Category Archives: Air Pollution

Air Pollution

Air pollution is the #1 topic on this website. Here is the entire collection of articles on this always-hot topic.

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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.

My Quest For The Ideal Air Purifier Is Over.

Update January 2016: My data below speaks for itself, but people should be aware of possible quality problems reported here.

I think I’m done testing air purifiers. Yes, I’ve said this before, and I admit I often enjoy crunching the numbers, but I think I mean it now. The only possible way I could be persuaded to test another model would be an amazing breakthrough in technology. But it would still have to be reasonably affordable, even under 1,000 RMB per machine. Crazy, no? Actually, it’s not, because I sold all my IQAirs and my Blueair and switched to six air purifiers which cost me a total of 5,200 RMB. That’s right; a total of 5,200 RMB, for six new machines. I hardly doubt I need to remind most of you that’s half the price of only one of the many popular machines on the market now. And as this article will show, I’ve proven that these new machines are keeping my family’s indoor PM2.5 under 10 ug/m3, which is the goal of the World Health Organization and thus should be everyone’s goal (read more about that here). So why would I even want to test anything else?

As to which machine, many readers probably already know my answer because I started to test them last year: the Xiaomi 小米空气净化器 at 899 RMB, covering 48 square meters (apparently there’s a newer, smaller model as well for only 699). I initially blogged about Xiaomi last year discussing my test results of indoor air purifiers under 1,000 RMB. After last year’s testing, I had replaced two imported models in bedrooms with the Xiaomi purifier, and my bedroom air has been just fine since the switch. Just a few weeks ago I made the really big leap, returning two large air purifiers in my front rooms with three Xiaomi purifiers (I needed one extra as the coverage was comparatively less). And for the record, I am getting no promotional money or free anything from Xiaomi.

In our nine years in China we’ve always used well known imported air purifiers to cover our front rooms, which are essentially one large open space covering 83 square meters. But the price gouging in China always galled me. So after last year’s testing of purifiers under 1,000 RMB, I was always thinking, why can’t I see if I can also replace these big machines in the front rooms with something of more value yet equally effective? The timing was perfect as we’ve had multiple December days way over 200, 300 — and the worst two days of pollution I’ve ever seen in Beijing in my nine years here, our 2015 Airpocalypse with readings over 600 and an eerily dark, yellow sky in the afternoon of December 1st.

Let’s jump right in to the only thing that matters: hard data. And maybe the most important test was Airpocalypse 2015. So here’s the bottom line: data from our Laser Egg PM2.5 monitor showed an outdoor average PM2.5 concentration of 465 ug/m3 over those couple days, and inside the front rooms only 25 ug/m3. That’s an astonishing 95% steady improvement over the worst pollution you’ll pretty much ever see here, and although it’s not at my ideal of 10 ug/m3, it’s still far below the 35 that most Chinese environmental teams shoot for anyway, and the 95% reduction is certainly way below the usually recommended 80% goal. And the machine’s weren’t even on the top, super-noisy speed but the middle speed, which is still fine for general conversation and TV watching. Here’s the graph from the Laser Egg app, with the red line showing outdoor PM2.5 and the blue line showing my living room PM2.5 (the left side axis is PM2.5 concentration in ug/m3 and the bottom axis is just the number of data entry points):

Red line: outdoor PM2.5 concentration (ug/m3). Blue line: indoor PM2.5 (Laser Egg PM2.5 monitor)
Red line: outdoor PM2.5 concentration, US Embassy (ug/m3). Blue line: indoor PM2.5 (Laser Egg PM2.5 monitor)

On more “normal” days the data was much closer to, and usually under, my aggressive goal of PM2.5 concentration under 10 ug/m3. Here’s the data from earlier November:

Red line: outdoor PM2.5 concentration, Beijing average (ug/m3). Blue line: indoor PM2.5 (Laser Egg PM2.5 monitor)

Even a quick glance at the graph above tells me everything I need to know: my living room air (the blue line) is generally exactly around 10, and always under 20 even as the outdoor air climbs up. And as Beijing’s annual average is 70-90 ug/m3, my three Xiaomi machines easily get me to my goal of 10 during those typical days — and at a nice auto setting with very comfortable sound level. For calculated averages over this week using the Excel spreadsheet data from the Laser Egg, my front room average was 6.1 ug/m3 when outdoor air was a relatively clean 61; 9.8 during heavier days averaging 136 outside; and a still respectful 14.4 when outdoor air tanked for a couple days at 220 (that’s an impressive 93% reduction).

