I’ve seen people ask if air purifiers depend on the placebo effect or do they actually work. I think this is a good question because we should not confuse the outcomes of (1) objectively reducing pollutants in the air versus (2) health effects.
There are two things we know about purifiers and one really important thing we don’t know.
1. We know that purifiers reduce PM2.5
Loads of tests have found this in labs and in real-world homes. I’ve tested the hell out of this, in my room, in bigger rooms, for 200 days of Beijing air, comparison tests against the most expensive machines available, with no one at home… purifiers reduce particulate pollution (and gas pollutants) in the home.
2. We know that PM2.5 kills people
Researchers estimate that air pollution causes one in six deaths—and one in four in places as polluted as India.
3. We don’t know whether purifiers make people healthier—until now
So you’d think that means we know air purifiers make people healthier, but research is surprisingly hard to find. Fortunately, researchers have just started to publish good studies testing this question.
Do purifiers have real, non-placebo health effects?
Researchers gave 35 college students in Shanghai real or fake purifiers and tracked several health markers for two days. Researchers tested the students’ lung function and several health markers in their blood.
First, reality check: did the purifiers reduce the number of particles in the air? The red line here is outdoor PM2.5. The blue line is indoor PM2.5 for students using the fake purifier.
Not surprisingly, fake purifiers don’t work. And note that these are bad levels of pollution. The World Health Organization’s annual limit is 10 micrograms. Without purifiers, these rooms were averaging 100 micrograms!
But check out PM2.5 levels in rooms with real purifiers (yellow).
PM2.5 levels were about half as high in rooms real purifiers.
But we already know purifiers reduce pollution. Did the purifier actually have effects on their bodies? Here’s what having a real purifier did to measures of blood inflammation:
The students had significantly lower inflammation in their blood vessels. They also had lower blood coagulation (a marker of blood clotting):
And they had less constricted blood veins:
One of the only things that did not improve significantly was their lung function:
Three Things I Didn’t Know Before This Study
1. The real harm of pollution isn’t in our lungs
My intuition with air pollution has always been that the biggest worry is lung problems like asthma and lung cancer. Air pollution does affect our lungs, but epidemiological studies are finding that effects on our lungs are far smaller than effects on our hearts and blood vessels. Air pollution kills about three times as many people through heart attacks and strokes than it does through lung cancer and other lung problems.
2. Health effects are real even for young, healthy college students
Remember that these aren’t old people. And they’re not people who joined the study because they already had problems. They’re young, healthy people. These results suggest that air pollution is taking a toll on our bodies, even if we are still mostly healthy.
3. The benefits were detectable within 48 hours
These students’ biomarkers of inflammation changed in just two days. Air pollution has long-term effects like cancer, but it also has short-term effects like blood inflammation and vein constriction (which probably help cause those long-term problems).
But this is only one study…
Because these results are compared to people who received a fake purifier, they cannot be explained by a placebo effect. And like any finding, we should be wary of making conclusions based on a single study, but there are at least four other recent studies out with similar findings (1, 2, 3, 4).
Bottom line: Air purifiers improve health markers in people living in polluted places, and these are not placebo effects.
Thomas is an Associate 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.