Beijing, China, has high levels of air pollution due to rapid industrial expansion and the increased number of automobiles on the road. Beijing's annual level of PM10 (particulate matter less
than 10 μm in diameter) exceeds 150 mg/m3, and is ranked the
sixth highest among the monitored cities in Chen et al., 2008. Studies have also reported high concentrations of polycyclic aromatic hydrocarbons, ranging from 28.53 to 362.15 ng/m3, particularly during winter months (Zhao et al., 2010). The Chinese government took steps during the Beijing Olympics and Paralym- pics to reduce air pollution and particulate matter levels in order to provide all athletes and guests with a cleaner atmospheric environment. Factories were temporarily closed across a large geo- graphic area and vehicle exhaust emissions were reduced by prevent- ing half of Beijing's 3.3 million cars from being driven on any given day. Consequently, the city's ambient air quality dramatically improved during the Olympics and particulate matter decreased to half of the pre-Olympic levels. After cessation of the control measures, particulate matter returned to pre-Olympic levels. These circumstances created a natural experiment with bi-directional change in particulate matter levels, allowing us to observe short-term biological responses to both decreases and increases in air pollution, and may be informative regarding the mechanisms potentiating long-term effect of exposure to particulate matter.