A MASSIVE UNPLANNED STUDY demonstrating the health impact of air pollution began when, in 1950, China’s Mao Zedong began providing free coal for winter heating to residents north of the Huai River. Because China imposed a household registration system that restricted mobility, people in the north breathed air that was 55% more polluted than that in the south, according to a recent analysis of meticulous government records in Proceedings of the National Academy of Sciences. The records also detailed the ages and causes of death in the static population, which allowed researchers to determine that for every increase of 100 micrograms of pollutant particles per cubic meter, people lose about three years of life to cardiovascular and respiratory disease. The 500 million people in northern China may lose an average of 5.5 years, for a total of 2.5 billion lost years of life.