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How To Choose An Air Purifier in China

Isn’t it a total hassle to buy an air purifier in China, especially for expats? Back in places like the USA, when we shop for anything we can luxuriously research using consumer magazines and other review sources, wisely shopping around online and in stores as educated consumers. Here, many of us struggle with Chinese so are at a loss on local shopping websites, forcing us to rely on word of mouth and a tiny handful of imported brand stores. This usually has meant that many of us end up buying the same legacy brands, usually quite reputable but also usually at incredibly marked up prices. Even if fluent in Chinese it’s still very difficult to research credible data. It’s a classic Economics 101 example of information asymmetry, where the typical consumer doesn’t have all the information that they need to properly research the marketplace, thus giving too much leverage to the seller — leading to a very unbalanced supply and demand curve where we pay much more than we should, for a machine that may not be exactly what we needed.

So in my continuing efforts to educate people about healthy living in China and elsewhere, I’ve written a basic how-to guide to buying an air purifier, similar to my pollution mask buyer’s guide. I won’t be discussing the need to buy one, as I’ve made it blindingly obvious in many other articles that an air purifier is essential for anyone living here, especially children.

Seal your windows and doors!

Pre-step: Hire An Independent Testing Company

I suggest that anyone with a villa, large home or a business should think about saving some time and possibly a lot of money first by hiring an environmental consultant to assess your site. You may think famous Brand X air purifier is great — but do you need one, two, three or more? And can you get equal benefits from a much cheaper model? And which speed setting do you need to use for general use? Let them figure out all the complicated cubic meters and assess how much machine(s) you need. They also do a great job looking for — and fixing — air leaks around windows and doors (which is cheap and effective advice for everyone, even in your one-window tiny dorm room). They also are experts on the brands of machines available. They should also come back for free for a specified time and double-check that all is OK. I’ve personally used two such companies, both run by expats: Environment Assured, which aim to get your indoor PM2.5 concentration under 10 ug/m3 (the WHO standard, and my goal at home), and also PureLiving China, whose indoor goals for PM2.5 follow the higher Chinese target of 35 ug/m3. I’m sure there are other companies out there, as this surely should be a booming industry. I personally think the evidence is overwhelming that your goal should be to keep your indoor PM2.5 under 10 ug/m3 all the time — even when the outside air is crazy bad. It can be done, and it doesn’t have to be as expensive as you fear.

Step 1: How much/many do you need? Let’s do some math

Kids' college funds, or air purifiers? Hm...
Kids’ college funds, or air purifiers? Hm…

If your needs are simple or small, or you’re only here a few months, you can definitely handle this on your own. First, in order to know how much or how many machines you need, you first need to do some math: you need to calculate your room’s volume. Most air purifier ads mention either recommended room size (适用面积) as square meters or airflow rate (风机空载风量), often written as CADR, and listed as cubic meters per hour. Here are the steps:

  • First, measure your room and get the area of your floor space, in square meters. For example, my dining room, living room and long hallway are collectively 84 square meters (“84 m2”).
  • You could be done already! Now you could just check the ads for the recommended room size (适用面积) and do the math. For example, I need 84 m2 of protection — a very large area. I don’t see any single machine rated that high but I see a few in the 40-70 m2 range. So in theory, two machines rated for 61 m2, with a combined area of 122 m2, should easily take care of my 84 m2.
  • That’s the easiest way — but I actually find it more accurate to use airflow and volume because it factors in the room height. A 20 m2 room with standard 2.6 meter high ceiling will need a lot less machine than the same floor space with cavernous 6 meter high ceilings, as many villa front rooms may need. So let’s do more math! Multiply your room area by the room height. For my front rooms: 84 m2 x 2.5m height = 210 cubic meters (210 m3) volume.
  • Since a proper air purifier needs to replace the entire room air at least five times an hour (5 Air Changes Per Hour, or ACH) to really be effective, you need air purifiers that collectively can cover the volume 5 times. So the total airflow (风机空载风量), or Clean Air Delivery Rate (CADR) you need = room volume x 5. For my front rooms, I need 210 x 5 = 1050 cubic meters per hour (m3/h) CADR. Since no single machine has this much power, I’ll need to buy a few machines.
  • Then you need to research air purifiers’ CADR — but here’s where it gets tricky: the published CADR applies only to max speed which is almost always not the speed you will be using 99% of the time. Most machines’ max speeds are far too noisy for normal use, and the speed you’ll actually be using may have airflow much less than the published CADR. The problem is that it’s hard to find published CADR for the lower speeds. I think as a general rule, you should cut the CADR in half to get a more accurate sense of how much coverage you’ll get.

Let’s continue with my example as a case study, now that I know I need to get to 1050 m3/h for my front rooms. Comparing a couple of very large machines:

  • Model A on max setting (5) = 783 m3/h. So I would need 1050/783 = 1.34 machines needed.
  • Model A on default setting (3) = 285 m3/h; 1050/285 = 3.7 machines needed.
  • Model B on max (6) = 510 m3/h; 1050/380 = 2.06 machines needed
  • Model B on medium (4)  = 289 m3/h; 1050/200 = 3.6 machines needed

Thus I could conclude that 2 Model A machines may be the most efficient choice for my needs, compared to 2 Model B machines, especially when it’s crazy bad outside and I want to crank up the machines to max speed for a few minutes. On the usual settings I would need an equal amount of machines, but they’re both equally expensive so I’d just start with two of Model A, take some data for a while with my Dylos, and see whether I need to buy a third one.

