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!


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Charlene

Can masks made with coffee filters double lined help? How do they compare with N95 masks?

Ethel - Smart Air

Hey Charlene, we’re working on testing coffee filters and should have a follow-up article out soon!

Mona

Thank you for this thorough post and data. It’s been the most informative out of the 10 articles I just read through on effectiveness of masks.

Heidi

Hi Thomas and Team,

Thank you for sharing this interesting data.

Could you share the specs of the surgical mask you used in your study please? It is the most accessible mask for the general population and there are many different types (e.g. 2ply vs 3ply, BFE vs PFE vs VFE). Many westerners just do not believe they are useful at all for tackling the spread of / preventing catching coronavirus. Your sharing of the specs of the surgical mask used would help clearing the uncertainty that many have.

Many thanks!

Great question Heidi! The surgical mask tested in these experiments was a ‘Weijian’ (稳健) surgical mask. It meets the YY0469 Chinese standard for surgical masks. This standard requires a BFE (Bacterial Filtration Efficiency, for particles 3.0 microns) above 95%, and a PFE (Particle Filtration Efficiency, for particles 0.1 microns) above 30%. Here’s the actual standards (written in Chinese): This certification requirement (a requirement to capture just 30% of particles 0.1 microns in size) is actually much lower than many of the international or US standards (for example the ASTM F2100 Level 1, Level 2 and Level 3). Most surgical masks… Read more »

Heidi

Hi Paddy,

Thank you very much for your reply!

Indeed, the US, together with Japan and Korea, probably have the most stringent requirements. On the other hands, the EU standard EN:14683 only specify BFE but no PFE, even though it is similar to ASTM F2100 as claimed by Nelson’s lab (https://www.nelsonlabs.com/en14683-harmonizes-bacterial-filtration-efficiency-and-differential-pressure-with-astm-f2100/). I am based in Europe hence my interested in the specs of your masks 🙂

One more question, are specs of the surgical masks tested here the same as the one from this experiment of yours (https://smartairfilters.com/en/blog/coronavirus-pollution-masks-n95-surgical-mask/?fbclid=IwAR0d3W-g6zl_G9cRygwCYt2J2PAfYYzg1cV9toacjwn6WBTt_mFqbyB1ikE)?

Many thanks!

No problem Heidi, I’m glad the data and info is useful to you! Yes, the specs of the surgical mask tested in that 2nd article you link to are the same. They’re all part of the same fit-test experiment undertaken by Smart Air and Dr Richard Cyr (a doctor in Beijing). Here’s a link to his original test results where he gives the specifications of all the masks. ‘Weijian’ is the surgical mask brand: http://www.myhealthbeijing.com/children/my-personal-fit-testing-heres-the-best-pollution-mask-for-me/

Dion

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!

samchrisp

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.”
http://www.myhealthbeijing.com/children/pollution-masks-children-n95/

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 »