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.
Three simple points
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
- 62% of the tiniest particles (0.3 to 1.0 mircons)
- 87% of the mid-sized particles (1.0 to 3.0 microns)
- 95% of the larger particles (3.0 to 10 microns).
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.
Summary
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.