  • Thus armed with such information for your own rooms, now you can properly research which machines may be best for your needs. Generally you’d want a higher airflow than you really need, so you can use the machine at a quieter level and still get effective airflow. Or maybe two smaller machines combined would still be a better value than one more expensive machine.

Step 2: How Much Budget?

Only 200 RMB
Only 200 RMB (or 6 Starbucks lattes)

Not everyone can afford 12,000 RMB ($2,000 USD) for an air purifier, and the wonderful news is that nobody should be paying such exorbitant prices. Certainly when I arrived in Beijing eight years ago those very few early entry, imported brands served an invaluable service for expats and others. But now, there are many reputable brands making perfectly fine HEPA filters in China at a far more reasonable price point. For example, I’m pretty sure almost every student in China could sacrifice a few days of their Starbucks latte and buy the 200 RMB do-it-yourself air purifier. I also just published my own data proving that a slew of air purifiers under 1,000 RMB are perfectly fine for smaller rooms. I’ve tested most of the expat-famous imported models and yes, many are quite good, but I wouldn’t call most of them a good value for the money — not anymore. For example, here is a list from of HEPA machines under 3,000 RMB which are rated for room sizes over 50 m2, including models from famous international brands such as Westinghouse, Philips, and Panasonic.

Step 3: Research data

In terms of features (工作原理), I strongly feel that you do not need ions (负离子) or ozone (臭氧) which actually can cause more lung harm than help — especially in the cheaper machines. Even UV lights (UV灯) are a bit gimmicky. I also don’t care much about killing bacteria (杀菌) and in my home formaldehyde (除甲醛) isn’t a big problem. Really, all you need is an awesome HEPA filter which clearly mentions it eliminates >99% of particles (过滤灰尘/花粉 (0.3 微米)), plus a strong fan speed which reflects in a high airflow/CADR. My second rank would be an activated charcoal filter (活性炭 滤网) which absorbs the sometimes serious indoor gases called VOCs, especially formaldehyde (甲醛) and benzene (甲苯).

In terms of independent reviews, English readers should definitely start with, which collects the most reputable reviews from multiple review sites such as Consumer Reports as well as reputable independent testers and also consumer reviews from Amazon. But this is limited data for us in China as many of the brands aren’t the same here. I dearly hope there’s a Chinese version of such a useful site for Chinese consumers! In the meantime, I’m aware of a couple of independent reviews in China which I’ve blogged about here; I also found another Chinese-only review here.

For other China-specific reviewers, I’ve blogged quite extensively about air purifiers and have reviewed many, which you can read here. My other favorite tests are from Thomas Talhelm, the creator of the 200 RMB Smart Air filter who also has extensive tests on his blog here.

The next level of research are the online shopping stores. Each model is overwhelming you with pretty pictures and a hard sales pitch, but it’s here that you’ll find the details of CADR, filters, room sizes, and extras such as carbon, formaldehyde, etc.

Step 4: Buying

In China, online shopping has quickly become the most efficient way to buy almost anything, including air purifiers (空气净化器). I recommend starting with Amazon China’s Clean Air Store, partly because on the left hand side you can instantly filter by square meters, price and features like HEPA. Also, for most English-speaking expats the Amazon store is much easier to use and very familiar to the USA version; they even have an English interface. My second choice online store would be Jingdong (, especially if you click on their own distribution brands (京东配送); they also have excellent filter options. My last choices would be Taobao and Tmall. Of course there are retail stores as well but for research and convenience, not to mention price, online is great.

Step 5: Testing

Dylos 1700 particle monitor
Dylos 1700 particle monitor

What good is spending all this money on machines if your indoor PM2.5 still isn’t under the goal 10 ug/m3 all the time? I’ve saved a huge amount of money and also know that my indoor air is clean because I invested a bit in a portable particle monitor. There are many brands but my favorite is the Dylos 1700, which also seems to be developing a fan club in China, led by the helpful FAQ on the fantastic website Apparently you can buy online on Chinese sites but it’s cheaper (and probably safer) to get in the USA directly from them or Amazon. I hear that a lot of groups are pitching in to buy one and share, which is a terrific idea. The goal with this Dylos is to get the data on the left side of the screen (PM0.5 but actually more represents PM2.5) always under 3,000, which correlates to an AQI of 50 (which itself means PM2.5 concentration under 12 ug/m3).

Step 6: Maintaining

Before and after the airpocalypse
Before and after the airpocalypse

It’s quite shocking just how many people buy machines and actually forget to change the filters, essentially making them totally ineffective if the filters get too clogged. It’s not only crucial to replace the filters on time (all machine replacement schedules are different), it’s also helpful to frequently wipe or vacuum the outside of the machine to get dust off the prefilters. I’ve had a Blueair for a long time — and totally forgot about the plastic prefilter screen underneath the machine, which was totally clogged with dirt and I’m sure was dramatically lowering the airflow. That was embarrassing!