You notice there are a few spikes on the blue line, but all of those are easily explainable as with any other air purifier I’ve had: either the windows were open while we were using our laundry dryer, or we were cooking, or the machines were accidentally off for a while and no one had noticed.

Here’s one more graph from my bedroom, again demonstrating even more clearly than last year’s test that a Xiaomi on quiet setting is perfectly effective, and easily just as effective as any machine I’ve ever tested, including all the far more expensive models. The calculated average PM2.5 concentration was 8.0 ug/m3, again nicely below my goal of 10. It was 8.6 ug/m3 when the outdoor air was a horrible 224 over two days; and 7.4 ug/m3 on two clearer days, when the outdoor air average was 69:

Bedroom data
Bedroom data, 4 nights (red line: outdoor PM2.5 concentration. Blue line: bedroom data)

So let me be perfectly blunt to those in China who say that an indoor PM2.5 of 10 ug/m3 is impossible and too expensive: I say that’s nonsense, especially now, certainly on the money side. A goal of 35 ug/m3 still gives everyone a 15% increased mortality risk, as the WHO specifically mentions in their Air Quality Guidelines. I do realize this is a sensitive concept for some readers, as many people have spent a lot of money on air purifiers, or even work for those companies. And it’s fair to point out that these Xiaomi replacement filters don’t last as long as others (three months, although they’re only 179 RMB), and their HEPA filter at 99.3% (rated H11) is not as efficient as most other higher-end machines’ filters, and maybe the machine’s solidity isn’t so great, and perhaps they may break down sooner than other machines. We simply don’t know yet, as these are new machines only a year on the market — which in itself should give people some pause. Also, the built-in sensor is quite inaccurate and pretty much useless to follow — but I’ve never found sensors ever reliable on any air purifier, and I always control all of them manually anyway (and none of the major brand machines ever had a sensor anyway). The sensor’s more concerning issue is that it thinks that a PM2.5 concentration of 75 ug/m3 and under is safe. That’s simply too high, and even the Chinese government rating of 35 would be more proper. I of course think 10 is best, but for a machine made in China I would think 35 is reasonable. So if people are using a Xiaomi and relying only on the sensor on auto speed, then indeed they still may be inhaling not great air. This again is why everyone, using any type of air purifier, should not be relying only on any machine’s auto setting ever (my personal opinion). You need to get a separate monitor and use that to test your air.) And yes, Xiaomi is mostly a cell phone company first.

But let’s be very clear here: I’ve mentioned many, many times that anyone living anywhere in polluted areas, whether China or India or Los Angeles, has an ultimate goal to get your indoor air pollution under the World Health Organization’s goal of PM2.5 under 10 ug/m3. The scientific data is very clear; any PM2.5 over 10 starts to have health effects. So people in polluted areas absolutely must have air purifiers indoors, where you spend 90% of your lives. At the very least, put one in your bedroom. And for heaven’s sake, make sure your children’s rooms are all protected.

So you’re more than welcome to get any air purifier machine you want, as long as you’re reaching your goal of PM2.5 under 10 ug/m3, and routinely monitoring your air afterwards (perhaps with the new Laser Egg, only 499 RMB) to make sure all is working fine. In terms of which air purifier is best, again I’ve mentioned often that all that anyone needs for indoor protection is a good filter attached to a good fan. I’ve tested many models over many years, in real world circumstances in my homes, and no particular model at any price point ever was so obviously better than the rest. This is why I am perfectly comfortable with my current setup.

