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.


Not great, 28% of particles blocked.

Next they tried a cheap surgical mask.


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.


Most were around 80%.

Then they tried several cheap 3M masks.


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:


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:


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.


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).


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


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!

Poor Man’s Fit Test

Which mask works best on your face? I was fortunate enough to visit a lab to do a super fancy fit test, but very few of us have access to this $10,000 machine. So what should normal folks do?

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While visiting the 3M lab, I learned about what I’m calling the poor man’s fit test. It’s not as accurate as a real fit test, but it will help you identify big leaks. It’s pretty simple:

  1. Put on the mask. Make sure the metal is bent tightly around your nose.
  1. If the mask has two straps, make sure one strap is below your ears and one above like this:maxresdefault
  2. Lightly hold the mask in place and inhale sharply. While inhaling, pay attention to see if you feel a sensation of air or coolness around the edge of the mask. Pay particular attention to the area around the nose.
  3. If you feel air leaking, adjust the mask and try again. If further adjustment does not solve the problem, try a different mask.


If your mask does not have an exhalation valve, you can also do the test while exhaling sharply.

Breathe safe!

Do ionizers actually clean air?

The other day, someone on Quora asked whether ionizer fans actually purify the air. This is an important question because ionizer purifiers are all over the place. For example, I was at a friend’s apartment in the US, and I saw his tower fan had an ionizer button on it:

Ionizers 1

It’s also important because several friends in China have sent me links to products like this:

Ionizers 2

 Amazing! A “miraculous purifier” that removes PM 2.5 and formaldehyde in just 30 seconds. And all that for far cheaper than regular purifiers and even cheaper than building your own purifier.

If this is true, my life in Beijing is now so much easier. But is it true?

So how do ionizers work? 

Here’s my bedroom, with an ionizer and bad particles in the air:

Ionizers 3

That ionizer shoots out negative ions:

Ionizers 4

Those ions cause the particles to stick to surfaces, like my bed, the wall, and the floor:

Ionizers 5

That’s the principle behind ion generators. It’s hard to see it happening with these tiny particles, but you’ve seen it on a visible scale if you’ve seen someone rub a balloon on their hair and then stick it to a wall.

Ionizers 6

But wait #1

A summary of scientific tests of air purifiers found that most ionizers have no noticeable effect on particulate levels (p. 8). Their conclusion is that most ionizers are too weak to have an effect. Studies do show an effect if they use very strong ionizers–much stronger than most ionizers on the market (p. 19).

But wait #2

OK, so regular ionizers don’t work, but we can use a big one! The problem is, when you put that many ions into the air, it produces ozone. Ozone is harmful, so that’s not good!

But wait #3

Even if we use a really strong ionizer and even if we can accept the ozone, you might have noticed that the ionizer didn’t actually filter out the particles. It just made them stick to my bed, wall, and floor.

First, that’s gross. Since the particles floating around here in Beijing include things like arsenic cadmium, and lead, I’d rather not have them stick to my pillow.

Second, they’re still a danger. The particles are just sticking to my bed. So let’s say Thomas comes home:

Ionizers 7

When I sit down on my bed, I’ll dislodge those particles, and they’ll float back into the air. Here’s my super scientific rendering of that process:

Ionizers 8

 Those problems are what led Consumer Reports to publish tests and warn people not to buy the Sharper Image Ionic Breeze. Sharper Image sued Consumer Reports; Consumer Reports won.

So when people send me links asking about these “miraculous” purifiers, I tell them to steer clear.

Careful not to overgeneralize

But let’s not draw too broad of a conclusion here. This doesn’t mean ALL air purifiers are junk. Instead, I use HEPA filters. HEPAs actually capture particles, and they are backed by empirical tests (1, 2, 3, 4, 5, 6). Here’s a little test I did with HEPA filters in Beijing:


cars delhi

A $3 billion pollution ‘solution’ in Delhi – but will it work?

Last week, the Ministry of Urban Development announced a Rs. 19,762 crore ($2.95 billion) solution to reduce vehicular pollution in Delhi. If approved, the proposal will seek to reduce emissions from the over 8.8 million vehicles in the city, mostly owned by the rising middle and upper classes.  Despite this class differential in vehicular emissions, some of the improvements sought include:

  • Seven pilot parking management districts
  • An integration of  207 metro stations with other forms of public transit systems
  • Construction of cycling tracks and footpaths with crossings at least every 250 meters, with first use of street space to pedestrians
  • Removal of choke points across the city
  • A procurement of 2,000 new buses immediately and 4,000 new buses in the next phase
  • Development of a Bus Rapid Transit System on high-density routes
  • Parking fees and congestion tax to discourage private vehicles

While several of these suggestions, namely parking management districts and imposing congestion taxes, may curb vehicle use, the rest of the plan largely ignores the intersection of class and the environment in the city. Academics such as Asher Ghertner and Sunalini Kumar have argued that past environmental efforts in Delhi have largely failed due to “bourgeois environmentalism” wherein middle class biases and interests take over environmental efforts that are genuinely in the public interest.