 My Bottom Line

There simply is no longer any good excuse for anybody in China, even those with limited resources, not to protect themselves with an indoor air purifier. I hope I’ve provided some helpful information for you to make informed decisions.

Safe Salmon in China? Yes!


I love the unique taste of salmon, which is fortunate for me as it’s truly one of nature’s superfoods. Salmon is packed with heart-healthy omega-3 oils EPA and DHA, protein and vitamin D and also is low in dangerous metals such as mercury. I oftentell people to eat oily fish such as salmon at least once a week to dramatically decrease their risk for heart disease. A 2006 review study in JAMA  shows that a daily dose of only 250-500 mg of omega-3 fatty oils can lower your risk of sudden death from heart disease by 36%, and from all-cause mortality by 17%; more than 500 mg daily actually provides very little extra benefit. And as 100 grams (3 ounces) of farmed salmon has over 2 grams of omega-3 (more than wild salmon has), even one serving a week may be enough because the healthy oils can remain in our tissues forweeks. This is all great news, right? But when I tell my patients in Beijing this fantastic news, they usually reply the same way: “I’d love to eat more fish here, but I never know which store I can trust.”589px-Salmon_sashimi

When my wife and I first arrived in Beijing nine years ago, we first bought our fish and other meat from the large international supermarkets Carrefour and Walmart, mostly because we assumed (for better or worse) that these stores would have superior quality control and safety standards, especially with imported foods. And that worked well for many years, especially as these markets slowly started to sell more organic options. Later on, we discovered the German-run Metro 麦德龙 hypermarket, and we immediately switched almost all our meat and produce purchases there, due to their outstanding logistics and traceable food chain. In other words, we trust them, and trust is a really big deal here in China. Metro’s salmon is mostly from farmed ponds in Faroe Island, a very safe area in the north Atlantic which is antibiotic-free and also certified by the non-profit Aquaculture Stewardship Council (ASC). Ikea, just up the street from Metro, also has an impressive selection of imported frozen salmon from Scandinavian waters, again all certified by the ASC or MSC (Marine Stewardship Council), and at very reasonable prices. Both stores sell their salmon for ~60-70 RMB/500g. So for those of you who don’t trust your fish in Beijing: there’s my answer.

omega-3 Fatty Acid Fish Safety Doses
Comparison of omega-3 and mercury levels in various fish

We still love Metro and Ikea but our #1 choice now for salmon is the monthly group buy, called GroupBuyByBianca, organized by the staff formerly from the Chef Too restaurant. Once a month they’ll trek to Beijing’s wholesale fish market; choose farmed salmon from Norway, Canada, or Faroe Island; de-bone and vacuum pack and then deliver to your door in chilled containers. It’s a fantastic service, and we usually get half a salmon every couple of months which we store in our freezer. Bianca and the team also sell imported cod and other meats in season. To sign up and order, follow their WeChat ID “GroupBuyByBianca” or email [email protected]. The cost depends on market prices but recently is usually ~45RMB/500g plus 10% and a flat 65 RMB processing fee.

Our other newer options for buying fish and meat are again online. The first is the wonderful local organic farm TooToo, which I’ve mentioned before as a very trustworthy, internationally certified local organic farm with a terrific distribution chain, easy online payment, professional delivery service and unbeatable value of organic produce. It’s an awesome resource for Beijingers — plus their website at has English and Chinese! You can buy 200g bags of Norwegian salmon for 36-50 RMB each. Besides salmon, they now offer a large selection of meats from many different sources — check out their long list of imported fish here. We’ve had particular success with shellfish from Europe — mussels from Scotland and shrimp from Ecuador were delicious.


Where? Cost (RMB) per jin Notes
Ikea 69/500g ASC certified, Atlantic
Metro market 60-70/500g ASC (Faroe Island: Bakkafrost)
Carrefour market 128/500g Faroe Island
Jingkelong market 78/500g online store 90/500g (36 RMB/200g) Norway
Group Buy by Bianca ~70/500g (~95/kg+10% + 65RMB) Farmed: Faroe Island, Norway or USA
April Gourmet 123/500g (245/kg) Norway


Besides TooToo, there are now a bewildering number of players in China selling foods online via apps and websites, with ridiculous amounts of investments from all the big internet players and finance companies. One such store my wife uses often is called yiguo (易果) at We liked them initially for their imported fruits but they also have a decent selection of meat, including a special section for imported beef. Other large sites like yihaodianWomai and’s grocery store are notable because they both have their own supply chains and distribution centers, which in theory could provide consumers better quality and more traceable products (with quicker deliveries, I’ve noticed). Amazon China also has their own online grocery store. All of these e-markets carry a big selection of imported foods of all types, far more than you would ever see in any local market.

