According to a comprehensive, two-year look at the effect of pollution on global health, it is responsible for about 9 million deaths annually. At 16% of deaths, that means it ranks much higher than the things we tend to fear more, like war or communicable diseases. Sadder still, it's the most vulnerable populations — the poor, the very young, and the very old — who are at highest risk of death caused by polluted air or water.

The report uses numbers from the Global Burden of Disease study in 2015. It found that 4.2 million people died from a type of air pollution called particulate matter in the ambient air, while 2.9 million died from indoor air pollution, from chemicals and burning cooking fuel and wood. Outdoor particulate matter and ground-level ozone come mostly from burning fossil fuels in power plants and vehicles. Unsafe water accounted for 1.8 million deaths, and occupational exposure carcinogens or particulates caused 800,000 deaths.