Simply Statistics A statistics blog by Rafa Irizarry, Roger Peng, and Jeff Leek

Welcome to the Smog-ocalypse

Beijing fog, 2013

Recent reports of air pollution levels out of Beijing are very very disturbing. Levels of fine particulate matter (PM2.5, or PM less than 2.5 microns in diameter) have reached unprecedented levels. So high are the levels that even the official media are allowed to mention it.

Here is a photograph of downtown Beijing during the day (Thanks to Sarah E. Burton for the photograph). Hourly levels of PM2.5 hit over 900 micrograms per cubic meter in some parts of the city and 24-hour average levels (the basis for most air quality standards) reached over 500 micrograms per cubic meter. Just for reference, the US national ambient air quality standard for the 24-hour average level of PM2.5 is 35 micrograms per cubic meter.

Below is a plot of the PM2.5 data taken from the US Embassy’s rooftop monitor.


The solid circles indicate the 24-hour average for the day. The red line is the median of the daily averages for the time period in the plot (about 6 weeks) and the dotted blue line is the US 24-hour national ambient air quality standard. The median for the period was about 91 micrograms per cubic meter.

First, it should be noted that a “typical” day of 91 micrograms per cubic meter is still crazy. But suppose we take 91 to be a typical day. Then in a city like Beijing, which has about 20 million people, if we assume that about 700 people die on a typical day, then the last 5 days alone would experience about 307 excess deaths from all causes. I get this from using a rough estimate of a 0.3% increase in all-cause mortality per 10 microgram per cubic meter increase in PM2.5 levels (studies from China and the US tend to report risks in roughly this area). The 700 deaths per day number is a fairly back-of-the-envelope number that I got simply using comparisons to other major cities.  Numbers for things like excess hospitalizations will be higher because both the risks and the baselines are higher. For example, in the US, we estimate about a 1.28% increase in heart failure hospitalization for a 10 microgram per cubic meter increase in PM2.5.

If you like, you can also translate current levels to numbers of cigarettes smoked. If you assume a typical adult inhales about 18-20 cubic meters of air per day, then in the last 5 days, the average Beijinger smoked about 3 cigarettes just by getting out of bed in the morning.

Lastly, I want to point to a nice series of photos that the Guardian has collected on the (in)famous London Fog of 1952. Although the levels were quite a bit worse back then (about 2-3 times worse, if you can believe it), the photos bear a striking resemblance to today’s Beijing.

At least in the US, the infamous smog episodes that occurred regularly only 60 years ago are pretty much non-existent. But in many places around the world, “crazy bad” air pollution is part of everyday life.