Physical inactivity has been identified as the fourth leading risk factor for global mortality.1 Globally, it accounts for about 6% of deaths, 21%–25% of breast and colon cancers, 27% of diabetes, and 30% of ischemic heart disease.1 In China, the economic costs of health diseases due to physical inactivity had reached 6.7 billion US dollars in 2007, which accounted for more than 15% of the yearly medical and non-medical costs caused by the main non-communicable diseases.2 Physical activity (PA) has thus been recommended and promoted by renowned organizations, such as World Health Organization (WHO), Centers for Disease Control and Prevention, and American College of Sports Medicine, due to its abundant health benefits.1, 3, 4 However, PA and national fitness promotion have been challenged and threatened by severe air pollution in China during its fast and massive industrialization and urbanization.5, 6 Among air pollutants, PM2.5 (particulate matter (PM) with a mean aerodynamic diameter of 2.5?μm or less), because of its debilitating effects on health,7, 8 has caused extensive public concern, received numerous negative media reports, and therefore presents another public health challenge in populous China.
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