This may very well be the case with the Ministry’s air pollution proposals, which largely focus on bus and metro expansion. In today’s age, car ownership is no longer a practical necessity but rather a symbol of class, prestige, and status. In 2001, Delhi had 900,000 registered private cars. Today, there are more than 2.6 million. Though small in comparison to the city’s population, the increasing use of cars in a deeply congested city is unlikely to be deterred by  building new bus and metro routes. In fact, the Supreme Court acknowledged the problem in January when it asked DMRC to explore the option of creating a ‘premium‘ class service on the Delhi Metro to make the train seem more friendly for the wealthy.

We’ve already seen a big failure in convincing the middle class to use public transport through the Bus Rapid Transit system in 2008, which displaced cars from three lanes to two and dedicated a special lane to bus use. Rather than getting support, the BRT received a hugely negative and critical media campaign by middle-class journalists who lived in colonies along the route. Their complaints centered on the fact that the special bus route increased car travel times by 20 minutes or more, leading to inconveniences for car users. This argument went to the Supreme Court when an activist argued that the BRT system ignored the “wealth creators” of the city who preferred cars. It seems that these reactions to public transport have gone unnoticed in the latest proposals.

Also ignored are larger contributors to air pollution than cars—trucks and two-wheelers—which contribute to 24-25% and 18% of PM 2.5, respectively. Cars, on the other hand, contribute to 14-15%. While public transport may attract owners of two-wheelers, typically lower-middle class, it won’t make big progress in changing truck usage. Policies considering trucks and two-wheelers seem absent from the proposals.

Ultimately, Delhi’s air pollution solutions need a broader perspective and incentive model that accounts for the behaviors driving modes of transport. If Delhi is to curb pollution, it needs to create marketing and norms to get managers, CEOs, members of parliament, and other middle- or upper-class individuals to prioritize public transport. The idea is not as crazy as it sounds; such consumers readily take on public transport in cities like London and New York. However, behavioral nudges must come along-side policies that disincentive car ownership by higher costs to purchasing cars, especially second cars. A prime example is Singapore’s Vehicle Quota System, which makes vehicles 3-5 times the actual cost, thereby incentivizing people to use public transport. The same must go for two-wheelers, and strict environmental regulations must be put on exhaust of all vehicles, including trucks.

It’s time to create norms that are set for everyone, not just the poor. Just as lower-class auto drivers have been forced into using CNG to curb pollution, and over 3 million squatters have been evicted from their land for ‘polluting’ the land, it’s time to create policies that promote the middle- and upper-classes to create a better environment. Though increasing public transport is well-intentioned, it is not the answer. The Ministry of Urban Development must take into account deeper considerations of culture, behavior, and norms and use the increasingly expanding world of marketing and behavioral economics to change what is normal.




Air pollution can break your heart

For years, air pollution has been linked to heart disease but scientists haven’t been able to understand how, exactly, it breaks your heart. Last week, researchers in the U.S. released results from a long-term study that shed light; they found that air pollution thickens blood and hardens arteries, accelerating atherosclerosis — a disease in which plaque (calcium, fat, cholesterol and other substances) builds up in arteries, preventing oxygen-rich blood from reaching organs or body parts. This leads to the development of other diseases based on which arteries and body parts are affected, including heart attacks, stroke, or even death.

The study tracked 6795 participants in 6 U.S. cities between the ages of 45-84, all without a previous history of cardiovascular disease. Participants ranged from four ethnicities and came from a variety of socioeconomic backgrounds. Researchers then scanned participants’ arteries over a course of 10 years.  At the same time, they measured concentrations of PM2.5 and traffic-related gases called nitrogen oxides (NOx) outside participants’ homes, while creating estimates for the concentrations of pollution participants inhaled during time spent indoors.

The study found that PM2.5 and NOx were strongly associated with the build up coronary artery calcium, which accelerates atherosclerosis. Joel Kaufman, the lead author and a professor at the University of Washington, explained to ThinkProgress that air pollution may trigger cell inflammation, affecting white blood cells that protect the body against infectious diseases. As white blood cells accumulate, they build up plaque, causing atherosclerosis. This mechanism may explain why air pollution has also been linked to cardiovascular problems and mortality.

