I learned a lot about air filtration during the recent pandemic. At some point, I wrote down and compared all the different standards used for air filtration. For example, what does HEPA actually mean, and how does it compare to the various members of the MERV clan? And how do all of those relate to N95? (Post 593, April 1, 2020).
The problem-du-jour isn’t about filtering tiny little viruses out of the air. Instead, it’s about filtering tiny little soot particles out of the air. In the U.S. Northeast, we’re now in an era of Canadian wildfires and the resulting air pollution alerts.
Nicely enough, I get to re-use what I think I already know about air filtration. The knowledge that applied to filtering aerosol droplets (droplets less than 5 microns in size) applies equally to filtering fine particulates such as the soot from wildfires. This soot typically falls into the PM 2.5 air pollution category, that is, any particulate matter in the air less than 2.5 microns in size. (For comparison, a human hair is typically around 70 microns thick.)
Except that there is one key difference between filtering the air for pandemic purposes, and filtering the air for forest fire purposes: Outdoor air is no longer our friend. In fact, outdoor air is now the enemy.
When filtering viruses, outdoor air could be assumed to be clean. The likely concentration of virus droplets in outdoor air was typically negligible. Disease transmission in outdoor settings was virtually unheard-of. You only had to worry about the “pollution” that you generated inside the indoor space.
But for forest fires, the entire problem IS the outdoor air. In some sense, the entire battle to keep indoor air breathable is about dealing with the dirty air outside.
To crystalize this, recall that one standard suggestion for improving the COVID safety of (e.g.) schools and other spaces was to open the windows. Perfectly rational if you’re dodging COVID. Not so smart if you’re trying to avoid forest fire smoke. So, from the get-go, you now see that dealing with forest fire smoke is a completely different engineering challenge from minimizing aerosolized COVID.
Why does this matter? Outside air is always leaking into indoor spaces. For the virus filtering, that was a good thing. Now it’s not, and you need to strategize your air filtration accordingly. Why? Because outdoor air doesn’t just leak into indoor spaces, it typically pours in. The minimum standard for houses, from a health standpoint, is that outdoor air should replace indoor air at least once every three hours (reference). But a typical well-constructed older home, in good shape (storm windows, caulked) would exchange all the indoor air with outdoor air once per hour (random reference). Leakier construction (no storm windows, caulk missing) would experience air exchanges more rapid than that.
Whatever air filtration setup you use, you now need to account for that.
My particular problem.
My daughter lives in New York City. Shown above, New York just had a bout of extremely bad air quality, due to Canadian forest fires. At the peak, PM 2.5 (particular matter 2.5 microns or smaller) reached 460 micrograms per cubic meter. This is way beyond what the EPA considers hazardous, and is maybe 10 to 20 times the typical value for that area.
Given that those fires continue to burn, I’m guessing this isn’t a one-and-done.
So I wanted to get her an effective air purifier for her apartment. (And even if forest fire soot does not return, a New York City apartment would probably benefit from having an air purifier).
My options were to go with a redneck air purifier (20″ box fan, and a high-quality electrostatic air filter), or to buy a purpose-made room-sized HEPA air purifier. The redneck air purifier is a variation on the “Corsi box”, a D-I-Y air purifier that was promoted as an easy fix for indoor air filtration during the pandemic.
You might normally say that HEPA must be better, because it’s a higher filtration standard. In theory, HEPA filters must remove 99.97% of fine particulates in a single pass (based on the Wikipedia entry for HEPA). In practice, I think 99.5% is more typically advertised. Whereas a high-end air filter (in this case, Filtrete 1900) only removes about 65% of fine particulates in a single pass. Seems like the case for HEPA is a no-brainer.
And if I were trying to filter the air in a hermetically-sealed box, HEPA probably is the better choice. The only drawback to HEPA is the back-pressure of doing that high level of filtration limits air flow, for a given power input. HEPA units advertised as “room-sized” air cleaners typically filter just a few hundred cubic feet of air per minute. By contrast, the entire selling point of the 3M Filtrete electrostatic filters is that they achieve a reasonable degree of fine-particle filtering with minimal back pressure. With a box fan on low, I can push 1000 cubic feet of air per minute, through a Filtrete filter.
My gut tells me that, for older construction, with a lot of air infiltration, the cheap setup (box fan and filter) is better than the equivalent purpose-built HEPA air filtration unit. A typical room-sized HEPA unit isn’t going to be able to “keep up with” the inflow of dirty outside air. Or, at least, not as well as the high-air-flow fan-and-filter setup. If a lot of air is flowing into the room, my gut tells me that that HEPA (high-filtration/low-volume) actually does a worse job than box-fan-and-filter (low-filtration/high-volume).
Many mathematical paths to the sea.
