Category Archives: Air Pollution Essentials

Here is my collection of evidence-based articles regarding air pollution’s effects, as well as what you can do to decrease your risks.

The Science of Air Purifiers and Better Health: Is There Any?

happy kid in the field

What’s the hottest gift this Chinese new year? Perhaps instead of moon cakes and red envelopes, you could give “the gift that keeps on giving”: an indoor air purifier. They certainly are all the rage in China since last year, with skyrocketing sales and sold-out inventories after the trio of highly publicized airpocalyptic crises. I think this is a good turn of events: plenty of independent testing, including mine, has documented that a good air purifier can dramatically improve your indoor PM2.5 by 80% or more. But is there any good data that proves that this actually makes you healthier? It seems logical, of course, that decreasing exposure to pollution would decrease harmful health effects. But medical history is filled with tales of common sense and tradition that later turn out to be worthless or harmful — like bloodletting, or the more modern tradition of multivitamins. A big percentage of people reading this article take a daily multivitamin, assuming it’s “healthier” to do so, but the best evidence shows they are worthless, and possibly harmful. Could air purifiers be the same?

In theory and in testing, a good purifier should improve a room’s pollution levels more than 80%; this 80% reduction is also what the private Association of Home Appliance Manufacturers (AHAM) uses in their clean air delivery rate (CADR) tests, which are widely cited in comparison charts of air purifiers. So let’s say you’ve installed a top-of-the-line purifier in your living room, feeling quite safe and cozy. But how much of your time is actually in that filtered room? Or maybe the purifier is too small for that room size, or the filters are old, or the fan speed is too low, or the windows are open? Even this commonly cited CADR test is just a lab test for only 20 minutes — what about in the real world? I want to take this conversation to the next level, seeking out proof that your health will improve when using these machines. I want to be able to tell my patients and readers that there are published research studies which followed people over many months or even years, compared them to a control group not using air purifiers, and measured their health to see if there was any improvement in heart and lung disease, cancers and death rates. Are there any such studies?

I searched the Pubmed scientific database to find the best studies, and I was disappointed but not surprised to find very little strong data. A properly designed research project like this would be very difficult and expensive. But there are a few attempts, especially studies looking at using HEPA filters to help children with asthma. One was a systematic review published in 2002, which found that air filters helped to improve asthma symptoms — but the effect was small, and there was wide variation between studies which made conclusive assessments difficult. A more recent, very well designed study published in Pediatrics in 2011 followed two hundred children with asthma who also were exposed to secondhand smoke at home, and gave half of the kids a true HEPA purifier and the other half a fake purifier for their bedrooms. After a year, the HEPA group of children had less doctor visits for asthma flares, which possibly — but not conclusively — could be due to the 25% decrease in PM2.5 in their homes.

Other studies have focused on allergies, including an interesting study from 2008 which assessed children with documented pet allergies, following them over a year and recording lung function and blood markers. After a year, those who used HEPA air purifiers showed no clear difference in lung function, use of allergy medicines, or blood markers of allergies. Another study back in 1990 was a bit more impressive, showing not only a 70% reduction in indoor PM0.3 but also improved patient symptoms of allergies.

All of these hint at health benefits, but they still dance around the edges of what I want to know for us in China and the developing world. In the USA, most of the air purifier marketing and testing focuses on allergies and asthma. But here in the developing countries, the air pollution is much more severe and thus the health risks are far more serious. We are worried about pollution’s long-term risks of death, heart and lung disease and cancer. These studies I just mentioned still aren’t answering that deeper question: can long-term use of indoor air purifiers prevent death, heart and lung disease, and cancer?

Happy Family Riding Bicycle With Cloud Background

The best study I found was published in January 2013 in Indoor Air. It was very well designed for this complicated type of study, being a randomized double-blind crossover study of 20 homes over three weeks, using an air purifier or a placebo purifier. Their main goal in this remote First Nations community in Canada was to assess whether air purifiers could improve cardiorespiratory health. As their abstract says,

“…each home received an electrostatic air filter and a placebo filter for 1 week in random order, and lung function, blood pressure, and endothelial function measures were collected at the beginning and end of each week… On average, air filter use was associated with a 217-ml increase in forced expiratory volume in 1 second, a 7.9-mm Hg decrease in systolic blood pressure, and a 4.5-mm Hg decrease in diastolic blood pressure. Consistent inverse associations were also observed between indoor PM2.5 and lung function. In general, our findings suggest that reducing indoor PM2.5 may contribute to improved lung function in First Nations communities.”

