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Do pollution masks 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.

Here are the two most frequent skepticisms I hear about 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:

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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.

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Not great, 28% of particles blocked.

Next they tried a cheap surgical mask.

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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.

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Most were around 80%.

Then they tried several cheap 3M masks.

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They all scored over 95%. Pretty good!

Conclusion: masks capture even very small particles.

  1. “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’re actually 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:

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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).

Beijing-based Dr. Richard Saint Cyr also tested masks, so I’ll combine my data with his. Here’s how well the masks worked on our faces:

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How well do masks work for the broader population?

It’s important to make clear: fit test results on my face won’t always be the same for other people’s faces. However, there is evidence from a broader population that masks fit most people well. A scientific study of 3M masks 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. And you certainly don’t need to buy the most expensive masks on the market to breathe clean air.

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A note on gases: Note that these tests are about particulate pollution. Most commercially available masks don’t target gas pollutants like NO2 and O3, so it’s not 100% protection.

  1. 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).

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

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Conclusion: Masks capture even the smallest particles—even while you’re wearing them. And they have documented health benefits. That should be enough to satisfy even the skeptics!

indoor vs outdoor

Shanghai Test: Is Indoor air better than outdoor air?

When I got my very first particle counter, I tested sites around Beijing to see whether indoor air was better than outdoor air. It was.

But that test had some limitations. My first particle counter didn’t have a battery, so I had to estimate outdoor particulate in some locations. I also didn’t look into any variables that could give some indoor locations better air than others.

Shanghai Test

Now I’ve got a fancy new Dylos DC1700 battery-power particle counter! Now I can easily take measurements indoors and outdoors. Here’s what it looks like:

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I took it to Shanghai and tested 14 times in 11 locations on August 27-29. While I was there, the AQI averaged 158 (70 micrograms). I tested mostly around the French Concession, although I also made it out to Fudan University. None of the locations use air purifiers.

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I tested in any type of place I could, and I mean any place. Here’s what I recorded in a public bathroom stall:

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Results

I focused on the smaller 0.5 micron particles (which are highly correlated with government PM 2.5 readings). Across the 14 samples, here’s what I found. The red line represents how many particles were in outdoor air.

http://smartairfilters.com/cn/

On average, indoor air had just 69% of the particles of outdoor air.

Why do some places have cleaner air?

This fits with my findings in Beijing–indoor air has less particulate than outdoor air. But next I wanted to see if I could figure out why some places had cleaner air than others. For example, my unpurified apartment air (unpurified because I just got home) had just 30% of particles compared to outdoors, but the public bathroom had 134% of particles compared to outdoors. Woah!

I tested the simplest explanation possible: Were the windows and doors open? Most particulate pollution comes from outside. And in closed rooms, particles will slowly fall to the ground. Thus, indoor air should be better when the windows are doors are closed.

To test that idea, I looked only at places that had doors or windows open:

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Yikes! If the doors or windows are open, I was breathing air that was basically as bad as outdoor air (92%). For example, here was how air compared indoors and outdoors in the public bathroom:

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But things were much better in places with the doors and windows closed:l

With the doors and windows closed, the air had 57% of the particles of outdoors. For example, here’s the air inside Fudan University’s Economics Institute versus outdoors:z

In places where the doors and windows were closed, the air was always better than outside. The one exception was the Yuanyuan Restaurant, at 115%. Those particles could be coming from the kitchen.

So what?

The conclusion here is simple: We’re usually breathing much less particulate indoors than outdoors–even without a purifier.

Clean Lung Tips

  1. Work out indoors if possible. I work out indoors in a gym rather than outdoors when I’m in China.
  2. Sit inside at cafes or bars (unless people are smoking).

Does that mean indoor air is safe?

Indoor air has less particulate, but remember that “less particulate” does not always mean “safe.” Out of all 14 tests, NONE of the numbers was below the WHO 24-hour PM 2.5 limit:

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One Last Exception

Finally, remember that pollutants can sometimes come from indoors. This is usually from the paints and chemicals used in remodeling and new furniture. If your home smells like paint or new furniture, you may be safer opening the windows (or at least using activated carbon).

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How Safe is Indoor Air?

I recently had a conversation in Beijing that went something like this:

Friend: I’m not sure if I can make badminton tomorrow. I have a basketball game in the day.

Me: Oh man, do you play outside?

Friend: Nah, it’s inside.

Me: Oh, phew. Good.

Friend: Wait, why do you say that?

Me: Oh, the air is way worse outside. I used to feel like I had asthma after playing basketball outside.

Friend: Really? No, they’re not that much different. I saw it’s just 20% different.

Seeing as how nerds cannot let matters of fact go, I started using my particle counter to take measurements of inside and outside air at different locations around Beijing. This answer is important: it tells you if it’s any safer to exercise indoors and how much damage you’re doing to your lungs by choosing that seat outdoors at your favorite cafe or restaurant.

So I took measurements in six locations around Beijing, in apartments, cafes, and my gym. I only chose bad days (pollution concentration above the WHO standard of 25), and I avoided days where it rained (because rain can cause quick changes in air quality). Here’s what I found:

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On average, indoor air had only 36% of the pollution outdoors.

Things were a little worse for the smaller .5 micron particles, but still much better than outside:

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On average, indoor air had only 51% of the .5 micron particulates of outside air. My guess is that the .5 micron data was worse than 2.5 micron data because it’s easier for smaller particles to get into your home and stay suspended in the air.

There is a lot of variation between places. For the 2.5 micron particles, the locations varied from 14% to 58%. Dr. Saint Cyr also found significant variation between two apartments he lived in, 50% to 70%.

Conclusion: In terms of particulate pollution, you’re safer snagging an indoor seat and working out indoors, particularly on bad days (I’ve seen some argue that we are particularly vulnerable when we work out because we breathe more deeply than normal).

But remember that doesn’t mean indoor air is safe, just better than outside. For example, if your air at home had 40% of Beijing’s concentration last night at 11pm (8/15), you would’ve had 64 g/m3 in your home, which is more than twice the WHO standard of 25.

As usual, I’m posting more on my methods and raw data below.

(more…)

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Independent Confirmation

Independent Confirmation. I’ve always suspected that what I’ve “discovered” is so basic that people must’ve known about this before. Lo and behold, doctors at the University of Michigan recommend that people who suffer from allergies make a purifier for $25 with a furnace filter and a box fan.

Getting independent confirmations of results is always important, and their live tests with a particle counter are convincing. However, their test is with a MERV filter, which catches less than HEPA filters. The upside is that they let more air flow through. I suspect that’s why his numbers are low, but not as low as my tests with a HEPA.