And this is why I am done with testing. Probably…

Checking Your Indoor PM2.5, Cheaply, With The Laser Egg

UPDATE January 4, 2016: Here’s a great review article about PM2.5 monitors: click here; they praise the Laser Egg but also discuss its limitations.
laser-eggI’ve mentioned often that I feel it’s important for people who use air purifiers to make sure your investments are working well. For me, this means always keeping your indoor air PM2.5 concentration under 10 ug/m3 (read my explanation here). The only way to know this is to have a PM2.5 monitor to check your air. We can spend so much money on air purifier machines — but how are we sure they’re on the correct speeds, or in the best part of the room, or that the filters aren’t clogged and need to be replaced early? And how do you know if you need to increase their speed when the AQI is airpocalyptic?

You certainly can’t rely on the built-in PM2.5 sensors that many air purifiers have, as the vast majority I’ve seen are inaccurate and basically useless. No, the answer has always been simple — and complicated. You’ve always needed to buy a separate PM2.5 monitor, but the best ones are wildly expensive. For a few years I’ve been using a popular, more economical choice from America called Dylos, but their data was uncommonly difficult to download and convert to something we layman could understand.

Now, finally, we’re starting to see some reasonable options, and I’m thinking specifically of the new Laser Egg, an air quality monitor from Beijing-based environmental company Origins, set up by an expat couple. Many of you are already using this Laser Egg, which is available for a wonderfully reasonable price of 499 RMB. I’ve been testing a few of these Eggs for a few weeks, and I can finally say that consumers now have a real option for testing your air at home (or work, or schools, etc etc).

laser-egg-appI have some highlights about this Laser Egg:

  1. The Egg already has saved me tens of thousands of RMB in air purifier costs, as I’ve recently replaced my home’s air purifiers with a far more economical machine (the Xiaomi), and the Laser Egg data proves that these new machines are very effective. (I will blog about this soon).
  2. Their app (Breathing Space) is very useful, as you can sync all of your Eggs to your home wifi, and you can access their data anytime, anywhere in the world.
  3. The app also shows the local outdoor air, so you can immediately compare indoor versus outdoor air. Very useful.
  4. You can choose to monitor the AQI from China or from the USA, or even better, the raw concentration of ug/m3. I always use concentration because I can quickly glance and see if it’s under 10 or not. Plus it’s the most evidenced based, as 10 ug/m3 is the official recommendation by the World Health Organization, and then you don’t have to deal with the politics of AQIs, which are totally different in every country.
  5. You can also use the app to export the data to your email, and create snazzy Excel graphs like the one below. For example, the Egg’s weekly data spreadsheet has 5-minute interval data on PM2.5, PM10, humidity and temperature. Data geeks like myself will love it.
  6. Its battery charges by USB so you can unplug and stuff in your bag and walk around town with it, checking out your favorite stores and also the outdoor air anywhere.
  7. You can keep it on 24/7, in sleep mode, and always access the data via your app or the screen.
  8. The app also can send you instant messages warning you if your air is suddenly getting worse. For example, I was at work and got a cell phone pop-up warning that my front room air was worsening, which I confirmed on the app’s graphics. I called my wife at home and she realized that all our Xiaomi purifiers had been shut off after a power outage. I turned them all back on (using another app from my air purifier), and sitting in my office I watched the app to see my home’s PM2.5 improving minute by minute. How cool is that?
  9. You can also rig it to check pollution masks, sort of. See the video I made below, using the Laser Egg to test an incredible new pollution mask I’m helping to design, called Freeair. It’s a battery-powered outdoor pollution mask that delivers pure air via a tiny fan and an astonishing new filter, classified ULPA (Ultra-Low Particulate Air), far superior to N95, N99 or HEPA filters. It filters basically 100% of PM2.5 and also filters much smaller ultrafine particles by 99.99999%. As you see in the video, literally every molecule of pollution is filtered out. You’ll definitely be hearing more about Freeair soon.
  10. You can quickly check for leaks around doors and windows. You all have already sealed your windows and doors with the very inexpensive rubber strips, yes? If not, read this and then come back. Anyway, one horrible night with AQI over 400 I noticed from the Laser Egg app that my baby’s bedroom PM2.5 was strangely high. And sure enough, the rubber sealing was coming off a part of a window. I resealed it, and voila! Happy baby.
  11. The Laser Egg looks pretty cool, actually.

