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.
A simple spreadsheet model
I noted today that I can’t drive to any local hardware store near me and pick up a couple of my preferred 3M Filtrete 1900 air filters.
This got me thinking about how good an air filter I actually needed, for my box-fan-and-air-filter setup.
It only took a moment to realize that’s not a black-and-white issue. The more accurate way to phrase it is, how much filtration can I achieve with different air filters and air flow rates.
In Post #1794, I worked up a little spreadsheet model of an air filter operating in a “typical” room. The room is 3200 cubic feet (20 x 20 x 8), and has one full air exchange per hour. (That is, outside air fully replaces the inside air once per hour). That rate of air infiltration is typical of tight older home construction in the U.S.
I focused on the tiniest particles (those 0.3 to 1.0 microns in diameter), as those are the hardest to filter out. And I assumed that the outside air was as polluted as it has every been, for fine particulates, in New Your City. That was an AQI of above 400 for fine particulates, with a concentration of 450 micrograms of fine particulates per cubic meter of air.
I used the model to show that high-volume/moderate-efficiency air filtration (box fan and 3M 1900 air filter) was much better than low-volume/high-efficiency filtration (a typical room-sized HEPA unit). The reason is that the slow air flow rates of a typical HEPA filter simply could not keep up with the constant inflow of dirty outside air. Better to filter a lot of air, pretty well, than to filter a small amount of air perfectly.
Now I’m going to re-run the model, using filters with different MERV ratings, and different air flows, and assuming different AQIs for the outside air.
MERV Recap.
MERV (Minimum Efficiency Reporting Value) is the industry standard for rating air filters, developed by ASHRAE. Oddly enough, different sources disagree on what, exactly, gets filtered out, at each MERV rating. So it’s best to go to the U.S. EPA for a reasonably definitive answer:
Anything above MERV 11 has some ability to capture the finest particles, those under one micron in diameter. As shown above, a MERV 11 filter must trap 20% of such particles, on one pass through the filter. The typically-recommended MERV 13 must trap at least 50%. And a MERV 16 must trap 95%.
Running the model
Above: EPA 2012 revision of the break points on the AQI scale for fine particulates (PM 2.5). You can find the full set of current breakpoints at this EPA web page.
Let me begin with a Code Maroon air quality alert. You would see an AQI number in the 400s, for fine particulates, and the EPA description is “hazardous”.That’s as bad as it gets, and that’s the level that New York City reached during the prior round of Canadian forest fire smoke this year.
Let me now filter that air with a box fan running on low or high, and air filters with various MERV ratings.
The takeaway is that when the exterior air is that bad, under the assumed conditions (20 x 20 room, one air exchange per hour), a single box fan plus filter will never get particulates down to the level the EPA considers healthy. At best, you can achieve moderate air quality with a MERV 13 filter and a box fan set on high (or two filters and two box fans on low).
That said, even the lowly MERV 11 filter gives you a material improvement, reducing the concentration of fine particulates in your indoor air by as much as 85%, if you run the box fan on high (or run two fans/two filters on low).
Now let me back off to a mere Code Purple air quality alert, an AQI somewhere in the 200s, and an EPA rating of Very Unhealthy.
In this situation, MERV 14 or better, and a box fan running on high, can clear the interior air all the way down to the “healthy” level. The MERV 13 filter just misses that (the cutoff is 12 micrograms per cubic meter, the MERV 13 gets it down to 15).
And once again, the lowly MERV 11 filter, with a box fan running on high, does quite a bit to clean up your interior air, bringing down from Very Unhealthy to Moderate.
Some generalizations
The model above is an accounting-style simulation. Minute by minute, it tracks the amount of air filtered, the amount of air entering the room, and that, together with the filter efficiency, determines the level of particulates in the indoor air.
A few things are evident, based on these two runs of the model.
Just by looking at the table, you can see that these setups achieve a given percentage reduction in particulates, relative to outdoor air. No matter how clean or dirty the outdoor air is. You can see that the percentage reduction columns in the two tables are identical to within rounding error.
But there are also a couple of non-obvious findings.
1: Your biggest bang-for-the-buck is your first air cleaning unit. Each additional unit makes the air cleaner still, but at a declining rate. Restated: Some air cleaner is a lot better than no air cleaner.
Here, the first fan+MERV 13 reduced particulates from 450 to 56, obviously a tremendous improvement. With three such units, running on low, you can get down to a particulate density of 20. And if you are willing to run five, you can get to the EPA “healthy” rating for your indoor air, with just 12 micrograms of fine particulates per cubic meter of air.
2: There are diminishing returns to using very-high-end filters. The ultra-high-end filters will make the air cleaner, but not a lot cleaner than you’d get with a MERV 13 filter.
In the simulation above, one fan on low with a MERV 13 filter got the interior air particulate level down to 56. If you pushed that all the way to a MERV 16 — and still kept up the same air flow — you’d get that down to about 31 micrograms per cubic meter. Given what the ultra-high-end filters cost, if you could even find one, you’d be better off running two fans with MERV 13s than one fan with a MERV 16.