In the study, participants’ pollutant concentrations between the years 2000 and 2010 ranged from 9·2 to 22·6 μg/m³ for PM2.5 and 7·2 to 139·2 parts per billion (ppb) for NOx. For every 5 μg/m³ increase in PM2·5 and for every 40 ppb increase in NOx, coronary calcium deposits progressed by about 20%.

Though the results of the study are sobering, it’s important to note that the study was done in the U.S. under air pollution levels that fall well below the World Health Organization’s recommended PM2.5 exposure of 25 μg/m³.

Comparatively, North Indian cities tend to have significantly higher annual average PM2.5 levels, with Gwalior at 176, Patna at 149, and Delhi at 122. Further research needs to be conducted to understand how such high levels impact the severity of plaque build up and heart disease. However, a comprehensive Global Burden of Diseases, Injuries, and Risk Factors Study calculated that about 5.5 million people prematurely died in 2013 because of indoor and outdoor air pollution.


Is it possible to lower the cost of clean air without sacrificing effectiveness?

A few months after I published my DIY tests, there were already Taobao stores up and running, selling DIYs even cheaper than my 200 RMB. I was intrigued. If we could really lower the cost of clean air, that’s a win!

But we have to be sure these cheaper machines are as effective. So I ordered the cheaper machines, tested them systematically, and found they performed much worse (2).

A New Way to Lower the Cost of Clean Air

But that doesn’t mean there aren’t ways to make HEPAs even more affordable. Recently I tested one way to do that: if we can shave 1 millimeter from the size of the HEPA, we can save 6% on the price. This new size still covers the fan opening, but does it still work as well?


To test this question, my collaborator Anna ran 10 overnight room tests with the 29 mm in her Beijing apartment and compared it to my earlier tests of the 30 mm HEPA in the same 15m2 room.


Like in my earlier tests, I calculated effectiveness as the percent reduction in particulate from the start of the test to the average of the last four hours. Here’s what one of those tests looks like with the reduction calculation laid out. The blue line is indoor 0.5 micron particles; the red line is outdoor PM 2.5



On average, the new 29mm HEPA reduced 0.5 micron particle levels in the room by 86% and 2.5 micron levels by 91%. These results were almost identical to my prior results with the 30mm HEPA.



This new HEPA lets us lower the cost of clean air without sacrificing any effectiveness. Win! We just lowered the price of Smart Air HEPAs from 80 RMB to 75 RMB.

HEPA Cost Comparison

Next I took that price and compared it to the two biggest brands out there, Blue Air and IQ Air. (Also check out the long-run cost comparison .)

Nerd Note on Replication

As a side note, this test is now the third series of room tests I’ve published with the Original DIY (early tests; 200-day longevity test). Add that to Dr. Saint Cyr’s tests, and I’d say that’s a satisfying amount of reproducibility.

For fellow data nerds, I’m posting the original data and more details about the test below.





Outdoor PM 2.5 Fluctuations

I like these long tests more than the common 20-minute tests (such as CADR tests) because this mimics how most people use purifiers. However, one drawback of these long tests is that outdoor PM 2.5 levels sometimes change over the course of 8 hours. If outdoor PM 2.5 drops, that can artificially inflate effectiveness. If outdoor PM 2.5 rises, that can artificially decrease effectiveness.

These changes should cancel out over 10 tests, but it’s worthwhile to re-run the analysis while excluding days with large outdoor changes. Among these 10 tests, two days (9/25 and 11/10) had large outdoor PM 2.5 changes. Without those two days, the averages were almost identical: 84% on 0.5 micron particles and 91% on 2.5 micron particles.

Average Outdoor PM 2.5 During Tests

It’s also worthwhile to check out bad outdoor PM 2.5 was during the tests to see if these days were representative of normal Beijing air. The average outdoor PM 2.5 was 116 micrograms. That’s about 20 micrograms higher than Beijing’s average PM 2.5 over the last five years (according to my analysis of the US Embassy’s PM 2.5 data). Thus, if anything, these tests are tougher than the Beijing average.



Feeling the heat? You’re feeling the pollution too.

Had trouble breathing lately? It might be because despite popular opinion, Delhi’s air pollution levels aren’t doing so great in the summer heat.

Our team regularly monitors air quality through the Air Quality Index Widget on our phone, which uses U.S. Embassy Air Quality data. The U.S. Embassy converts P.M. 2.5 (mg per cubic meter) to an Air Quality Index (AQI) to easily guide health decisions. It is fairly typical for Delhi figures to be in the orange or red zone, considered unhealthy at an AQI of 151 to 200, in the summer. Winters typically see figures in the ‘Very Unhealthy’ violet category with an AQI ranging from 201 to 300.

US embassy AQI
U.S. Mission NowCast Air Quality Key


Though April AQIs have been, on average, showing moderate AQIs below 100, the pollution levels in the last week of April spiked.