Many expats get their salmon and meat from the small international markets such as April Gourmet or Jenny Lou’s, and that’s fine of course, and it’s certainly convenient for many on the way home from work. I just think the prices can be a lot higher than other options (see the comparison chart below), and I also worry about low sales volumes in small markets in terms of food safety. Many people also buy salmon at local markets like the popular Sanyuanli market, but I personally feel they have extremely inadequate food safety there; most vendors’ meats sit in the open air at room temperature, uncovered, on wooden slabs, with flies buzzing around. Do I really need to break down how many violations of basic food safety I just mentioned in that one sentence? I wouldn’t recommend buying meat from any market anywhere in the world if it’s sitting at room temperature for more than two hours.

Notice the ASC certification label…

Besides making your own salmon, eating in restaurants is definitely the next best option. All you sushi lovers can easily get your weekly omega-3 fix with even a few slices of salmon. Beijing is blessed with plenty of excellent Japanese restaurants and salmon dishes. Our favorite sushi place is a small Japanese market called yuqing (鱼清) just next to Yotsuba along the Liangma canal waterfront across from the Four Seasons Hotel; you can choose your raw fish from their shelf and the chef will prepare it right there for you to eat in the store.

What about the big percentage of readers who take a daily supplement of fish oil, including myself? This indeed has been long recommended even by the American Heart Association, but unfortunately the most recent studies, much larger than earlier studies, disturbingly show very little benefit from the supplement. There must be something else besides omega-3 in the actual fish that provides the heart-healthy benefit. Anyway, when my supply runs out, I won’t be continuing that anymore.

So there you have it; I hope I’ve convinced some of you that healthy fish = healthy heart. And for Beijingers, it’s not nearly as hard as you may have thought to add safe salmon into your diet, even at a reasonable price. For those of you in China out of the tier one cities or not near a good market, now there are plenty of online options to get salmon delivered right to your door. If you’re really worried about trust, sustainable fishing, and seafood free of chemicals and antibiotics, just stick with vendors that have ASC, BAP or MSC certification stickers on the fish packaging — Ikea and Metro would be your safest bets.

In terms of general value, here’s a nice graph from the JAMA review showing relative money spent on different types of fish to get your daily 250 mg of omega-3:

Estimated Costs of Consuming the equivalent of 250 mg/d EPA + DHA From Fish


Air Pollution or Smoking: Which Is Worse? A Letter From Dr Pope


The following is a reprint of a letter from Dr. C. Arden Pope III addressing the recent controversy whether Beijing’s daily air pollution exposure PM2.5 is equivalent to 1/6 of a cigarette (my 2013 estimate, which you can read here, using Dr. Pope’s research),  or to 38 cigarettes, as recently suggested by a new study with the accompanying media commentary and press releases by Dr. Richard Muller at Berkeley Earth. Dr. Muller, Dr. Pope and I have been corresponding via email this week and Dr. Muller has also already reviewed Dr Pope’s letter below and agrees with the general conclusions. Dr. Pope has given permission to publish this letter. I am preparing a blog article discussing this discrepancy but thought my readers should read this letter first.

You can skip to the “Discrepancy in excess risk…” section below to get to the most important parts, but otherwise the take-home message is, as Dr Pope says below, “air pollution is associated with a much higher excess risk and loss of life expectancy compared to cigarette smoking than would be expected based on the comparative dose of fine PM.” He offers three potential explanations for this discrepancy, all of which would require further research.

How can burden of disease from exposure to air pollution be comparable to cigarette smoking given enormous dose differences?

C. Arden Pope III, PhD
Mary Lou Fulton Professor,
Brigham Young University


Recent studies estimate the burden of disease from air pollution in some parts of the world (such as highly polluted cities in China) to be comparable with that from cigarette smoking. For example, the recent global burden of disease (GBD) estimated that 3.2 million deaths per year worldwide were attributed to ambient fine particulate matter (PM) air pollution (Lim et al. 2012). A recent study from Berkeley Earth (Rohde and Muller, 2015) estimates that in China alone fine PM air pollution contributes to approximately 1.6 million deaths (or roughly 17% of all deaths in China). These estimates of air pollution’s contribution to loss of life are comparable with, and for some areas, even greater than estimates from cigarette smoking. Is this possible given huge differences in dose to the body from mainstream tobacco smoke from active smoking versus breathing air pollution?

Doses of fine PM from different sources

Active cigarette smoking

When a smoker smokes a cigarette, the cigarette smoke, including fine PM and gases, are sucked through the tobacco rod and the filter. The fine PM from this cigarette smoke, excluding the nicotine and water, is commonly referred to as “tar”. A standardized smoking-machine test using the Federal Trade Commission (FTC) protocol has been used to estimate the doses (or yields) of tar and nicotine from various cigarettes. Sales-weighted tar yields, since the mid-1990s, based on this protocol have been approximately 12 mg (or 12,000 μg) per cigarette (National Cancer Institute 2001). A range of somewhat similar estimates of the dose of tar or fine PM come from multiple analyses (Martin et al. 1997; Djordjevic et al. 2000; Repace 2007). Cigarette yield estimates clearly depend on type of cigarette and other cigarette characteristics. Furthermore, the average daily dose of fine PM from active smoking is also highly dependent on more than just the cigarette yields estimates. Individual smoking patterns and habits play a critical role in determining actual fine PM exposures (National Cancer Institute 2001).

