Note: I wrote this in much more of a hurry than I normally write things, because I have noticed that I get way too meticulous, so it takes forever and I eventually give up. I hope it is still readable/useful!

Like many of us living in the Bay Area and other parts of the Western US, I have been dealing with smoke, and like most people who live and work indoors, I sometimes deal with stuffy, high-CO2 air. Smoke is bad for health, mood, productivity, sleep, and comfort. CO2 is bad for all the same things except perhaps health. Even people who live in places without seasonal wildfires might benefit from being able to filter out pollution or allergens without closing off their house and building up CO2.

The standard way to handle smoke is to buy air purifiers that pull air from inside the room, run it through a HEPA filter to remove particulates, and push it back out into the room. These work well for keeping particulate levels low when the air is not too smoky. However, if the air is very smoky, the only way they can keep up is if only a small amount of air is entering the house from outside. This means either closing the windows or buying more air purifiers. The trouble with closing all the windows is that CO2 created by people in the house will build up, and the trouble with buying more air purifiers is that it is expensive and impractical for high levels of particulates.

The solution is to filter air as it enters the house. An air filter that is pulling in smoky air from outside removes more particulates per minute for the same throughput than one that is filtering less particulate-filled air. An additional benefit of this is that it serves the purpose of actively cycling air into the house, rather than allowing it to passively happen through an open window. I spent some time searching for filters that work this way, and I was unable to find one. I also looked a bit for an in-window air conditioner that included a HEPA filter, but did not find one. So I built one myself. It was actually quite easy and inexpensive.

The filter consists of a cardboard box, a HEPA filter, an in-line duct fan, a short length of flexible ducting, and a cardboard flap. The box has a large rectangular hole cut into one side and a 6" circular hole cut into another side. The fan is attached to the 6" hole using duct tape (I used "all-weather" duct tape, since some of it would be exposed to heat, UV, and dust, though I do not know if it actually makes any difference), the HEPA filter is taped over the rectangular hole, and the duct is attached to the fan. The box is placed in the window with the filter facing outward and the fan pointing to the side. The duct is bent so that it blows filtered air into the room (or into my face, as is sometimes the case when I am working at my desk). The flap covers the rest of the window to limit the amount of unfiltered air coming into the apartment. All the parts combined cost less than $100. It may not be hard to get it below $50, depending on how much throughput you want and what kind of filters you can find.

The air purifier installed in my apartment

I actually built this a couple weeks ago, but I did not get a chance to measure its performance until my air quality monitor arrived in the mail and there was enough smoke outside to actually test it. I did test it, and it does seem to mostly work.

CO2 and particulates inside and outside my house, while changing various parameters

This data does not entirely make sense to me. in particular, the whole thing seems to be performing substantially worse toward the end while I'm out of the house, which is also significantly worse than it was for most of today.

It generally works better for removing particulates and worse for cycling out CO2 than I'd thought and hoped that it might. The fan seems to be moving less air than it is rated for, but I do not think it is designed for operating with much of a pressure difference. I will probably order a bit beefier fan and rebuild the whole thing with a more durable box.

All in all, I'm happy with it. The parts were cheap and it only took about 30 min to assemble. I'm happy to answer questions if anyone else is interested in building one.

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10 comments, sorted by Click to highlight new comments since: Today at 2:42 AM

Hey, I've been looking into air quality quite a bit recently. I have several questions.

What air quality sensor are you using? How are you getting outdoor data?

I suspect some of the confusion in the results may be due to circulation within the home and monitor placement. Have you thought much about circulation?

Additionally, it looks like indoor PM2.5 is tracking outdoor PM2.5. Have you thought much about other sources of ventilation?

What air quality sensor are you using? How are you getting outdoor data?

I'm using one of these:

I get the outdoor data from PurpleAir. There are two stations within a few blocks of me, and they both seem to report very similar numbers, though I have not done anything to formally estimate the error/variance.

Have you thought much about circulation?

Yes, and it does seem to depend somewhat on where the sensor is placed and what the circulation is. I took notes on where it was sitting, but I haven't tried to notice any patterns yet. I had a fan running to circulate air in the room the whole time that data was being recorded. I did not open the door very many times.

Additionally, it looks like indoor PM2.5 is tracking outdoor PM2.5. Have you thought much about other sources of ventilation?

Some, yes. I do expect the indoor PM2.5 to track it at least a little, since I would (naively, at least) expect the filtration system to work linearly (that is, it removes a percentage of particulates that is not dependent on the particulate concentration. But it does not seem to be linear, since it was removing a smaller fraction later in the day when the outdoor concentration was higher. It does seem like this might be explained by a leak somewhere?

I've been considering how hard it would be to build a system that can maintain a positive pressure difference in the house, so that it will reduce particulate inflow through cracks, other windows, etc. I'm not sure how hard that is to achieve.

Have you done experiments of your own?

Here are some details that I left out of the main post:

The fan I'm using is this: The filter is this: (a bit expensive, but I figured it made sense to buy filters I can use with my commercial purifier, if I they did not work with the homemade one) This is the duct:

If you're buying a fan, I recommend getting one that does at least 200-250 CFM and is rated for at least 40W. This is what I'm using and it seems okay. You will probably get better results with a more powerful fan. I haven't looked in too much detail into which fans will handle the pressure difference across the filter well, but my impression is that centrifugal fans are better for this. I will likely replace the fan soon. If you want to estimate how much throughput you need, you can use a copy of this sheet (look at the column 'Outdoor air exchange rate required for CO2 steady state (m^3/minute)'):

You don't need to be super meticulous about sealing everything off with tape, except maybe on the parts of the box that are outside. The air pressure pushes the filter against the box, so that helps. I taped the carbon odor filter against the HEPA filter, mainly to give it a bit of protection from larger pieces of stuff outside.

One other perk is that, as assembled, the whole thing makes white noise (or maybe it's pink noise) that is, to my ear, quite pleasant and good for working or sleeping. It's a little annoying if I'm trying to listen to something through speakers.

Why the 6in fan rather than the 8in one? Would seem to move a lot more air for nearly the same price.

I think I was just trying to match the CFM of my Coway purifier, since I was using the same filters. I was also worried it would be harder to properly mate a larger/heavier fan to a box. Now that I've actually built the thing, I would say the larger fan is probably better.

This is cool! Just to check, is there a reason why a normal AC doesn't work? I've installed an AC unit in my window when the smoke started, and the air coming out of it seems both low CO2 and to have basically no particulate matter in it (the sensor I places right in front of it consistently reads a very low level).

Window AC units don't actually pull air from outside.

Oops, yep, that makes total sense now that I thought about the physics for 2 minutes.

If you are pulling air from outside do you also have some exhaust outlet to keep internal pressure largely constant? If not, could something be going on with building a positive pressure inside that then creates problems with your intake air flow?

I cracked the window next to the one with the filter installed, mainly for the reasons you're describing. It doesn't seem to make a huge difference, which makes me think my apartment is fairly leaky.