A lot of people get this wrong: the answer is winter is worst. People look at those endless hazy skies of ozone smog in mid-August and usually think that is the worst season. But actually, the deep summer is the cleanest time of year in regards to the deadlier pollutants like particulates (PM2.5 and PM10), sulfur, nitrates and carbon monoxides. Why is winter worst? It’s mostly from all the coal burning, both residential and commercial, to keep us warm. Here is the actual monthly data from 2006, published in a United Nations Environmental Protection paper just before the Olympics:

As you can see, the first graph shows that particulate matter (PM10) peaks in the winter and especially the spring, probably due to the sand storms. Most research suggests that particulate matter does the most health damage, which is why PM is the major number that most countries use as the #1 marker of environmental health.

The other two graphs above, showing sulfur (SO2) and nitrogen compounds (NO2), also peak in November-January, and both are lowest in July/August. Again, this is most likely from the coal-burning power plants and hutong houses trying to stay warm.

Here’s another graph, discussing another common toxin, carbon monoxide:

Interesting data, yes? You can see a lot more tables, graphs and photos below, from my online slide show presentation on air pollution. You can also view this full-screen; click on the “menu” button on the slide’s lower left corner.