Soon after I started making DIYs, people emailed me asking if we could make a DIY for their car. My initial reaction was, “Great idea!” On the highways, we’re surrounded by all of that awful pollution. Best to get a car air purifier.
But as a methodical nerd, I wanted to do some tests first. The results of those tests led me to a conclusion I didn’t expect: you don’t need a car purifier. Here’s how I got to that conclusion.
The number on the left is the number of particles 0.5 microns and above. For that channel, “safe” is somewhere around 2,500. So the air that day wasn’t pretty. The number on the right is larger particles—2.5 microns and above.
Next, I got into an Uber and kept taking readings. Here’s what the air was like when I just got into the car:
Way better! The 0.5 micron particles were only about 25% of outdoor air.
And a few minutes later:
Look, ma, no purifier! We were down to 11% of outdoor air.
So why’s the air so much cleaner?
It turns out car air systems have filters in them already, and apparently they do a pretty good job of filtering out particulate pollution. Here’s an example of a car filter on Amazon for only around $10.
Tests in a Regular Car
This simple test suggested to me that the car’s built-in air filtration system reduced particulate levels significantly—even the dreaded particles smaller than 2.5 microns. But was it something special about that car?
To be sure, I teamed up with Ohad, a Smart Air aficionado to do tests in his Mazda in Israel. He drives a regular 2010 Mazda 3–no special filters.
Ohad took his Dylos particle counter with him when he drove his car. When he got in the car, he turned on the AC, used “recirculate air” mode, and kept the windows closed. That’s it. Ohad took measurements every minute for five minutes. That’s not very long, so this is a fairly conservative test of the car cabin.
But wait, does Israel really have bad air pollution? It’s no Beijing, but several parts of Israel have recently experienced dangerous levels of smog. In Ohad’s tests, baseline 0.5 micron counts ranged from 2,464 (near the WHO limit) to over 10,000 (four times the limit). So the air has plenty of particulate to do tests.
I averaged across all 8 tests, and here’s what I found:
The Mazda’s built-in air filtration system drastically reduced the number of particles. When averaged over the course of 5 minutes with the fan on its maximum setting, the results showed a staggering 97% reduction of 0.5 micron particles and 99% drop in 2.5 micron particles!
But wait, savvy readers might ask: how do we know it’s the car’s air system and not just that there’s a person breathing in all those particles? Or maybe it’s because particles settle over time?
Ohad also ran an identical test with the car air system turned off. That way we can know if the reduction is truly because of the air system.
With the air system off (the red line), particle levels drop a bit, probably because of settling.But with the air system on (blue line), particulate levels dropped much faster.
Across 5 tests with the AC off, particulate went down (right), but not nearly as much compared to the 8 tests with the AC on high (left):
After seeing the data, I’m convinced we don’t need special car purifiers to protect against particulate pollution. The air systems already in cars dramatically reduce particulate pollution.Here’s how to do it:
1. Keep the car windows closed.
2. Use the “recirculate air” mode for the highest particulate drop.
3. Turn the fan on high (at least at the beginning of the trip).
4. Check and replace your filters every few months.
5. Think twice about whether you really need a car purifier.
Exception: Gas pollution
There is one exception to these tests: Car filters capture particles, but they won’t capture gases, like NO2. Thus, it may still be worthwhile to consider using a carbon filter. (Does carbon really work?).
Are you saying no one should ever use a car purifier?
If I owned a car in China, I wouldn’t bother buying a car purifier, but I won’t say no one should buy one. For example, people who are extra sensitive to pollution or parents with small children might consider using a purifier to get extra protection.
As always, I’m making the raw data and more testing details publicly available for fellow nerds. Check that test out for a full comparison of AC high, AC low, and AC off tests.
Thomas is a new Assistant Professor of Behavior Science at the University of Chicago Booth School of Business and the founder of, a social enterprise to help people in China breathe clean air without shelling out thousands of dollars for expensive purifiers.