Ambient air pollution

The average daily dose of fine PM to the lung from breathing air pollution is dependent primarily on two factors: 1) the concentration of fine PM in the air being breathed (typically measured as in μg/m3 of fine PM) and 2) the daily inhalation rates (m3/day). Measured concentrations of long-term mean ambient fine PM range from as low as approximately 5 (or less) μg/m3 in very clean communities to as high as approximately 100 (or more) μg/m3 in highly polluted communities. Inhalation rates vary depending on age, sex, body size, activity levels and other factors, but estimates average volume of air inhaled by adults ranges from approximately 13 to 23 m3/day (Allan et al. 2008; Brochu et al. 2006; Layton 1993; Stifelman 2007; USEPA 1997). Assuming an inhalation rate of 18 m3/day, the approximate average dose of fine PM from air ambient air pollution could range from 90 μg (5 x 18) to 1,800 (100 x 18) μg per day.

Second hand smoke

Similar to exposures from ambient air pollution, the average daily dose of fine PM to the lung from breathing second hand smoke (SHS) is dependent primarily on two factors: 1) the average increase in concentration of fine PM from being exposure to SHS and 2) the daily inhalation rates (m3/day). There is limited information regarding the increase in concentrations of fine PM that comes from SHS, but one study suggests that homes with a smoker of one pack of cigarettes per day contributed about 20 μg/m3 to 24-hr indoor fine PM concentrations (Spengler 1991). Again, assuming an inhalation rate of 18 m3/day, the approximate average dose of fine PM from living with a smoker that smokes one pack of cigarettes per day would equal approximately 360 (20 x 18) μg per day.

Discrepancy in excess risk from smoking versus air pollution:

Comparing these estimated doses demonstrates that the doses of fine PM from air pollution or second-hand cigarette smoke are only a small fraction of the dose associated with active cigarette smoking. Yet, empirical studies of smoking indicate that smoking 20-40 cigarettes per day increases mortality risk by approximately 100 % (U.S. Department of Health and Human Services 2010; Pope et al. 2009; Pope et al. 2011) reducing life expectancy by approximately 8 years (Pope and Dockery 2013; Dockery and Pope 2014). Long-term exposure to fine particulate air pollution in highly polluted cities increases mortality risk by approximately 30% and reduces life expectancy by as much as approximately 3 years (Pope et al. 2002; Pope, Ezzati, Dockery 2009; Pope et al. 2011; Pope and Dockery 2013; Dockery and Pope 2014). Air pollution is associated with a much higher excess risk and loss of life expectancy compared to cigarette smoking than would be expected based on the comparative dose of fine PM.

It is informative to note that a similar dilemma also occurs for SHS exposures which are also only a small fraction of doses associated with active cigarette smoking. The excess risks of cardiopulmonary disease mortality from SHS (of about 20 to 30) are similarly disproportionately larger than would be expected based simple on comparative dose. In fact, the elevated fine PM exposures and excess mortality risks for SHS and air pollution are remarkably similar.

Potential explanations for this discrepancy

Supralinear exposure-response function

Recent studies that have integrated information from exposures to air pollution, SHS, and active smoking indicate that the exposure-response relationship, especially for cardiopulmonary diseases, is non-linear with a steep increase in risk at low exposures, flattening out at higher exposures (Pope et al. 2009; Pope et al. 2011; Burnett et al. 2014). There is evidence of a “saturation phenomenon” where relevant biological pathways for cardiopulmonary disease may be activated at low levels of exposure, and that increasing exposure further increases risk, but at a decreasing marginal rate.

Differential toxicity

Although the potential differential toxicity of fine particulate matter air pollution from various sources is not fully understood, fine PM from the burning of coal, diesel, and other fossil fuels as well as high temperature industrial processes may be more toxic than particles from the burning of tobacco. However, this potential explanation does not explain the disproportional excess risk from SHS and some have even suggested differential toxicity regarding SHS exposure versus mainstream exposures.

Ubiquitous exposure to air pollution

Essentially all individuals living in polluted communities are exposed, including the most vulnerable individuals such as infants, children, persons with existing coronary artery disease, chronic obstructive pulmonary disease, etc.

Combination of factors

It is likely that all three of the above potential explanations are relevant and play a role. Further discussion can be found in Pope et al. 2009; Pope et al. 2011; Burnett et al. 2014.



