Listening to the radio the other day, I learned something interesting about the risks of inhaling smoky air. The smallest particulates in wildfire smoke are less than 2.5 µm in diameter. This is incredibly tiny—it’s about 30 times smaller than the thickness of human hair. Such a minuscule size means these particulates can get deep into your lungs when inhaled, and even work their way into your bloodstream¹. I found that unsettling enough to dig out an old plug-in air purifier and load it up with a new HEPA filter within a day.

As wildfire smoke blanketed parts of North America, it seems I was fortunate to already have an air purifier on hand. According to The Canadian Press, demand for them has increased. They are expensive, so some people have instead constructed themselves a Corsi-Rosenthal box, a homemade solution created and popularized early in the pandemic.

It occurred to me that this wildfire situation was a lot like the 2020-21 period, where air purifiers were also flying off the shelves. Wildfires come and go, but COVID never stopped being airborne, and no other air-related health concern went away. But, an air filter needs replenishment every 6 months or so, as does one’s desire to pursue such matters as clean indoor air. (And look at me, I am just as guilty for letting it slip!)

Learning requires repetition; will this most recent lesson on IAQ stick? Make no mistake, the smoky air can be very dangerous, but in some way it’s a good reminder that indoor air quality is indoor air /health/. It’s a boring topic, and in a normal situation most people probably don’t realize they ought to care about it. The reality is that the air quality of most indoor environments could be improved substantially. This is the reason behind all those new sales in air purifiers. Every one of them is purchased to address some deficiency in someone’s home or workplace, and in principle they would be unnecessary if the deficiency were fixed permanently. However, IAQ is not just about whether or not air has been sufficiently filtered. The concept ideally involves all aspects of the indoor environment; for example, the relative humidity is also important². Now, the typical air quality deficiencies in a home may not be worth the trouble to fix, but we sure put up with a lot of them. All I can say is, there are alternatives to collectively buying truckloads of air purifiers, humidifiers, and dehumidifiers.

Newer homes are more sealed up than ever before, so outdoor air is increasingly delivered through HVAC. This means air is filtered first, which is a good thing. Likely many people with a central HVAC system were thankful for that. Check out what happened to a friend’s filter (left) after a day or two of the worst quality air we had in Ottawa.

But most people don’t change their air filters very often, anyway. For that reason alone, IAQ in the home will probably always be a crapshoot.

Outside the home, something to be optimistic about is the news last month that the U.S. CDC updated their recommendations for building ventilation. These recommendations, motivated by COVID-19, were based off ASHRAE technical resources and call for a minimum of five air changes per hour for occupied spaces, and use of MERV-13 filters. MERV-13 is not quite HEPA, but it’s still pretty good. HEPA is not usually practical to use in HVAC systems anyway, as the filters are expensive and cause a relatively large pressure drop airside. The pressure drop requires more fan energy to push the air through, so the operating cost goes up a fair amount.

Given some time, what the CDC has recommended could lead to some real improvements (even here in Canada) in many of the buildings where people spend so much of the day, such as schools and offices.

Anyway, please change your HVAC filter.

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