This same Canadian research team had earlierpublished a similar study, testing 45 non-smokers for 7 days in 20 homes that used wood stoves, comparing health effects with or without HEPA purifiers. The people using the filters showed improved endothelial function and biomarkers of inflammation such as CRP. As most pollution researchers now see pollution as a pro-inflammatory disease, testing for such biomarkers could indeed be an accurate surrogate for later health problems. This approach is also being used in studies of air pollution masks, which I recently reviewed.

My take from these studies? Firstly, they all confirm what we already know: air purifiers can reduce the levels of indoor PM2.5, but with a wide range of effectiveness. Secondly are the more important results looking at health markers. I think the most encouraging finding was the First Nation study showing improvement in lung function, even in such a short amount of time (less than a month). Their data was a bit less convincing on blood pressure improvements, but perhaps a larger study would help confirm their initial findings of a slight improvement.

None of these studies are slam-dunk proof for me, but I honestly don’t know whether we ever will get many more well designed studies like these, unless governmental researchers or Gates-type philanthropists fund them. But until better studies come along, we must rely on what we do know:

  • Air pollution contains many chemicals, but PM2.5 is considered to be the most harmful to health.
  • There is no such thing as a “safe” level of  PM2.5. Lower is always better.
  • Worsening PM2.5 causes deaths from all causes, especially heart and lung diseases and cancers. Many studies have shown this, including this 2013 meta-analysis of the population in China.
  • On the brighter side, long-term improvements in PM2.5 do help to decrease mortality. The best study was a huge epidemiological analysis of entire populations in American cities as the air improved from the 1970’s to 1990’s. Lifespans improved for everyone, for a multitude of reasons, and they estimate that 15% of the improved life expectancy was due to cleaner air.
  • Shorter studies have also shown improvements in health from better outdoor air pollution. The best designed study I’ve seen on this happened right here in Beijing, during the 2008 Olympics. A team of researchers followed 125 healthy young doctors before, during and after the Olympics, and found improved blood pressure, heart rate and other biomarkers of inflammation during those lovely days of improved air pollution. Another encouraging study followed pregnant women and their babies in Tongliang, China both before and after a heavily polluting coal-fired power plant was forced to shut down in 2004, and found improved neurodevelopmental scores in newborns at age 2 years.

Is all of this enough to convince you to use an indoor purifier? For me, I was already convinced years ago — it’s not just common sense, it actually makes biochemical sense and also perfectly fits with the precautionary principle: “When an activity raises threats of harm to the environment or human health, precautionary measures should be taken even if some cause and effect relationships are not fully established scientifically.”

Do Pollution Masks Really Work?

Do pollution masks actually help to protect you? This winter has already seen harsh pollution spikes first in Harbin, and recently in Shanghai and eastern China. By coincidence I had been taking the train from Shanghai to Beijing during one of those days, and I sat mesmerized as we barreled through endless vistas of ghostly cities disappearing into the shroud.

Double Protected -- Totobobo mask and bike helmet
Double Protected — Totobobo mask and bike helmet

Such events, along with Beijing’s own “airpocalypse” last January, have dramatically increased public awareness and discussion about air pollution in China. Online stores are flooded with anti-pollution products; chat rooms and coffee shop conversations are filled with personal comments about their adventures with masks. Even in convenience stores such as 7-11 you can now find a big display of pollution masks stamped with N95, 99% or something similarly impressive on their labels. Let’s step back from this marketing hype — is there any evidence that any of these masks really do any good? This question is deadly relevant to me and my family, as well as to the hundreds of patients and readers who have asked me this question over the years.

What is my answer? I usually tell them, as I now tell you: yes, there is evidence of benefit — sort of.

There’s no medical doubt that smaller particles — the ones smaller than 2.5 microns (called PM2.5) — cause major inflammatory damage that leads to heart disease, lung disease and cancers. Quite a few studies have also shown encouraging results that lowering an area’s air pollution improves the health of the community, lowering ER visits and overall mortality. Lower your exposure, improve your health: yes, this seems obvious, but research is filled with assumptions that were proven wrong. Not in this case.