April 28th, 29th, and 30th averaged at ‘very unhealthy’ levels of air pollution with average AQIs of 248, 246, and 300, respectively. Such high levels of pollution can cause significant aggregation for those with heart or lung problems.

The highest hourly averages on these days were above 500, with the maximum of 592 on 29th April. To put this into perspective, the U.S. Embassy Air Quality key maxes out at 500, which marks the maximum in the worst category, ‘hazardous.’ Hazardous levels of air pollution may cause serious heart and lung risks, even leading to mortality amongst those with cardiopulmonary disease. At levels above an AQI of 500, these effects may worsen.

Though there are is no evidence to help us understand the unusual spike, the high amount of air pollution may be a result of two on-going activities. One, the end of April brings an end to the wheat season. To clear the land of wheat stubbles in preparation of planting other crops, farmers often burn the wheat residue. According to Umendra Dutt of the Kheti Virasat Mission, however, only 16% of wheat reside is burned, so it is unclear how strongly the activity contributes to the PM 2.5 levels. Two, water-parched areas in Uttarakhand have resulted in an unusual number of forest fires. As of last weekend, 427 fires were simultaneously burning in the region, with reports of air pollution and poor air visibility.

Though the reasons for the poor air quality is unknown, Delhi government has yet to put precautions in place to protect public health. Contrastingly, China has frequently triggered red alerts in cities across the country when smog levels rise to unusual levels. Red alerts, which include recommendations that people should stay indoors and vehicles should be restricted, are issued when regions see an AQI of 300 or above. Delhi is seeing almost double this number during peak pollution hours, yet the government has taken limited precautions beyond odd-even. Until institutional factors are not addressed, air pollution will persist in high amounts. Such factors include industrial policies in Delhi and neighboring regions, implementation of policies restricting crop burning, and measures to reduce negative health and environmental impact of droughts.

Does Spring bring better air to Delhi?

Spring in Delhi, is the air clean?
Spring in Delhi, is the air clean?

Delhi Winters are notoriously known for bad levels of air pollution, but we often hear that air gets cleaner come Spring. We were curious what was happening in our own backyard, so we put out our Dylos Particle Counter on a rooftop near our Saket office.  The Particle Counter measures particle counts between 0.5 to 2.5 micron in size per 0.01 cubic ft. From this number, we can estimate PM 2.5, which are microns approximately 30 times smaller than human hair. PM 2.5 is considered harmful to human health since these particles can get lodged deep into the lungs and cause respiratory and cardiovascular problems.

Here is the data from November 2015 – March 2016. Though air pollution levels do drop in March, notice that the month still had zero days of ‘good’ or ‘moderate’ quality air. Due to fluctuations in hourly air quality, some of the maximums in March are higher than the average pollution levels between November and February.

Though Delhi air may get better in the Spring, it doesn’t improve enough to be considered clean or healthy. Keep those masks and air filters handy!

Smart Air Filters featured in The Hindu

On March 31, Smart Air Filters was exclusively featured in The Hindu’s Business Line. In “Making cleaner air affordable the DIY way,” Chetna Mehra highlighted our commitment to providing clean air at a low-cost. Here’s an excerpt from the article – you can read more here.

If you are living in Delhi, you are already deprived of clean air, and may be ready to pay a hefty price to get lungfuls of this precious resource. But how much should clean air cost? Most will say it should be free; but the most affordable tag on it now is ₹3,400.

Social innovation firm Smart Air is selling affordable DIY (do-it-yourself) air-purifiers that clean indoor air as well as any expensive air-purifier available in Indian market in the ₹16,000-30,000 price range.

To find out more about our range of clean air products, check out our online shop.

DIYer Makes Cannon Quieter

Bill is an expat living in the hills northwest of Beijing, and he uses his DIY Cannon to cut the particulate in his home.

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But Bill wanted to improve on the biggest downside of the Cannon–it’s a bit loud on high:


So Bill designed a sound dampening box for the Cannon out of foam blocks:

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There are openings in the front and back to let air through, although my guess is this lowers effectiveness by at least a few percent.

To further cut down on noise, Bill wrapped the Cannon with a blanket:

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With all that padding, Bill measured a 6 decibel drop in noise! If you have a Cannon already, here’s an idea of what that’s like: That’s 2 decibels quieter than the regular Cannon on low (52 db).

If the noise gets to you and you don’t mind a craft project, I sanction it! However, keep in mind two things:

  1. Monitor the fan temperature. Make sure your design does not allow the fan to overheat.
  2. I suspect this will lower effectiveness by at least a few percentage points. I can’t say more precisely without testing it.

Great work, Bill!