PM 2.5 : A barrier to fitness and health promotion in China.

1.Introduction Physical inactivity has been identified as the fourth leading risk factor for global mortality.~1Globally,it accounts for about6%of deaths,21%–25%of breast and colon cancers,27%of diabetes,and 30%of ischemic heart disease.~1In China,the economic costs of health diseases due to physical inactivity had reached 6.7 billion US dollars in 2007,which accounted for