Allan M, Richardson GM, Jones-Otazo H. 2008. Probability density functions describing 24-hour inhalation rates for use in human health risk assessments: an update and comparison. Hum Ecol Risk Assessment 14:372-391.
Brochu P, Ducré-Robitaille J, Brodeur J. 2006. Physiological daily inhalation rates for free-living individuals aged 1 month to 96 years, using data from doubly labeled water measurements: a proposal for air quality criteria, standard calculations and health risk assessment. Hum Ecol Risk Assessment 12:675-701.
Burnett RT, Pope CA III, Ezzati M, Olives C, Lim SS, Mehta S, Shin HH, Singh G, Hubbell B, Brauer M, Anderson HR, Smith KR, Balmes JR, Bruce NG, Kan H, Laden F, Pruss-Ustun, A, Turner MC, Gapstur, SM, Diver WR, Cohen A. An integrated risk function for estimating the global burden of disease attributable to ambient fine particulate matter exposure. Environmental Health Perspectives 2014;122:397-403.
Djordjevic MV, Stellman SD, Zang. 2000. Doses of nicotine and lung coarcinogens delivered to cigarette smokers. Journal of the National Cancer Institute. Vol. 92, No. 2, pp 106-111.
Dockery DW, Pope CA III. Lost life expectancy due to air pollution in China. Risk Dialogue Magazine January 16, 2014.
Layton DW. 1993. Metabolically consistent breathing rates for use in dose assessments. Health Phys 64:23-36.
Lim SS, Vos T, Flaxman AD, et al. A comparative risk assessment of burden of disease and injury attributable to 67 risk factors and risk factor clusters in 21 regions, 1990-2010: a systematic analysis for the Global Burden of Disease Study 2010. Lancet. 2012 Dec 15;380(9859):2224-60. doi: 10.1016/S0140-6736(12)61766-8.
Martin P, Heavner DL, Nelson PR, et al. 1997. Environmental tobacco Smoke (ETS): A market cigarette study. Environment International. Vol 23, pp. 75-90.
National Cancer Institute. 2001. Risks associated with smoking cigarettes with low machine-measured yields of tar and nicotine. Smoking and Tobacco Control Monograph No. 13. NIH Pub. No. 02-5047 Bethesda, Maryland: National Institutes of Health.
Pope CA III, Burnett RT, Thun MJ, Calle EE, Krewski D, Ito K, Thurston GD. Lung cancer, cardiopulmonary mortality and long-term exposure to fine particulate air pollution. Journal of the American Medical Association 2002;287:1132-1141.
Pope CA III, Burnett RT, Krewski D, Jerrett M, Shi Y, Calle EE, Thun MJ. Cardiovascular mortality and exposure to airborne fine particulate matter and cigarette smoke: shape of the exposure-response relationship. Circulation. 2009;120:941-948.
Pope CA III, Burnett RT, Turner MC, Cohen A, Krewski D, Jerrett M, Gapstur SM, Thun MJ. Lung cancer and cardiovascular disease mortality associated with ambient air pollution and cigarette smoke: shape of the exposure-response relationships. Environmental Health Perspectives 2011;119:1616-1621.
Pope CA III, Dockery DW. Air pollution and life expectancy in China and beyond. (Invited commentary). Proceedings of the National Academy of Sciences of the United States of America (PNAS). 2013;110(32):12861-12862.
Pope CA III, Ezzati M, Dockery DW. Fine-particulate air pollution and life expectancy in the United States. New England Journal of Medicine 2009;360:376-386.
Repace, JL. 2007. Exposure to secondhand smoke. Chapter 9 in Ott, Steinemann, and Wallace, eds., Exposure Analysis, CRC-Press, Taylor & Francis Group, Boca Rotan, FL.
Rohde RA, Muller RA. 2015. Air pollution in China: Mapping of concentrations and Sources. Draft copy. Berkeley Earth.
Spengler JD. 1991. Indoor Air Pollution: A Health Perspective. Baltimore: Johns Hopkins University Press, pp 33-67.
Stifelman M. 2007. Using doubly-labeled water measurements of human energy expenditure to estimate inhalation rates. Sci Total Environ 373:585-590.
U.S. EPA. 1997. Exposure Factors Handbook. EPA/600/C-99/001. Office of Research and development, Washington, DC, USA.

Did Beijing’s Pollution Cause My Asthma?

Now that months have passed and I’m basking in our summer sun, I can safely confess that I had a miserably unhealthy winter.

It started in November with my first ever broken bone, a silly bike-vs-oil-patch accident which broke my clavicle and brought me surprisingly distressing pain for more than a month. But far worse was when I was diagnosed with asthma last December and needed two inhalers to breathe better. It started insidiously, when I began to wake up deep in the night with achy chest pains. I initially thought it was just rib bruising from my bike tumble, but then I also started feeling short of breath. One morning I woke up suddenly gasping for air, and I finally went to a colleague at my clinic. My chest x-ray was normal but I took a breathing test which showed my lung function only 60% of normal, and she said I probably had asthma. I’ll never forget those moments after taking those first two puffs of albuterol: in just a few minutes, that elephant-like pressure on my chest for a month quickly lifted away, and I filled my lungs with precious, polluted Beijing air, its acrid smell never tasting sweeter.

So I was fairly certain I had asthma. And while I was incredibly relieved to feel better, I was shocked and disturbed by my diagnosis. It’s not common at all for adults to suddenly get asthma, and of course my overwhelming thought was to blame it on air pollution. Finally, after eight years in Beijing, gasping through multiple airpocalypses, and despite all of my obsessive attempts to shield myself from air pollution, I believed the inevitable had caught up to me. I felt like a fool for ever thinking I could avoid pollution’s long-term health effects. All of my blogging about masks and purifiers; my TEDx talk about healthy living in China; my book discussing healthy lifestyles in China — all of it suddenly felt like sugar-coated wishful thinking, and my rose-tinted glasses finally shattered to reveal the truly ashen hues of my city’s “yellow fog”.

