Do you need a HEPA filter in your car?

With atrocious air quality in California right now, I’m doing everything I can to keep my lungs clear of fine particulate matter, aka PM2.5. Chronic exposure to even moderate levels of PM2.5 or acute exposure to high levels significantly increases all-cause mortality. I’ve previously written about how running HEPA filters in your house is a cost-effective way to improve your health. 

This week I started wondering if I could get even better protection from PM2.5 by replacing the non-HEPA cabin air filter in my girlfriend’s car with a HEPA filter. This detailed Quora answer documents that the stock, non-HEPA cabin air filter in a Mazda 3 effectively clears the cabin of fine particulate matter. I decided to check those results in our 2017 Subaru Forester using my Temtop M10 Air Quality Monitor. 

I have no way of checking the accuracy of my Temtop, so I ran a simple test to make sure it can at least differentiate higher and lower levels of PM2.5. First, I placed one of my HEPA filter right next to the door, where the air in our apartment is dirtiest, with the monitor on top and the fan off. I got a reading of 43 ug/m3. Then I turned on the fan and the reading dropped to 8 ug/m3 within about 30 seconds. This simple test confirms that the filter is lowering PM2.5 and that the monitor can tell, even if these exact values of 43 and 8 aren’t correct.

Next, I went out to our parking garage, where the monitor registered an astronomical 430 ug/m3, deep into the “hazardous” range.  

Inside the car, the air quality was sitting at a not-as-terrible value of 156 ug/m3. Then I ran the fan in recirculation mode on high with all the vents open. After five minutes, the PM2.5 had dropped to 122 ug/m3. After 10 minutes, it had dropped to 103 ug/m3. That drop is meaningful, though not nearly as good as the person on Quora documented for a Mazda 3. It’s also possible the PM2.5 was just dropping as the dust settled after I closed the car door.

As a control, I opened the door for a few seconds to let in more PM2.5, then watched it drop for five minutes without the car’s fan running. It dropped almost exactly as fast as it had with the fan running. So it appears the non-HEPA filter in the Subaru Forester is not doing anything to remove PM2.5 from the cabin. If the different results for the Mazda 3 were correct, it isn’t possible to make a general statement about all cabin air filters removing PM2.5 or not. For our car, I’ll be looking into replacing the cabin air filter with a HEPA filter.