More than 500 million Chinese urban residents (14% of the global urban population) are imperiled by fine particulate hazard

China's urbanization and the subsequent public vulnerability to degenerated environment is important to global public health. Among the environmental problems, fine particulate (PM_2.5) pollution has become a serious hazard in rapidly urbanizing China. However, quantitative information remains inadequate. We thus collected PM_2.5 concentrations and population census records, to illustrate the spatial patterns and changes in the PM_2.5 hazard levels in China, and to quantify public vulnerability to the hazard during 2000-2010, following the air quality standards of World Health Organization. We found that 28% (2.72 million km²) of China's territory, including 78% of cities (154 cities) with a population of >1 million, was exposed to PM_2.5 hazard in 2010; a 15% increase (1.47 million km²) from 2000 to 2010. The hazards potentially impacted the health of 72% of the total population (942 million) in 2010, including 70% of the young (206 million) and 76% of the old (71 million). This was a significant increase from the 42% of total the population (279 million) exposed in 2000. Of the total urban residents, 76% (501 million) were affected in 2010. Along with PM_2.5 concentration increase, massive number of rural to urban migration also contributed greatly to China's urban public health vulnerability.