Do Pollution Masks Really Work?

When a billion people in China (and quite a few expats) woke up to the severe air pollution in almost every city in China, it forced a billion people to become experts in a complicated scientific question: Do masks work?

Since then, I’ve given talks with hundreds of people all around China about how to protect themselves from air pollution. In those talks, I’ve heard doubts from smart, skeptical people. Here I’ll answer those doubts because, fortunately, smart, skeptical scientists (plus one dedicated nerd—yours truly) have empirically tested these questions. 

Update: for air-breathers in India, we did new tests with some of the most common masks available in India, including kids’ masks!


1. “There’s no way they capture the really small particles.”


The skeptic case: 

The most dangerous particles are the smallest particles, but masks are so thin. How could they possibly get the smallest particles?


The scientific test:

Researchers from the University of Edinburgh tested different common masks by running a diesel generator (to mimic car exhaust) and piping the exhaust through different masks. They used a particle counter to see how many particles made it through the mask. Here’s my super scientific rendering of the setup:



One important detail: the particle counter they used measures down to .007 microns. We’re talking about truly tiny particles here!

First they tried a simple cotton handkerchief. Sometimes I see bikers in China wearing these.


Cotton mask effectiveness smog haze pollution PM2.5


Not great, 28% of particles blocked.

Next they tried a cheap surgical mask.


Surgical mask PM2.5 effectiveness haze smog


Surprisingly good! (Fit tests generally show lower results–see below–but still a lot higher than most people’s intuition.)

Next they tried several bike masks.


Respro PM2.5 test review data effectiveness


Most were around 80%.

Then they tried several cheap 3M masks.


N95 mask surgical mask Respro particle capture test data


They all scored over 95%. Pretty good!

Conclusion: Masks capture even very small particles.


2. “OK, they capture the small particles, but when you wear them, all the air just leaks in the side.” 


The skeptic case: 

Masks work in theory, but those tests aren’t on real faces! When you actually wear them, you can’t get a good enough fit, so they’re basically useless.

The scientific test:

This question is tougher to answer because you have to measure the mask while you’reactually wearing it. For that, you need a really expensive fit test machine. Fortunately, I begged and begged 3M until they let me use their lab in Beijing:


Pollution mask N95 fit test TSI 3M


The blue tube is sampling air outside the mask, while the white tube is sampling air from inside the mask (more details on the methods here ). 

Smart Air co-founder Anna Guo and Beijing-based Dr. Richard Saint Cyr also tested masks, so I combined all of our data. Here’s how well the masks worked on our faces:


Pollution mask respirator fit test data haze


How well do masks work for the broader population? 

It’s important to make clear: masks that fit my face well might not fit other people’s faces well. However, there is evidence from a broader population that masks fit most people well. A scientific study of 3M models on 22 Chinese people found a median fit score of 99.5%–essentially the same as the top results from Dr. Saint Cyr and me.

Best yet, effective masks don’t cost a lot of money.


Air pollution mask effectiveness versus price data review


Exception: Gas Pollutants

Respirators are great, but note that these are tests on particulate pollution (such as PM2.5). Most commercially available masks don’t target gas pollutants like NO2 and O3, so masks are not giving 100% protection.


3. Is there a documented health benefit of wearing a mask?

This is probably the hardest question to answer. However, there are two solid studies that have randomly assigned people in Beijing to wear masks or not and measured their heart rate and blood pressure (1 2). 


Pollution mask health blood pressure study


While wearing masks, people had lower blood pressure and better-regulated heart rates.


Pollution mask health blood pressure smog


Conclusion: Masks capture even the smallest particles—even while people are wearing them—and they have documented health benefits. That should be enough to satisfy even the skeptics!


The Sqair air purifier Kickstarter

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Can you share how long each of the masks in your test last? I hear the cheaper 3M masks, whilst they are very effective, can only be used once.

Hey Dion, we’ve actually done these tests. We’ve tested an 11 day old mask to see if it lasts, as well as a 20 and 30 day old masks to see if they are still effective. Check out those posts for more information!


Hey Thomas & team,

Richard Saint Cyr says, “The other major issue here is fit: even a small amount of air leaking around the corners makes any mask basically worthless.”

It sounds like you disagree, and you’re saying that even with small air leaks, a good mask is still probably cutting out a decent amount (say 50%) of pollution. Is that right?

Hey Sam, Paddy here – I’ll reply on behalf of Thomas as he’s out running nerdy tests right now! It’s true that even if a mask doesn’t fit 100%, it’ll still be doing something to reduce air pollution. One great example is the surgical mask data in this article. You’ll see how in the fit test it managed to capture 63% of PM2.5. That’s great for a mask which is so ‘leaky’! In practice other masks are much much better sealed than surgical masks, so I think you can expect to get very high filter rates. Most of the masks… Read more »


Thanks for the awesome blog, pls keep writing


Thank you so much for such thorough investigation! I have been using a 3M 9211 as I am cycling around Los Angeles for more than a year and I can definitely say that they are effective. I have two questions that I can not find the answer anywhere! How often is recommended to change the mask? Also, although I can not smell car exhaustions (vs not wearing it) I can definitely smell tobacco (or weed, it’s LA you see) when I am stopped at the traffic light and a pedestrian passes by smoking. The smell is clean so does that… Read more »

Glad you’ve found the data useful Vangelis! We had exactly the same question on how long you can use your mask. That’s why Smart Air nerd also tested a 14 day old mask during his fit tests. You can find the data in our mask lifespan writeup. As for your second question: this data shows that masks are great at capturing PM (particulate matter), but that says nothing about how well they capture gases. Smells, formaldehyde, ozone and more all fall into that category, and your typical 3M masks can’t capture these gases. There are some ‘carbon’ or ‘charcoal’ masks… Read more »


I started wearing a 3M 7502 (7500 Series) respirator. I have two pratical options available plus a super safe one.
1) with the 2097 filter (P100 filter class plus carbon). Advantage: filter class highest. Or
2) the 6001 CN (carbon filter) plus the 5N11 CN (N95 filter class). Advantage: easier to breathe and huge carbon filter.
3) There’s also the possibility to combine the 6001 with the 2097 but its quite heavy and hard to breathe. Advantage highest particulate filter class plus big carbon filter. Disadvantage: no sports, e.g. riding bike, comfortably possible.

What do you think about that?


I forgot to mention that the best advantage of the 3M 7500 respirators over the foldable 9332 is the fit. There is absolutely no air passing the face seal.