So how reliable is the Laser Egg, and is the price too good to be true? For accuracy, you can check out the company’s own comparison testing with the US Embassy’s monitor, but perhaps you’d rather see more independent tests such as this reviewer’s comparative testing with much more expensive sensors. It’s in Chinese but just from his photos you see they’re generally the same. I’ve discussed the Egg with some pollution experts in Beijing and they’re generally supportive but mention that humidity can mess with the numbers a bit, and also putting Eggs side to side will show slightly different numbers. But in general, on a consumer level, it certainly does what it says it does.

I personally think the Egg could be improved, but it’s accurate enough for me and very impressive for a first version. And right now I definitely have stopped using the Dylos and will only be using the Egg from now on. If you’re interested in the Egg, you can buy on their website here or also via their Wechat directly (ID: originstech). I’ve also seen it behind the cash registers at April Gourmet and Jenny Lou’s. Think Christmas stocking stuffer…

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.

Move Over, PM2.5; Ozone is the New Black

ozone FAQ chinaJust when I was getting hopeful about the dramatic improvement this year in Beijing’s PM2.5 levels, along comes a summer deluge of local press coverage about what they’re claiming is air pollution’s number two threat: ground-level ozone (臭氧). Recent press reports and a campaign by the China National Environmental Monitoring Center have highlighted the fact that on some hot, sunny days this summer, the worst pollutant has been ozone and not the usual PM2.5. But is ozone really as much a threat to our health as the well-documented PM2.5? And if so, do our usual masks and indoor air purifiers help in any way?

Most of us know ozone when discussing global warming, as the thinning ozone layer in our upper atmosphere protects us here on the ground. But at ground level, ozone is quite destructive to our lungs, causing both immediate and long-term harm as well as increased death rates. Ozone levels are always highest during sunny summer afternoons, as the sun’s ultraviolet light creates ozone from airborne chemicals, especially from vehicle exhaust — most notoriously from old diesel trucks spewing out their toxic cocktail of fumes. The World Health Organization’s Air Quality Guidelines from 2005 recommend safest ozone levels under 100 ug/m3 (using 8-hour exposure limits), while over 160 ug/m3 would be considered unhealthy for vulnerable people such as children and people with lung disease, and over 240 ug/m3 especially unhealthy to all people. China’s MEP guidelines follow the WHO’s interim target of 160 ug/m3, while the US EPA’s AQI “green zone” is under 120 ug/m3. During this week’s heat wave in Beijing, afternoon ozone regularly peaked over all three thresholds, commonly over 200 ug/m3 and peaking at 299 ug/m3. The Los Angeles area, most polluted in the USA, still regularly has ozone days over 200 ug/m3 but is far better than a few decades ago when levels frequently hit over 400 ug/m3. So while ozone levels in Beijing aren’t comparatively as high as PM2.5 is, certainly they are frequently at levels that would be considered unhealthy.

In terms of symptoms, ozone seems to cause much more immediate effects than PM2.5 does. You may have noticed while outside during this hot summer that your eyes sting, your head pounds, your throat burns, you cough and feel a bit short of breath. I’ve certainly had such symptoms this summer during some of my bike rides to my clinic, especially returning home early evening, which is usually when ozone peaks. I’m fortunate that I haven’t had more serious symptoms such as severe asthma flares, but I definitely worry about children’s exposure to ozone, especially those with asthma who live near busy roads in sunny places. Healthy children are also at risk; one sobering study in 2002 followed a group of healthy students around smoggy southern California, and those who spent the most hours outside playing sports had a three-fold increased risk of developing asthma from ozone exposure, compared to those who mostly stayed indoors.