Let’s return to the masks. Construction sites all over the world can be filled with dust, heavy metals, and dangerous gases. These workers, for decades, have relied on protective masks made by companies such as 3M, with proven test results from independent labs that they reduce inhaled exposure by at least 95%. This is exactly what N95 means, a term we see often: an N95 mask, if officially certified by the National Institute for Occupational Safety and Health (NIOSH) in the USA, has passed the fit-test results on a human face. During this test, a person is wearing the mask for about 15 minutes, moving around in many positions, with the mask attached to special machines that record the concentration inside and outside the mask. If the air inside the mask is more than 95% reduced in particles, that mask passes the N95 test. Quite a few masks have this certification, and it’s for this exact reason that I am sure that these same masks can dramatically decrease all of our exposure to air pollution’s most dangerous particles, those PM0.3-PM2.5.

The main problem is jumping to the next level of assumptions — that by wearing the masks, you can improve your health. Yes, again this seems logical, but to date no studies have been done which follow such mask-wearers over months and years, testing their health and looking for improvements. Most likely these tests may never be done, unless a governmental agency such as the NIH pays for it. So in the face of lack of terrific data, the best data we have are a pair of short term studies from the same team of researchers, using subjects walking around the streets right here in Beijing. These researchers, from Cambridge as well as Beijing, first tested s couple dozen healthy young subjects and had them walk around the side streets around Second Ring Road, one of our many notoriously congested highways. When they wore a mask (the 3M 8812 model), they had a large improvement in blood pressure, 7 points on the scale.

But their follow up study, published last year in Environmental Health Perspectives, was much more relevant for me, as a family doctor. This time they tested people who already had heart disease — stable, but on medicines. Almost 100 people were tested this time, both with and without the same 3M mask for more than 24 hours. With the mask on, they showed improvement in heart rate variability, blood pressure, and stress markers for ischemia. All of these are considered risk factors for heart disease problems such as heart attacks and strokes, so any improvement in this objective data could have a major positive impact.

Why is this so important? Because air pollution doesn’t just cause lung problems, it causes heart attacks and chest pain, and strokes. And besides small children, I’m most worried about older people with heart disease, and this study proves that wearing a mask on those super bad pollution days just may save their life, preventing a heart attack.

These studies do have their flaws, especially their admitted fact that the control group wasn’t wearing a “placebo” mask, and perhaps the reported benefits are partly psychological as the subjects maybe were more relaxed while wearing the mask. But these are the best studies we have so far on the subject of masks, and I certainly am not going to wait around for better designed studies. I’m quite convinced that the underlying principle  is quite robust and well proven: that decreasing exposure to air pollution improves your health. And we already know, from decades of construction work use, that N95-rated masks are designed exactly to decrease PM2.5 exposure, and that this PM2.5 is currently considered the most dangerous of the many harmful chemicals in the air.

What more does one need to know?

N95 Pollution Masks: A Buyer’s Guide

Little girl with toys wearing a protective mask

Update March 2015: I no longer recommend any commercial masks mentioned below except for government-certified masks such as 3M and perhaps Vogmask. Please read my personal mask test results and a review of three independent tests of masks.


I’m a big fan of using air pollution masks when the AQI is over 200 (read my articles herehere and here, and the Chinese market has a lot of new competition since last January’s airpocalypse. Even in convenience stores such as 7-11 you can now find a big display of pollution masks stamped with N95, 99% or something similarly impressive on their labels. “Wow, this 5 RMB surgical mask says it’s 99% effective — plus, it’s imported from Japan!” Not so fast. Any marketing person can throw any numbers on these packages — we consumers need to dig a bit deeper and find out just where these numbers are coming from. Air pollution is such a serious issue here in China that we really need to stick with the best masks — with proven results from independent labs, not just TV advertorials and website testimonials. Here’s my advice on how to find the best mask.

What The Heck Is N95, Anyway?

We hear and read about this “N95” so much — are you sure you know what it means? Here’s a nice synopsis of what N95 certification means, from Nelson Labs, an independent testing center in the USA:

“…If you are a manufacturer of respirators, one of the most important processes you must go through is to receive certification from the National Institute for Occupational Safety and Health (NIOSH).  NIOSH respirator certification is necessary in order to label your respirator with one of the three “N” certification categories: N95, N99 or N100.  N95 respirators are used to filter contaminants such as dust, fumes, mists, as well as microbial agents including tuberculosis bacteria and flu virus. They are certified to filter greater than or equal to 95 percent of all challenged particles free of oil and greater than 0.3 microns in size.  N99 respirators filter greater than or equal to 99 percent…”

Let’s say that again: a mask rated N95 filters out more than 95% of particles (PM) larger than 0.3 microns — that’s much smaller even than PM2.5 microns. This is a good thing, since most of air pollution’s bad health effects are caused by particles 2.5 microns and smaller (the width of a human hair is 70 microns). These masks really work.

