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You don't really want to use HEPA either, you want to maximize airflow.

PC fans with low MERV type filter do great since the smaller the particle (I think this effect kicks in below 5 microns) the better it is at filtering it so if it can pass 10 times more air than a hepa filter it's as effective as one while being able to filter more air faster and keeping the particles airbone.

The only downside is that small range of particles where lower merv filters aren't good enough to filter so upwards of 70% of the particles pass through

That sounds worth knowing; however when I looked MERV up, it seems that it's a rating system, not a type of filter. Could you be more specific abot the kind of filter you mean?

HEPA is typically just one type of filter with True HEPA as an offspring, MERV is a range which allows you to filter exactly what you need at the highest airflow. It really depends on what kind of pollution you have at your home.

If you just have a lot of dust then you want highest airflow possible (around MERV 9-10) if you want to filter things that cause allergies you need to go as high as MERV 14 since MERV 9-10 effectiveness is super low in that specific range.

Agreed. MERV 11-14 can be far more effective than HEPA.

If you need to filter "one and done" (like pumping air into a hospital operating room), that's where you need HEPA. Most home air purifiers mix the clean air back into the same room, so MERV is closer to the ideal sweet spot.

It's also important to buy reputable brands of MERV filter, ideally ones which have a large number of folds (surface area) like the 3M 1900 MPR. In recent testing about half of filter brands scored well below their claimed MERV rating:

https://www.youtube.com/watch?v=FKAVek1YaSQ

> Most people dramatically under-size their air purifiers, or run them on a very low fan setting, and then they throw up their hands and just say that air purifiers don't work.

I believe something is better than nothing here. One of the biggest complaints against air filters is noise so maybe a good compromise would be to run them at full speed and full noise for a certain amount of time or something when nobody is in the room?

The way to get air purifiers to really work well is to install them at the air intake, i.e. in the windows, or where the central air intake is, so all incoming air passes through them. I use indoor air purifiers too, but not as a substitute for ones at the intake. Note that tires and diesel fumes are prominent neighborhood sources of harmful particulates.

It is not expensive to run intake fans in the spring and fall seasons when active heating or cooling are not required.

That helps for pollution that comes from outside (traffic, pollen, wood smoke), but most of the microplastics are generated by moving/wearing synthetic textiles inside the home.

Positive pressure systems are great, love 'em, but there's a quantitative mismatch in this case. Above ~1 ACH your HVAC costs will go through the roof (even with heat/humidity recovery), but for effective filtration you need 6-8 ACH to catch the larger dust before your lungs do.

And for pm2.5, at least in Australia, it’s entirely generated from cooking. Outside air is very clean.

A false sense of security can be worse than nothing, because it prevents you from seeking out actually effective options.

I too would like such a "shy" air purifier, but manufacturers always seem to go the other way: when occupancy is detected they increase the fan speed.

Best option IMO is just to get an air purifier with a good CADR-to-decibel ratio and then (again) size it correctly. A surprisingly good option is something called the Airfanta 3 Pro, which is basically like those wildfire filter boxes except it uses PC fans.

bigger filter = less noise to move more air

The therm "microplactics" includes particles up to 5mm. And I think the bulk of the material is probably made of these larger particles. But I guess you're less likely to inhale something that large. So while air filters will remove most of the plastic you might inhale, you will still have to clean up most of the mass of microplastics in your house.