But which pollutant is truly more serious, PM2.5 or ozone? Air pollution action plans for schools (such as the one below) across the USA rank PM2.5 and ozone as equally dangerous pollutants for children, and an AQI over 200 for either PM2.5 or ozone (equivalent to 8-hour ozone of 115 ppb, or 225 ug/m3) would warrant all children staying indoors; for those with asthma, a stricter cutoff AQI of 100-150 could be even more protective:

AQI School Guide San Joaquin Valley RAAN PM2.5 Ozone
AQI School Guide — from San Joaquin Valley and other California Districts

But does any of this mean we really should change our behavior here in Beijing, any differently than we already act right now with PM2.5? I’m honestly not too convinced; while the WHO Guidelines and a more recent US EPA review do mention data showing increased mortality from higher ozone, and quite strong connections with lung disease, the long-term data just isn’t as impressive to me as it is for PM2.5. For example, PM2.5 clearly causes cancers, and is officially on the WHO’s list of carcinogens, while ozone’s data shows no strong correlation to cancers. Also, PM2.5’s direct damage to the heart is extremely well documented, while data for ozone is far less certain of a connection.

But while ozone’s long-term risks may be less than PM2.5, short-term symptoms seem to affect all of us much more easily, especially right now during summer. Perhaps in this case any one of us who is at higher risk, or who commutes in high ozone areas — that includes anyone sitting on a bus, taxi or subway — could consider an anti-ozone mask, especially if you’ve already noticed symptoms.  Also, anyone who feels symptoms when ozone is high, even if you are young and healthy, should certainly consider using an anti-ozone mask.

But the main problem here is that most masks’ filter material against PM2.5 is quite impotent against the far smaller ozone molecules, reducing ozone by only one-third. The only way to effectively remove ozone via a mask is to add a layer of carbon to the mask. These specialized anti-ozone masks using carbon were initially designed especially for welders, not consumers. Welding is a particularly dangerous occupation as the heat can create dangerously high levels of ozone (over 1,200 parts per billion, more than 12 times the US OSHA standard), and welders have much higher rates of lung disease than the average population, including asthma. Based on this specialized technology, a few consumer-level carbon masks are available. Studies have shown that disposable masks with carbon (in this case, the 3M 9913, also 90% effective against PM2.5), if fitted properly, can eliminate up to 98% of ozone, which I feel is quite impressive. Even more importantly, lung function was stable with the carbon mask but decreased with the no-carbon mask. After 40 hours of use, the carbon filter still worked perfectly, which means a person like myself could wear these masks for at least a couple of weeks of normal commuting activity, and simply throw them away once they get too dirty — or the rubber strap breaks, which usually happens first.

Indoor Air

Ozone inside a house usually is only 40-50% as much as outdoors, but since we all spend much more time indoors than out, it’s estimated that 25-60% of our total exposure to ozone comes from indoors. Fortunately, the answer to this problem is one you’re probably already taking care of: an indoor air purifier. You may have noticed that most of the decent indoor air purifiers already include a carbon filter (活性炭滤网) along with a true HEPA filter (HEPA过滤网) which eliminates PM2.5. Ozone indoors is still probably a less serious hazard to most of us than PM2.5, as well as from dangerous gases like benzene (甲苯) and formaldehyde (甲醛), which causes cancer. These and other volatile gases are frequently elevated in newly renovated Chinese apartments with poorly made furniture, walls and flooring. The carbon filters can work extremely well against ozone as well as all of these gases.

My Bottom Line

I recently had pretty bad asthma attacks, which are now resolved, but I’m still quite nervous about recurrence. So while I’m relieved that I haven’t had any asthma spells this summer, all this recent press about ozone has empowered me to finally get up to speed on ozone research. I’ve decided that the only life change I needed to make was to use a carbon mask (combined with N95) during bad ozone days when commuting to work. Now when I glance at my pollution app on my mobile phone, I will look at ozone concentration as well as PM2.5. Otherwise, I’m already safe at home as my indoor air purifiers already include both carbon and HEPA filters.

But I’m also more cautious this summer with my children and asthmatic patients in clinic, making sure they’re also more aware of ozone’s specific threats to them on these hot sunny days. As usual, living healthy in Beijing always requires a combination of common sense and education on public health issues. And with ozone, just like PM2.5, a bit of the same preventive strategies should keep most of us healthy.