At some level, I realize that I’m trying to solve a classic calculus problem. Typically, it’s a water tank with inputs and outputs. Here, it’s a room with leaks and a filter. There must be a classic closed-form solution that would tell me the final concentration of particulates in the air. Just plug in the parameters, and view the output.
Somewhere along the line, I have lost the ability to cast a problem such as this into that classic format. No problem. Much of what used to require actual intelligence and insight can now be done with brute-force computing power.
In this case, all I need to do is the simple numerical simulation, in a spreadsheet. Start with a room full of dirty air, with a known rate of infiltration of outside air. Turn on an air cleaner with known properties. And just do the accounting, minute-by-minute. Air in, air cleaned, air out. And track the resulting concentration of pollutants in the air.
Does anybody ever care about the details of methodology?
Answer: Only if they disagree with the results. By contrast, if I end up saying something you agree with, you won’t care how I arrived at that. That’s just human nature. People just want to say Amen and move on.
My redneck air purifier consists of a box fan pushing 1000 CFM (on low), through a 3M Filtrete (r) filter. The Filtrete is a 3M “1900”, rated at MERV 13. It grabs 65% of the PM 2.5-sized particles with each pass, and about 95% of PM 10-sized particles. For this simulation, I’m only tracking PM 2.5.
B
The only practical detail you should care about is that if you do this — a single filter stuck on the back of a fan — you must use a low-back-pressure electrostatic filter. Otherwise, if you want to use el-cheapo MERV 13 filters, you need to go to the trouble of actually constructing a literal Corsi box, using four filters taped together. That’s because with cheap filters, you need all that surface area to avoid the high back pressure that would starve the fan of air. See this reference for Corsi box. Even with that, my take on it is that it provides slower air movement than using a single Filtrete 1900 placed on the back of a fan.
Below is a literal Corsi box, via Wikipedia. If you use cheap filters, go that route.
By contrast, my theoretical HEPA filter pushes 100 CFM through a filter that grabs (say) 99.5% of PM 2.5 particles. (Separately, I’ll show the results for a filter pushing higher rates of air flow).
The room is 20′ x 20′, with an 8′ ceiling, and has one full air exchange per hour, typical for sound older construction. That is, enough outside air enter the room through various leaks and cracks that it would be enough to replace the air in the room once per hour.
The outside air is at 450 micrograms per cubic meter of PM 2.5, the peak of the air pollution in the New York City area.
Based on this, I’ve written the spreadsheet that does the accounting. Air in, air purified, air out.
Results.
The results of my Excel-based numerical simulation validate what my gut was telling me. Due to the high rate of air infiltration typical in older construction, filtering the air rapidly is far more important that filtering the air extremely well.
On the left, you see the results for my redneck, box-fan-plus-Filtrete air filtration unit. It passes 1000 cubic feet per minute, but only filters out 65% of the finest (PM 2.5) particles. On the right, you see the results for a slower HEPA unit. It passes one-tenth of the air per minute, but it filters it more than 10x better.
The equilibrium level of particulates in the room is vastly lower with the high-volume, lower-efficiency filter (left graph above). Why? Because the slow pace of the HEPA filter (right graph) can’t keep up with the level of outside-air infiltration that is typical in older construction.
To get the HEPA filter to work almost as well as the simple Filtrete (r) 1900 plus box fan, in this typical leaky room, you’d have to crank it up to a much higher air-volume throughput.
A HEPA filter is a beautiful device. It would work wonderfully in a hermetically-sealed room. But in an actual room, with high-volume exchange of air between inside and exterior, it just can’t keep up. You’re better off using a cheap box fan on low (1000 CFM) and a low-back-pressure air electostatic air filter, such as a 3M Filtrete (r) filter.
Is this a fair comparison? Judging from what I see on Amazon, I’d say so. When they even bother to show the approximate air flow rate, HEPA units offered as whole-room units typically run at:
- Example 1: One air exchange per hour.
- Example 2: 130 cubic feet/minute.
- Example 3: One air change per hour.
Whereas the box-fan-and-filter turns over the air in my example room about 20 times per hour, at roughly 1000 cubic feet per minute.
Further, it makes almost no difference whether I use 99.5% efficiency or 99.9% efficiency for the HEPA unit. At slow rates of air turnover, the HEPA filter gets overwhelmed by the infiltration of outside air.
Conclusion
I just sent my daughter two Filtrete 1900 filters. Plus, oxymoronically, a stylish 20″ box fan. Hoping that on low, the fan will be quiet enough not to be bothersome.
My final finding is that the folks who run Amazon don’t miss a trick. If you search for a stylish box fan, Amazon suggests a few packs of MERV-13 filters, as an add-on purchase.
My conclusion from the above is that, between viruses and soot, a whole lot of people have figured out that the best and cheapest way to filter indoor air is with some form of “Corsi box”. So these days, as soon as you pick your fan, Amazon is right there, suggesting the add-ons you need to do that.
