The study is based on hourly air quality measurements from 1,500 sites across China and Hong Kong over a four month period last year. During that period (April to August 2014), 92 percent of China's population experienced more than 120 hours of "unhealthy air," based on US standards, while 38 percent were exposed to average concentrations that were unhealthy. Authors Robert Rohde and Richard Muller, of the research organization Berkeley Earth, calculated that outdoor air pollution contributes to 1.6 million deaths in China every year, or 4,400 per day.

As the South China Morning Post reports, Rohde and Muller have now applied their methodology to develop a Google Maps plug-in that displays air pollution levels across China in near real-time. The map shows levels of air particulates measuring less than 2.5 microns in diameter (PM2.5), which are considered to be most dangerous to human health. Levels are displayed along a colored gradient, like a weather radar map, with low levels in green and unhealthy levels in red. Orange areas signify pollution levels that are dangerous to sensitive populations. On Thursday morning, the highest smog levels were in Shanghai, along China's eastern coast, and near the northeastern province of Shandong.

The Chinese government has taken steps to mitigate the country's air pollution, in response to public outcry, though it has retained tight control over public data and media reports. When a documentary on China's smog crisis went viral earlier this year, government censors quickly blocked access to it.