I felt trapped, helpless against the choking evil oozing invisibly and inexorably through window and door cracks, always finding a new hole after my frantically plugging another one. Anxiety filled my days, distracting me at work and home. I was no longer fully present with my family, my patients. I frantically retested all my air purifiers, added one in my office, and upgraded from N95 to N99 masks for my bike commute. Incense at our home during meditation suddenly devolved from a relaxing tool to an anxiety-provoking source of PM2.5. I even considered the previously unappealing but blindingly obvious “cure”: fleeing from China.

I didn’t take it very well, as you can see. “Disease produces much selfishness”, as Samuel Johnson once said. “A man in pain is looking after ease.” I even wrote a long blog article about my new illness and its profound impact on my life here, chronicling my desperate attempts to shield myself from pollution. I felt a massive release of catharsis after finishing the final draft, satisfied that it perfectly captured my state. And then I held off publishing it so I could revise later.

Now, a few months later, I’m relieved I never published that article, because what was diagnosed as asthma is now completely gone, for many months already. And now I know that my symptoms may well have had nothing to do with China’s air pollution — it had all been an infection, the sort one likely could contract anywhere in the world.

A stunning turn of events led to this discovery. I actually had been feeling much better after a few weeks with my inhalers and steroids, but mid-February I started again to get wheezy, along with very strange and seemingly disconnected symptoms such as muscle aches and frequent headaches. Then the night aches came back, and on Chinese New Years Eve I woke up gasping for breath yet again, this time with fever and headache. So instead of preparing dumplings and watching the annual TV gala, my family spent much of the night with me in my hospital’s emergency room. There I was diagnosed with an atypical pneumonia and started on antibiotics. Seven days later, all of my symptoms were gone — including the symptoms of asthma. I haven’t touched an inhaler since then.

Antibiotics kill bacteria. So as this medicine completely cured not only my pneumonia but also my supposed asthma, it’s apparent now that I had been walking around for months with a bacterial infection in my lungs, causing all of my symptoms from the chest pains at night all the way up to the more traditional pneumonia symptoms at the end — including my wheezing and asthma.

Looking back, it certainly wasn’t an illogical assumption for me and my colleagues initially to blame air pollution, as my initial symptoms had none of the typical features of a pneumonia infection. And the evidence is quite strong that air pollution can worsen asthma — but there’s actually less clear proof that it can cause new asthma in an otherwise healthy person like myself. Yes, many studies do show an increase in hospital admissions for pneumonia during pollution spikes, so perhaps from this indirect pathway, air pollution was still partly to blame for my illness — although last winter’s air pollution was in fact much better than previous winters.


As I now reflect on those rough months, I’m disturbed how I was far too ready to play the popular “blame China” game. It’s such an ingrained reflex for Beijingers, both foreign and local, to complain about our many environmental troubles. Scandalous stories are so common that we’re hard to shock and easy to believe the worst. So of course, it seemed totally natural to me, my colleagues and my friends to think that air pollution caused my suspected asthma. But we were wrong.

My unpublished article thus has transformed both in tone and intent. No longer a simplistic screed, it’s become a more nuanced debate on environmental risks versus epigenetic predestination. But more importantly, it has become — at least for me — a cautionary tale about a person’s unpredictable reactions to pain and illness and the vulnerabilities it exposes. During my most serious illnesses ever, I was anxious and needy, retreating into a shell of survival. I was desperately searching to find some meaning, some positive outcome to my unexpected sickness. Looking back, I am a bit disappointed in myself, for reacting so negatively to what was honestly a not-so-serious diagnosis, especially in comparison to so much of the suffering I see in my own patients in clinic. I found that my emotional reserves in the face of illness weren’t as deep as I had hoped.

But from this humbling, grounding experience I have found more than a few positive sprouts, and thus the entire ordeal has proven to be an unexpected blessing. I now have a deeper compassion for others with illness, and I understand how a person’s perception of their illness is perhaps even more important for a doctor to “heal” than the actual illness. I’m more aware than ever of the deep connections between mind and body, between physical and mental health, both intertwined and inseparable.

I also I never want to be so unprepared again for pain and illness, and I continue to reflect how I can improve whatever inner strengths I may need in reserve, even on a spiritual level. As The New York Times columnist David Brooks says in his new book, “The Road to Character”, suffering “drags you deeper into yourself.” And as I now again revel in the pure joy of my wife and playing along with our two miraculous sons, I am filled with gratitude at everyone’s good health, now knowing how fleeting that can last.


This article was edited and translated by Jonathan Ansfield and Ke Xu originally for the New York Times Chinese edition, published there in my health column at

Indoor PM2.5 Under 10: A Noble Goal


window sealing PM2.5 pollution
Window sealing

I’ve recently become quite smugly satisfied that my home’s indoor air is always 80% better than the outdoor air, thanks to a quartet of air purifiers working 24/7. But I no longer think my 80% reduction is good enough, and I now have a much more ambitious goal — to keep my home’s PM2.5 concentration under 10-12 µg/m3, all the time — even when the pollution is crazy bad. This target of 10-12 µg/m3 (equivalent to an AQI of 42-50, using the US EPA AQI conversion) may very well be a tilting-at-windmills fantasy, but that is now my goal — backed up by science.

