Air pollution is hard to see, but it poses a serious threat to people that live in high-risk areas. According to a report from the St. Louis Post Dispatch, as many as 3.3 million people lose their lives each year from illnesses caused by air pollution.

Researchers from Germany, Cyprus, Saudi Arabia, and Harvard University reached this number after one of the most comprehensive air pollution studies to date. The study predicted that if current rates of air pollution aren’t abated, the impact on human life could double before the middle of the century to 6.6 million.

The study was published last week in the journal Nature, and used health statistics and complex computer models to show how many people were dying of air pollution, and where. Roughly three quarters of air pollution related deaths were caused by heart attacks and strokes, according the head author Jos Lilieveld at the Max Planck Institute for Chemistry in Germany. The findings are consistent with other studies that examined the effects of air pollution on health and mortality across the globe.

“About 6 percent of all global deaths each year occur prematurely due to exposure to ambient air pollution. This number is higher than most experts would have expected, say, 10 years ago,” said Jason West, a professor of environmental science at the University of North Carolina.

According to Lilieveld, air pollution kills more people than HIV and malaria combined. China is notorious for having issues with air quality, largely due to their rapid economic growth and widespread use of coal to produce energy. In China, roughly 1.4 million people die from breathing polluted air each year. India follows with 645,000 deaths a year, and Pakistan with 110,000 deaths caused annually by air pollution related illnesses.

The United States lost 54,905 citizens from illnesses related to soot and smog, and ranks the seventh highest in the world for deaths related to air pollution. Oddly enough, agriculture was responsible for 16,221 of those deaths, second only to deaths related to breathing air pollution caused by power plants, which killed 16,929 people in the same year.

Farms produce ammonia from fertilizer and animal waste, which combines with sulfates from coal energy and nitrates from car exhaust to form large soot particles that can wreak havoc on the respiratory and circulatory systems. Agricultural emissions are currently not regulated, and contribute to a shocking number of fatalities across the United States.

In the central U.S., the majority of air pollution deaths are caused by power plants, while the bulk of deaths from air pollution on the West Coast can be attributed to traffic emissions.

Reducing air pollution is not only good for public health, it’s good for the climate too. As regulators seek to reduce CO2 emissions, other pollutants will inevitably be slashed as well.