Beijing - Air pollution kills about 4 000 Chinese people a day, US researchers said, citing coal burning as a key contributor.Bad air contributed to 1.6 million deaths a year or roughly 17% of all deaths in China, according to the scientific paper published Thursday by independent research group Berkeley Earth.The average air quality for 38% of Chinese was "unhealthy" by US standards based on four months of hourly data from 1 500 Chinese ground stations, the authors outlined before the planned publication of the study.Some 92% of China's population experienced at least 120 hours of unhealthy air between April 5 2014 and August 5 2015, Richard Muller and Robert Rohde said in a press release."When I was last in Beijing, pollution was at the hazardous level. Every hour of exposure reduced my life expectancy by 20 minutes," said Muller, scientific director of Berkeley Earth."It's as if every man, woman and child smoked 1.5 cigarettes each hour," he said.Beijing may struggle to clean up in time for the 2022 Olympics because the sources of its air pollution are not local, they warned.The capital city "receives much of its pollution from distant industrial areas, particularly Shijiazhuang, 322km to the south-west," Rohde said.China's national safety standard for daily exposure to harmful PM2.5 pollution, particulate matter 2.5 microns and smaller is 35 micrograms per cubic metre, making Chinese air quality limits four times higher than those recommended by the World Health Organisation.PM2.5 pollution penetrates the lungs and can trigger heart attacks, stroke, lung cancer and asthma."The fact that sources of PM2.5 match those of sulfur implies that most of the pollution comes from coal," the press release said.