I mention this because my home’s environmental testing team has an indoor target of 10 µg/m3 which is the lowest I’ve heard. Before this, I was more familiar with an indoor air target of 35 µg/m3 (AQI of 100), which is what many testing agencies and air purifier vendors are advising. This 35 may be a fine goal for many, as long as you as an informed consumer realize that chronic exposure to 35 µg/m3 of PM2.5  still leads to long term health problems and is a compromise between economics and health, while under 10 truly is the number where health effects are approaching zero. Perhaps even more importantly, under 10 also is the official recommendation from the World Health Organization. Given all this uncertainty about ideal targets, I thought I’d try to walk my readers through the evidence, and you can come to your own conclusions as to which target you’d like to achieve.

Dirty pre-filter…

First, there actually is almost no such thing in the real world as a safe level of air pollution. Even with an extraordinarily low PM2.5 under 7 µg/m3 (AQI 30), the data shows an uptick in deaths, cancers and heart disease. As the WHO states in their 2005 WHO Air Quality Guidelines Global Update:

The risk for various outcomes has been shown to increase with exposure and there is little evidence to suggest a threshold below which no adverse health effects would be anticipated. In fact, the low end of the range of concentrations at which adverse health effects has been demonstrated is not greatly above the background concentration, which for particles smaller than 2.5 μm (PM2.5) has been estimated to be 3–5 μg/m3 in both the United States and western Europe.

The WHO updated this guideline in 2013, and with eight more years of research they are even stronger in their assertions:

Thresholds: For short-term exposure studies, there is substantial evidence on associations observed down to very low levels of PM2.5. The data clearly suggest the absence of a threshold below which no one would be affected. Likewise long-term studies give no evidence of a threshold. Some recent studies have reported effects on mortality at concentrations below an annual average of 10 µg/m3.

The WHO Guidelines for Indoor Air Quality (page 4) explain why their indoor air and outdoor air recommendations are the same:

The steering group assisting WHO in designing the indoor air quality guidelines concluded that there is no convincing evidence of a difference in the hazardous nature of particulate matter from indoor sources as compared with those from outdoors and that the indoor levels of PM10 and PM2.5, in the presence of indoor sources of PM, are usually higher than the outdoor PM levels. Therefore, the air quality guidelines for particulate matter recommended by the 2005 global update are also applicable to indoor spaces

Those italics are mine because this is very important for people to realize: your indoor air goal is the same as the outdoor air goal — and again, that means getting your PM2.5 under 10 µg/m3.

Much of the WHO’s research is based on a couple of famous, very large cohort studies involving hundreds of thousands of people, including the Harvard Six Cities Study and the American Cancer Society Cancer Prevention II Study. These studies show clear increases in death rates from all causes, as well as from heart disease and lung cancers, as air pollution rises. (It’s important to note that all of the data points in these studies, from dozens of cities, had a PM2.5 range from 10 to a maximum of 30 — far lower than most cities in developing countries across Asia now.) All make it very clear that after ~7 ug/m3, the health effects increase. Here’s the graph from the ACS Study:

Figure 2. Nonparametric Smoothed Exposure Response Relationship

Below is another graph from another famous article published in the New England Journal of Medicine in 2009, showing how life expectancy in US cities from 1997-2001 decreased with PM2.5 levels above 5 ug/m3:

Life Expectancies in 51 Metropolitan Areas in the US, 1997–2001
Life Expectancies in 51 Metropolitan Areas in the US, 1997–2001

Because of this and other data, the WHO’s Global Burden of Disease research uses a PM2.5 annual concentration of 7.5 µg/m3 as their counterfactual — the “control” number which would assume to have no health effects. All of their relative risk assessments, including their most recent reanalysis of household air pollution, use 7.5 µg/m3 as the ideal baseline — so why shouldn’t it be our personal goal as well?

Some may still argue that 35 µg/m3 is still the more reasonable goal, as even the WHO officially states that developing countries such as China could use looser guidelines, called Interim Targets. Interim Target-1 states an annual PM2.5 of 35 µg/m3 as the target for annual exposure. Also, this 35 is currently China’s target goal for urban areas (15 for rural areas). And getting under 35 is actually a significant achievement in places such as Beijing, with annual PM2.5 last year of 89.5 µg/m3. But as the WHO states in their Table 1 (below), a level of 35 is “associated with about a 15% higher long-term mortality risk relative to the AQG level” — which again is 10 µg/m3.

WHO air quality guidelines for annual PM2.5
WHO air quality guidelines for annual PM2.5

The data seems clear to me, and yet here we are in the trenches, still with many differences of opinion. I’m convinced of the science and also have no intentions of waiting years for stronger data and a more unified opinion. Besides, it’s just common sense, isn’t it? Lower is better. For the sake of my wife and new son, I want my home’s indoor PM2.5 under 10 — always. If I can get there, I can literally breathe easier.