In a nutshell: There is no hard cutoff. More air filtration is better. But there are clearly diminishing returns to buying the ultra-high-end air filters.
The bottom line is that, when it comes to the current air quality alerts, some air filtration is a whole lot better than no air filtration.
With smoke from the Canadian forest fires continuing to generate air pollution alerts in the U.S., my wife suggested that I re-up my articles on using a box fan as an air cleaner.
This is a re-telling of Post #1792 and Post #1794. Refer to those posts if you want more background information.
Point 1: A standard 20″ box fan and a high-end 3M Filtrete HVAC filter together make a simple and effective air cleaner. Get a 3M 1900 filter (rated MERV 13), place it on the back of the fan, and turn the fan on.
The key here is that the 3M electrostatic filters produce little “back pressure” or resistance to air flow. That’s why you can have the low-powered fan draw air through that filter and still have significant air flow.
You can do the same thing with standard high-resistance MERV 13 filters, but you would need to construct a “Corsi Box” to provide enough surface area. That is, tape four together into a hollow box, to provide enough surface area to allow for adequate air flow.
The 3M filters are expensive, but in my experience they last for months. Arguably, this being almost July, you’d only need one for the entire summer.
Point 2: This is more effective than a typical room-sized HEPA filter. The reason is that with heavily-polluted outdoor air, filtering a lot of air reasonably well (fan + filter) beats filtering a small amount of air extremely well (HEPA unit).
Above is the labeling on that Filtrete (r) 1900 filter. In a single pass through the filter, it removes
That’s nowhere near as good as a HEPA filter, which removes on-order-of 99.97% of all such particles in a single pass.
So why does the fan + filter win?
First, outdoor air infiltrates into indoor spaces at a fairly rapid rate. Typical tight older construction has one air exchange per hour. That is, every hour, enough outdoor air enters the building to replace the entire volume of indoor air.
In the current situation, that means smoky outdoor air is more-or-less pouring into your living space, continuously. Even with the windows and doors shut.
Second, a box fan moves a lot more air per minute than a typical room-sized HEPA unit. A box fan on high can move about 2000 cubic feet of air per minute. Depending on the fan, a box fan on low can move on order of 1000 cubic feet per minute. A typical room-sized HEPA unit might move just over 100 cubic feet per minute.
The end result is that the slower HEPA filter can’t keep up with the steady inflow of dirty air. Or, more properly, can’t keep up as well as the fan-and-filter combination.
On the left, you see the results of a numerical simulation of the two types of filtration. Left is the box-and-filter, right is a typical HEPA unit. Horizontal axis is time, vertical axis is the density of particulates in the air. (See prior post for full details of simulation).
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.
Point 3: Availability. As we learned during the pandemic, if there’s a sudden surge in demand (e.g., for N95 respirators), the shelves are soon stripped bare. So if everybody goes out looking for an air cleaning device, those will soon become unobtainable.
As of today, my local Home Depot has well over 100 20″ box fans in stock, on the floor, ready to be purchased. By contrast, they have just five room-sized HEPA units in stock.
Which makes sense. Those fans are commodity items costing about $25 each. The Honeywell HEPA unit, by contrast, goes for just about $300. Home Depot couldn’t afford to keep 100 of those in stock, on the off chance that there might be a run on air cleaners.
Sometimes, simple and cheap is what you want. In this case, a box fan and a 3M 1900 air filter together cost much less than a room-sized HEPA filter. And in this situation — where you are trying to filter pollution arriving from outdoor air — the much higher air flow of the fan-and-filter combination actually works better than a typical HEPA air cleaner.
Nothing prevents you from dealing with this problem by wearing an N95 respirator inside. But note from the simulation above, the fan-and-filter combination provides air that is almost as clean as you would get, breathing through an N95 respirator. So you get almost the full benefit of that, without the hassle of wearing a mask 24/7.
As a bonus, while the mask protects your lungs, the fan-and-filter combination protects both your lungs and your eyes. If eye irritation is an issue for you, filtering the indoor air is the only way to go.
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 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).
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.
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.
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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.
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:
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.
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.
