Addendum October 2015: please also read a special followup article here, as the below article’s conclusion is far more simplistic than the actual mortality data shows.
One of life’s great mysteries is finally answered: “living in polluted City XYZ is equivalent to smoking how many cigarettes a day?” OK, it’s not on everyone’s top ten, but I’ve been asked that question many times by patients and by the media — and now I know what to tell them: a day in Beijing is like smoking one sixth of a cigarette. More specifically, on an average day in Beijing an average adult inhales a total of 1.8mg of PM2.5 particles from air pollution, which is 1/6 of the average 12mg of PM2.5 particles inhaled from an average cigarette. Yes, that’s a very strange number, but if I’ve done the math correctly, it is indeed true. And if it is true…well, it’s surprisingly low, isn’t it? First, let’s walk through the steps so we can all agree on the facts.
Step 1: Read Two Crucial Papers
The last two months have been amazing for anyone interested in public health in China. There has literally been an explosion of air pollution information in all forms of media, not only promoting the US Embassy’s PM2.5 monitor but actively discussing and criticizing many official air pollution stats. In other words, regular Chinese people have finally discovered what many expats in Beijing have already known for three years; the official PM10 daily pollution numbers were not nearly as helpful or as realistic as the US Embassy’s PM2.5 hourly feed.
This wonderful data explosion has culminated with two extraordinarily useful and detailed articles from two of my heroes of public health: Steven Andrews and Doctor C. Arden Pope III. Andrews wrote a dramatic research paper in 2008 discussing official pollution data in China, and last week on Chinadialogue he posted what is easily the most researched analysis to date comparing the US Embassy’s data with official data. It’s a must read.
The second is an even more scholarly article from Dr Pope, who has authored or co-authored most of the most important research on air pollution over the last 20 years. In his latest spectacularly useful research from Environmental Health Perspectives, he analyzes death rates from heart disease and lung cancers caused by smoking and air pollution. The detail is stunningly useful, and it also provided me with the data I needed (finally!) to actually measure mortality risk of air pollution, as well as to answer my original question above.
Step 2: Do The Math
First, just what is an average day in Beijing? We get rightfully scared about these occasional 500’s, but those are rare peaks. From Andrew’s paper, we get the data that the official average PM10 level from 2010 is 121 µg/m³, which converts to an AQI of 84, which is “Moderate” on the US EPA’s guideline. Andrew also did us all an enormous benefit and calculated that the US Embassy’s average daily PM2.5 over the last two years is 100 µg/m³ (which converts to an AQI of 171, “Unhealthy” for the US EPA).
Now we know the daily data. So how much of these particles do we breathe in a day? First, we need to know how much air an average person breathes. This can vary in the research literature, but our esteemed Dr Pope mentions that most studies use an average adult inhalation of 18m³/day of air. Now we can do our calculations. If we use:
- PM10 data: 121 µg/m³ x 18m³/day (÷1,000 to convert µg to mg) = 2.2mg/day of inhaled PM10
- PM2.5 data: 100 µg/m³ x 18m³/day (÷1,000 to convert µg to mg) = 1.8mg/day of inhaled PM2.5
Since Pope’s studies mostly use PM2.5 data for comparisons, we will focus mostly on PM2.5 and not the “official” PM10. As you can see, it’s not a stretch to use the US Embassy’s “unofficial” data, as 2.2mg and 1.8mg are relatively close numbers, in my opinion. Also, since it’s estimated that Beijing’s PM2.5 concentration is roughly 85% of PM10, then the numbers mostly are equal.
Step 3: Compare To A Cigarette
Pope also uses the research standard that an average cigarette contains 12mg of inhaled PM2.5. Yes, there are differences in smoking habits (some of us even claim “I didn’t inhale”…) but this seems to be the standard in most research literature.
So if we compare the above numbers:
- PM2.5 data: 1.8 mg/d air pollution ÷ 12 mg cigarette = 15% of one cigarette
That means that a daily dose of Beijing’s average air pollution is equivalent to 1/6 of one cigarette. Do we agree? If so, let’s continue analyzing this shocking finding.
What About The Really Bad Days?
So what about the really bad days when the US Embassy’s 24-hour data hits “Very Unhealthy” 200; “Hazardous” 300 or “Crazy Bad/Beyond Index” 500? Let’s find out. First, you must convert the AQI to PM2.5 concentration, which is easy to do with the online calculator:
- AQI200 = 150µg/m³ x 18m³/day (÷1,000) = 2.7mg/d ÷12 = 23% of one cigarette a day
- AQI300 = 250µg/m³ x 18m³/day (÷1,000) = 4.5mg/d ÷12 = 38% of one cigarette a day
- AQI500 = 500µg/m³ x 18m³/day (÷1,000) = 9.0mg/d ÷12 = 75% of one cigarette a day
This means that even with an extremely rare average of 500, that’s still equivalent to smoking less than one cigarette a day!
What About Your City?
What about Shanghai, or Mexico City, or LA? Just find out the average daily PM2.5 concentration or AQI and plug in the numbers! For example:
- City X: PM2.5 concentration = __ µg/m³ x 18m³/day (÷1,000) = 0.27mg/d ÷12 = __ % of one cigarette a day
So What Does This Mean?
Honestly, I was quite surprised by these numbers (assuming I did the math correctly). I had grown up hearing apocalyptic quotes such as “living in Mexico City is like smoking a pack a day”. Well, that just seems preposterous right now, in any city. According to Pope, a pack a day equals 240mg of PM2.5, which would be equivalent to a daily PM2.5 concentration of 13,333µg/m³, which is essentially impossible! So how on earth did such misleading urban legends start? And how can no scientist or reporter have answered this common question when these stats have been in Pope’s research papers for years?
My next reaction, as I’m sure many of you feel right now, was this: does this mean that air pollution isn’t as serious as we thought? I say no, it doesn’t mean that, but this data certainly puts things in a new perspective.
My Bottom Line
Once the shock wore off, a few major points lingered with me:
- Yes, this number is a lot lower than I would have assumed. But this absolutely does not mean that air pollution is now less dangerous. Hundreds of studies, many by Dr Pope, have documented the very real health risks of air pollution. And many studies have documented improved community health conditions after their air was cleaned up.
- But my #1 take-home message is that smoking is an astonishingly toxic and destructive addiction, and is far, far worse than the worst air pollution in the world. Even one cigarette a day is more toxic than a day in any city in the world! And China, with limited public health budgets, would get magnitudes more national health benefit if they focused on eliminating tobacco use when compared to eliminating high air pollution.
There is, of course, a much more important question than the parlor trick above: how much does air pollution increase mortality risk? Find out here in my follow-up article. Here’s a sneak preview of the crucial data from Pope’s article, discussing mortality risks of lung cancer (on the left) and heart disease (on the right) from PM2.5:
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