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By Luke Sanford
In December 1952 London experienced a smog event that killed 4,000 people over 4 days, and arguably another 8,000 in the months that followed. In Early January 2014, Beijing experienced a smog event dubbed the “airpocalypse” that shut down major highways in the northeast of the country.
Are these two events comparable? Not really.
The WHO estimates that each year in China approximately 1 million people die per year of smoking-related illnesses. A study on air pollution educed mortality estimates that nearly 1 million people per year die prematurely in China as a result of air pollution.
Does this mean that living in China is just as bad as regularly smoking cigarettes? No.
A few days ago here in Chengdu, a city fabled to have the most beautiful fair-skinned ladies in all of China (probably because the sky is never clear enough to get a tan), the air pollution got so bad that I started searching Baidu for masks and in-home air filters. This got me thinking about the question: how many cigarettes am I smoking each day just living in this city? Finding the real answer turned out to be surprisingly difficult.
Part 1: The Haze
A search for the terms “air pollution” and “cigarettes smoked per day” returns a few results like these:
1) Living in Beijing is equivalent to smoking 21 cigarettes per day
2) A day in Oxford is like smoking 60 cigarettes (as of 2004); London between 25 and 30
3) Milan’s air is worth roughly 15 cigarettes per day
4) Houston’s air is worth a pack.
If your only experience with China’s air pollution has been through the pictures and videos in the western media, this probably seems perfectly plausible. Even for me it often feels that way: seeing the brown stuff accumulate on exposed surfaces, feeling my breathing become more labored, and the ever-present brown snot that makes me feel a little bit like I’m in the dust bowl.
While researching the potential hazards of living and breathing in China, I made the interesting discovery that, compared with other countries, air filtration machines are outrageously expensive in China. The price of a single replacement filter for a device certified at international standards is often more expensive than the whole device would be in the US or Europe. This is potentially the subject of a fascinating research paper—if you happen to write that paper, please let me know.
None of the articles cited above have any information detailing how their calculations were performed or whether their information came from a reliable source. Equally troubling is that, if taken at face value, these articles would give you the impression that Oxford’s air is three times as bad as Beijing’s, that Houston’s is just as bad, and that Milan’s is only marginally better. This all seems highly implausible, and definitely warrants a closer look.
Part 2: Clarity
A few weeks ago this article came out, adding to my conviction that people don’t really understand how to compare various forms of air pollution. It is a review of a recent paper about the effects of exposure to second-hand smoke, and the tagline that the media picked up was that living with a smoker is just as bad as living in a polluted city like “Beijing or London.” Two things here should stand out: first, that second-hand smoke (rather than smoking itself) was being compared to pollution; and second, that pollution in Beijing was being compared to pollution in London. While London has had its share of bad days (more on the great smog disaster later), its average pollution levels are slightly more than a tenth of those recorded in Beijing. Hardly comparable. I was now beginning to seriously doubt that the answer was as clear-cut as the “pack-a-day” articles make it out to be.
A little more poking around led me to a great website run by Richard Saint Cyr, an American doctor who lives in Beijing and has written several good articles about comparing pollution to smoking. Dr. Saint Cyr recently cut straight to the chase and contacted the foremost expert (Dr. Arden Pope III) on the health effects of air pollution and smoking, documenting the thought-provoking answers he received in an article here.
“On an average day in Beijing, the number of particulates you inhale are the equivalent of smoking only 1/6 of a cigarette.”
The surprising conclusion: even during Beijing’s “Airpocalypse,” the amount of pm2.5s that a person inhales in a day was about the equivalent of smoking a single cigarette.
I’ll take a moment here to explain a few details that many of the existing articles on this subject, scholarly or not, often take for granted. PM2.5’s are particulates that are less than 2.5 micrometers across (1/30th the width of a human hair), and are thought to be the most deadly component of air pollution. Their relative lethality comes from their tendency to lodge deep in the lungs, where many of them are absorbed directly into the bloodstream. This results not only an increased risk of lung cancer, but also elevated rates of heart disease and other conditions (some recent studies claim potential links with psychological disorders). Furthermore, it seems that most PM2.5s act in generally the same way, no matter what they are composed of. This means that even though the particles that you inhale from smoking are very different from the particles inhaled from the exhaust fumes of a diesel truck, their effects on your body are comparable.
On the most polluted days, going outside without any sort of protection is equivalent to smoking a single cigarette, and on an average day only amounts to 1/6th of a cigarette in Beijing. That’s a hugely different result than 21 cigarettes per day in Beijing cited above. Why?