But here is an important point: almost every mask you see on the Chinese market, except for 3M, isn’t officially certified N95. Only the USA’s governmental agency NIOSH gives out the N95 certification, and if it’s not on their lists here or database here, then it’s not certified N95. But just because it’s not on this list does not mean it’s a crappy mask — a handful of masks not on this official list actually have proven 95% effectiveness from testing. It’s actually very expensive and difficult to jump through all those government hoops for official NIOSH certification. N95 is more for industrial worker safety, and mega-companies such as 3M can easily afford those tests, while smaller consumer start-ups can barely afford local testing, much less certification. Other countries have their own ratings which you may see on packages, such as the European EN149 FFP2 or Chinese YY-0469, both equivalent to N95.

Not All Tests Are Created Equal air pollution masks china review

My second major point in this article is that there are two important but extremely different tests of a masks’ effectiveness. The first tests the material itself, pushing a microscopic chemicals such as latex through a swatch of the fabric and seeing what percentage make it to the other side. These are called particle challenge tests. Quite a few of the most popular masks have results way over 99%. Vogmask gets 99.978%; Totobobo 99.85%; Lvdun 绿盾PM2.5口罩 99.45% … Those are incredible numbers, yes?

Yes, they are — but the problem is this: this test means nothing in the real world. You could do a particle test on a rock and get 100% efficiency, but you’re not going to wear it on your face. The best fabric in the world is totally useless if the mask itself doesn’t fit your face well, with air leaking all around it. This is why it’s far more important to make sure your mask has fit test results. During these tests, an actual human being is wearing the mask, and special sensors are sticking out of it, measuring efficiency over a span of 15 minutes. There are two types of fit tests: qualitative, which are personal impressions of a masks’ fit; and quantitative, the hard data showing efficiency at filtering out PM2.5, PM0.3 and even smaller particles. Of these two, the quantitative data is far more useful for us. A person could love the comfort and give the quantitative results an A+, but if the hard data only shows 50% filtration, then who cares how comfortable it is?

Why is this important? Because a bunch of mask companies are bragging about being 99% effective — but they’re only talking about their particle challenge results, not fit test results.  This is the major reason why I am not thrilled with Respro masks, which have impressive 99% results on particle challenge but are only 88% effective on their fit test results. Why buy a mask like this when there are a handful of other masks which filter better (and also are cheaper)? For the same reason, I am not a big fan of the newly popular Chinese branded mask, Lvdun 绿盾PM2.5口罩, available at 7-11. Their particle challenge results are just as impressive as the rest, again far above 99%, so their filter fabric is excellent. But I don’t see a fit test report on their website or their Weibo. Plus this filter is not the entire mask at all, it’s a smallish rectangle which you insert inside the fabric of the mask. I personally tried this for a few days and noticed air leakage and street smells, so I highly doubt they could get a fit test of 95% or above. Maybe they can, but again, given the gravity of our air pollution, why buy any mask that doesn’t have a fit test report over 95% when so many others do?

In the same vein, there are a bunch of surgical masks and other masks online and in stores bragging about 95% or 99% on their shiny packages, many claiming to be from Korea and Japan. But again, there’s almost no way any of these throwaway surgical masks are truly 95-99% effective in the real world, as it’s obvious there’s massive air leaking around most of these. OK, they are better than a simple cotton mask, as one study showed surgical mask effectiveness at 80%, compared to 28% with a simple cotton handkerchief. But if you’re buying cheap disposables anyway, why not stick with 3M, which are proven much more effective?

Fit test Nelson labs air filter
Fit Testing (Quantitative)

So Which Have These Fit Test Reports?

There are four brands that I am aware of which have both particle challenge results over 99% and quantitative fit test results over 95% for PM2.5:

I personally have used all of these and feel that all are good choices. I continue to use them on different occasions, and I have no strong preference for any of them. I have them stashed at work, at home, in my briefcase and in my bike saddle bags.

So take your pick! If you don’t like the fit of one, try another. Experiment, and leave your own personal feedback below in the comments section. An educated consumer is always a better consumer.

If you want more information on masks, you can read a few key earlier articles (herehere and here) as well as wander around my air pollution archives.

What About Kid Sizes?

As of this month there are finally good options for kids. The Vogmask company now has two versions that fit a kid’s smaller face; they are first on the market to pass fit test results on children, showing over 95% effectiveness on a 9 year old boy. Totobobo also has a new model which you can custom mold to faces using hot water, which you can trim down to a child’s face. This custom molding may lead to better fit and thus higher effectiveness.

