - a bit like running AC at full blast with open windows.

They do to a degree but overall composition of the atmosphere has so much oxygen and so little co2 that the dynamics are quite different.

There’s something like 50x as much oxygen in the atmosphere as co2 so a relative change of 1% nominal value will correct itself fifty times faster for oxygen than for co2 between outside and inside (ok a wild simplification, but still)

The result is that without really intense sealing it’s basically impossible to make a difference in indoor oxygen levels whereas with even a cracked window i can maintain co2 levels two to three times outdoor concentrations in my apartment.

Running a co2 fixer with the place shut up would be more like... running the air conditioner full blast with the place shut up. Yes there would be losses but they would be relatively contained.

Typical Passivhaus-Buildings are very airtight and feature air exchange systems often even with HEPA-Filters. That should do the trick.

Could you get a few more plants around the house? Plants can remove things like benzene, formaldehyde, xylene, and toluene. Maybe not the fastest but certainly don't cost much once you have them.

Unfortunately this myth, which I also once believed, has been debunked. You just need waaaay too many plants to actually have an effect. The tests the myth were based on all assumed a closed system, which just isn't realistic.

From one randomly googled article on the myth of house plants cleaning the air:

"To remove toxins, you would need at least 10 plants per square foot."

To remove how many "toxins" exactly? Like, let's say I actually have a shitload of plants, like my ceiling is just lined with peace lilies. Presumably there is some % of something being removed, I'd be curious to hear what that is.

Well "toxins" is ill defined.

But CO2... to simplify imagine being a vegetarian, how much plant matter would you eat in a day? How much plant matter grew in one day in your houseplants? The only way plants are going to fix carbon is by adding it to their own mass. Just think about a conservation of carbon atoms. There's no way any plants in your apartment are going to make a dent in the carbon cycle of your personal environment.

There are arguments to be made for lots of things sticking to or getting sucked into the leaves of plants... maybe if there were a whole lot of plants a meaningful amount of VOCs or other pollutants could be removed... maybe.

If you think about it you have to grow an amount of plant matter comparable to the amount of food you eat in order to maintain levels (considerably less of course because a lot of carbon goes down the toilet, but still a fixed fraction, this would take full sun an a considerable area, like a fraction of an acre)

That's weird. I can easily get CO2 levels way below 800 with windows closed, just not completely sealed.

And then air purifier can keep PM2.5 levels below 10 on the lowest setting while outside is above 30

What kind of home do you live in? I think a lot of the new construction is sealed very well and there are few leaks besides the actual interfaces.

Block of flats. Apartment is fairly well sealed. If I seal the windows I can drive up CO2 levels to 1500. I have plastic windows with seals that have "unsealed" mode. In this mode window is just barely opened. Like 1mm gap between a window and a frame.

And one window cracked like that is enough to drop CO2 levels below 800 while there's one person in the apartment.

note that the effects of periodically higher co2 concentrations on overall health is inconclusive at best (life evolved with significant co2 around). particulate matter and atmospheric pollutants (like voc’s), however, are most assuredly detrimental to our health, so it makes sense to act mainly on the latter (with air filters/purifiers) over the former.

I've worked in offices where the CO2 levels regularly spiked to levels not seen since... pants first started colonizing the land.

It's been, what, 30? 50? million years since atmospheric CO2 levels matched current outdoor levels, much less what happens inside.

What kind of pants are best for removing CO2?

The kind with lots of chlorophyll... all the rage with the kids these days (sigh... plants... plants)

the point is that humans (and life in general) have entirely evolved with varying levels of co2, so the excess levels found in a fairly closed room is unlikely to be a really serious risk (vs. a sealed room with much higher concentrations, where it's obviously a serious asphyxiation risk). this risk, according to research, is currently inconclusive at best.

the concentrations and types of particulate matter, however, is unprecedented, and as such, presents a higher inate risk, something that scientific research is really only starting to grapple with but so far is showing to be likely a serious risk to many (most?) species on earth.

with that said, i'm not trying to discourage your project in any way, just saying that for most people, addressing particulate (plus voc) risk is likely going to be the overwhelming majority of the benefits to be found in relation to indoor air quality. reducing co2 could be worthwhile if you have an especially airtight room/house, but nearly no houses are that airtight (passiv houses are meant to be so, however).

What about acetone or ethanol or whatever else we exhale that builds up?

shrug big effects first.

You might be interested in this paywalled paper (or similar ones): https://pubs.acs.org/doi/10.1021/acs.iecr.0c02255

Reply on news.ycombinator.com