Part 3: Filtration
Disclaimer: The math to get results that are scientifically reproducible is much harder than the math that I’ve done. That’s because, for your sake and mine, I’m just calculating rough point estimates, not the size of the error terms.
I’ll start with the simple calculation that is used to get the ≈1 cigarette number from the end of the last section. PM quantities are reported in concentrations of the unit μg/m3, or migrograms per cubic meter. At its worst, Beijing had PM2.5 concentrations of about 800 μg/m3. People breathe approximately 18m3 of air per 24 hour period (this varies between children, adults, and by activity, but 18 is the number most commonly used). If you multiply those numbers together, you get about 16.4 mg of PM2.5 (that’s milligrams, 1/1000 of a gram). The paper by Dr. Pope (who has multiple papers with well over 1,000 citations, for those of you keeping track) that is cited above records the amount of PM2.5s inhaled in the smoking of one cigarette as 12mg. Thus, just over one cigarette on the worst pollution days. This also assumes that people are outside all day, which is not true for most people. Rather, people spend the majority of their time inside where the PM2.5 concentrations are between 60 and 70% of the outdoor levels, and sometimes much lower or higher depending on if they keep windows open or have an in-house filtration system.
I extrapolated this result to compare some of the most polluted cities in the world in terms of number of cigarettes smoked:
|Location||Average annual PM2.5 Concentration (μg/m3)||PM2.5
|Xingtai (most polluted city in China)||191||3.438||0.287|
|Ahvaz, Iran (most polluted city in the world)||372||6.696||0.558|
|Beijing, Airpocalypse ||800||14.4||1.2|
|London, Smog Disaster||4,000-10,000||72-180||6-15|
This raises a few more important questions including:
1) If the London Smog Disaster was only worth 15 cigarettes per day, why did so many people die? The answer to this is a combination of two things. First, if you took the entire population of any city and forced everyone, including infants, sick people, and the elderly, to smoke 6-15 cigarettes per day for four straight days, that you would inevitably see some deaths. In fact, forcing people to smoke a certain number of cigarettes is a pretty good way to think of air pollution—basically as second-hand smoke for the entire population of a region. The second reason is that there were likely a variety of other suspended gasses and particles contained in the Smog Disaster’s pollution that had effects outside of those caused by PM2.5s alone.
2) Why does it look so terrible outside on high pollution days? Why does my snot turn brown? Most of the particles that make it hard to see and turn your mucus brown are substantially larger than the PM2.5s that cause the most harm. Particles of 10 micrometers contribute more to visible pollution because of their size, but are less easily absorbed into the body for the same reason.
3) Why is air pollution such a big deal if the worst places aren’t even worth 1 cigarette per day? The answer to this question is complicated and has to do with how pollution affects the body, how mortality rates are calculated, and even what the best ways to study the effects of pollution on populations are.
“It is very difficult to get an objective measure of how air pollution affects a society.”
Let me start with the example of second-hand smoke. It turns out that on average, the PM2.5 concentrations in a smoker’s house are approximately 30 μg/m3 than in a non-smoker’s house. The literature detailing the health effects of second hand smoke is extensive and universally accepted, despite the fact that people in those situations only inhale a tiny fraction of a cigarette’s worth of smoke per day. Air pollution is substantially worse than second hand smoke in nearly all of China.
That being said, it is very difficult to get an objective measure of how air pollution affects a society, because it’s impossible to accurately estimate what that society would be like without air pollution. For example, try guessing how many people would get sick or die each year from things like lung cancer or heart disease in the US if there was no air pollution. The problem is that it’s impossible to tell how many of those illnesses are caused by air pollution, and how many are caused by other things (like smoking, or unhealthy diets). A number of studies have tried to do this for different places, but the assumptions that they make about the baseline (no pollution) case have major effects on their predictions.
“The first few milligrams of particulates you are exposed to have a much greater impact on your health than those inhaled later.”