Pollution Masks For Children: A Review

My adorable son is now four months old, still a tiny bundle of joy. I look forward to us venturing outside for summer fun, but what if the pollution is bad that day? I desperately hope we don’t have another January-ish “airpocalypse” of extreme air pollution keeping our kids inside. I hope everyone is taking long summer breaks amid cleaner and greener pastures. But for us staying here in Beijing, what pollution guidelines should we follow when kids are at camp or playing outside?

child-n95One major debate is choosing when to keep your children inside. I think a good baseline is to follow the action plans that most international schools use, with a usual AQI cutoff of 200 triggering cancellation of all outdoor sports — assuming no protective masks would be worn. We use these cutoffs because children’s lungs are still developing until around age 18, and as they exercise they breathe much deeper than usual, thus potentially causing more damage.

What about air pollution masks? Since I last wrote about air pollution for BeijingKids, there was a very interesting new study published which provided the best evidence so far that wearing an N95 mask does indeed help to reduce air pollution’s unhealthy effects. They tested people walking around major roads in Beijing and had half of the group wear a 3M N95 mask. Those who wore this mask had improved blood pressure as well as heart rate variability, both of which are signs of damage from pollution.

I think this and other studies should convince parents to consider buying masks for their children, especially when air pollution levels are over 200 AQI. The key is to use a real N95-rated mask. N95 literally means that mask is certified (in the USA) to filter out 95% of particles (PM) smaller than 2.5 microns — usually much smaller, at PM0.3. This means if the AQI is a crazy-bad 500, your mask is filtering 95% of that, and your actual breathed air is much healthier. But you must find a mask that is rated N95, not just a cotton mask or even just a blue surgical mask.

The other major issue here is fit: even a small amount of air leaking around the corners makes any mask basically worthless. Unfortunately, there is no easy answer as to the best brand and model for kids.  The 3M N95 rated masks are always considered the first choice “gold standard” in research tests. April Gourmet, Jenny Lou’s and many other places sell them, as do all of our clinic pharmacies. The 3M company has an 1860S child size, but I have only seen this in the USA. Many parents have tried Totobobo masks down to toddler years as they can be cut down to children’s faces, and they also aren’t as scary looking as a standard mask. Others have tried Respro, but their own blog doesn’t recommend any of their models for children under 11. Others seem to like the Vogmask series as they are comfortable for smaller faces. However, many parents report varying degrees of success for all of these. But don’t fret, as I am personally aware of at least two companies feverishly working on children’s masks. Even as we go to press, there may be at least one company making them. (UPDATE: starting next month, Vogmask will be selling the first consumer line of masks designed for children of all ages, officially tested over 95% protection. Stay tuned for a review…)

If your child is planning any overnight trips or camping adventures, I think it’s wise to give them a good air pollution mask just in case. And if their day trip plans come into question because the air is bad, you don’t always have to cancel their fun — consider wearing a mask while they play. I know this entire idea of wearing masks is very tiresome and even controversial, and it certainly puts a damper on outdoor play. But as parents it’s our moral responsibility to safeguard our children’s health while living here in China.

(This article was originally printed in the current edition of Beijing Kids magazine, where I am a contributing editor. You can click here to read the rest of my BeijingKids “The Doc Is In” columns.)

Finally, Great Data on Air Purifiers in China!

UPDATE, March 2015: Please check out my buyer’s guide to air purifiers; my 2014 review of the science behind air purifiers; my 2015 tests of air purifiers under 1,000 RMB plus my 2014 review of two dozen top air purifier models in China. 

Which is the best air purifier for China? I get asked this question all the time, and my articles discussing my own test results still are the most popular articles on my website (here, here and here). But I’m just one data point and can’t try all the models available. For objective tests, I’ve always been telling people to read the independent Consumer Reports test results, as well as the reviews from Consumer Search. But finally we have new data points specific to the machines available here in China. Mentioned in Shanghai Daily, they report on the Shanghai Consumer Rights Protection Commission’s new analysis of 22 air purifier models. They focus on PM2.5 and formaldehyde, arguably the two most serious indoor air pollutants on our health. Quite a few brands didn’t do so well! Anyway, the original article is in Chinese but I’ve Google Translated the table below. How did your model do? I’m happy as my home’s Blueair did great, removing 97-99% of both formaldehyde and PM2.5; I don’t see results for my IQAir here. But there are many others: Panasonic, Daikin, Sharp, Yadu, Honeywell…
air purifier filter tests 1

air purifier test results