In an unfortunate intersection of trends, the rise of running in China has coincided with the country's air pollution disaster. Marathons are growing fast, with participation hitting record numbers that are outpaced only by air pollution levels — a hashtag that translates roughly to "I don't want to be a human vacuum cleaner" has become one of the most popular phrases on Sina Weibo, the Chinese version of Twitter.
A standard air quality index is the measure of inhalable particulates, specifically PM 2.5, or fine particulate matter with a diameter smaller than 2.5 microns; the WHO warns against daily levels exceeding 25 micrograms per cubic meter. The small size of these particles makes them the most hazardous, since they can pass through the lungs and into the circulatory system, causing cancer and cardiopulmonary disease. Runners breathe at 10 to 20 times the rate of a person at rest, increasing exposure -- and leaving China in a frantic race to clean up.
Top image: Participants run on the Bund near Huangpu River as they compete in the Shanghai International Marathon. (Carlos Barria/Reuters)