The second issue here has to do with how health risks are calculated. When researchers calculate the effects of pollution on the human body, they are looking for Relative Risk (RR) rates for different health conditions. These represent the increased risk of getting X if you are exposed to N amount of Y, or in our case, the increased risk of getting lung cancer, heart disease, or a few other conditions (our X’s) when one is exposed to an additional 10 μg/m3 (our N) of PM2.5s (our Y). For lung cancer, the RR is about 1.1, for heart disease the RR is 1.2, for cardiovascular disease the RR is 1.1, and for cardiopulmonary diseases the RR is 1.1:
|Approx. 1.5 Cigarettes||18||10.4||1.61||1.58||1.72|
This is where things get a little more complicated. If you were to live in an environment where you were exposed to 100 μg/m3 on a daily basis, you might expect that your RR of lung cancer would be 10 x 1.14, or 11.4. Actually, because each dose multiplies your risk by 1.14, what you would really be looking for is (1.14)10, or 3.71. However, you would only have been exposed to 1.8 mg of PM2.5, or just 1/10 of the amount you get from 1.5 cigarettes, but still over 1/3 of the risk. The issue here is that studies show that the RR function isn’t linear—in fact it is much steeper at lower levels of exposure than higher ones.Put differently, the first few milligrams of particulates you are exposed to have a much greater impact on your health than those inhaled later. Graphically, it looks like this:
This graphic is taken from the Pope et al 2011 paper, which conducted a comprehensive literature review of studies of smoking and air pollution. Of particular note is the zoom-box, where air pollution values are depicted with diamonds and second-hand smoke values are depicted with stars. It is unclear why air pollution seems to carry a higher RR at those levels (also evident in lung cancer RR, not shown here), perhaps a topic meriting further study.
The more PM2.5s you are exposed to, the smaller the impact that each successive dose has on your health. This explains why air pollution still has a very measurable effect, even though smoking exposes your body to many, many more PM2.5s.
The results…show an average 3 year decrease in lifespan per increase of 30 μg/m3
The final step is to go from RR, a fairly abstract concept, to life expectancy. To do so, you need a table that shows the mortality rates at every age without pollution. Using this table, it is possible to estimate how many additional people would die each year from each of the conditions caused by pollution, and then to estimate the effect on overall life expectancy. Fortunately, Dr. Pope has done this using the best studies that could be applied to China (paper available here). The average decrease in life expectancy for a moderate smoker is 7.8 years. The results of the China study show an average 3 year decrease in lifespan for an increase of 30 μg/m3 between the areas studied. Though this number is fraught with uncertainty, it is the best one available. Further uncertainties come from a lack of research on how many of the particulates are actually absorbed into the lungs and bloodstream, and whether the timing and continuity of the dose affects its health consequences.
In conclusion, we can definitively say that 1) Living in Beijing is not equivalent to smoking a pack of cigarettes per day. 2) Even though air pollution results in the inhalation of roughly an order of magnitude less particulate matter than smoking does, it’s the small(er) initial exposures that do the most damage. 3) Air pollution has a very significant effect on life expectancy, even at low levels (by Chinese standards).
- The US EPA’s standard for PM2.5 requires an annual average of less than 20 μg/m3 per year. Almost everywhere in the US meets this standard, with the main exceptions being dusty cities in the Californian central valley.
- Only one Chinese city has average PM2.5 levels below 20, and only 21/200 have average PM2.5 levels below 50.
- None of the top 10 most polluted cities in the world are in China.
- In addition to their negative health effects, fine particulates can have huge climate effects (depending on their color) through directly heating the atmosphere by absorbing sunlight, landing on snowy surfaces and reducing their reflective potential, or by inducing precipitation. These particles can have well above 500 times the global warming potential of CO2 by weight.
Photo by Steve Boland
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Pope, C. A., and D. W. Dockery. “Air Pollution and Life Expectancy in China and beyond.” Proceedings of the National Academy of Sciences 110, no. 32 (August 6, 2013): 12861–62. doi:10.1073/pnas.1310925110.
Pope, C. Arden, Richard T. Burnett, Michelle C. Turner, Aaron Cohen, Daniel Krewski, Michael Jerrett, Susan M. Gapstur, and Michael J. Thun. “Lung Cancer and Cardiovascular Disease Mortality Associated with Ambient Air Pollution and Cigarette Smoke: Shape of the Exposure–Response Relationships.” Environmental Health Perspectives 119, no. 11 (July 19, 2011): 1616–21. doi:10.1289/ehp.1103639.
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Semple, S., A. Apsley, T. Azmina Ibrahim, S. W. Turner, and J. W. Cherrie. “Fine Particulate Matter Concentrations in Smoking Households: Just How Much Secondhand Smoke Do You Breathe in If You Live with a Smoker Who Smokes Indoors?” Tobacco Control, October 20, 2014. doi:10.1136/tobaccocontrol-2014-051635.
 All Chinese air quality data is taken from numbers reported by the US embassy and consulates across the country, a measure implemented by former US ambassador Gary Locke. This was a very controversial move in China, but ended up garnering the support of much of the public. Air pollution data can be found here: http://www.stateair.net/web/post/1/1.html
 The PM2.5 levels are estimated from overall PM10 levels, not directly measured.