Air pollution causes 3.3 million deaths worldwide each year, mostly from heart attacks and strokes, according to a new study from the Harvard School of Public Health published in the journal Nature.

Harvard researchers, along with scientists from Germany, Cyprus and Saudi Arabia, used health statistics and computer modeling to calculate air pollution’s global toll. They predicted the number of deaths could double to 6.6 million by 2050 if emissions continue at the current rate.

The leading cause of air pollution deaths worldwide was the use of biofuels for indoor cooking, with China topping the list at 1.4 million deaths a year, followed by India with 645,000 and Pakistan with 110,000. (The United States was seventh on that list with 54,905 deaths a year.)

The second highest cause of air pollution deaths, at 664,100 deaths, was agriculture — specifically, ammonia from fertilizer and animal waste combing with car exhaust and sulfates from power plants to form harmful soot particles. In the U.S. Northeast, all of Europe, Russia, Japan and South Korea, agriculture is the leading cause of soot deaths, according to the study. According to the Associated Press:

The problem with farms is ammonia from fertilizer and animal waste, [lead study author Jos] Lelieveld said. That ammonia then combines with sulfates from coal-fired power plants and nitrates from car exhaust to form the soot particles that are the big air pollution killers, he said. In London, for example, the pollution from traffic takes time to be converted into soot, and then it is mixed with ammonia and transported downwind to the next city, he said. "We were very surprised, but in the end it makes sense," Lelieveld said. He said the scientists had assumed that traffic and power plants would be the biggest cause of deadly soot and smog.

Here are three ways you can help reduce air pollution:

1. Make your car greener. You can reduce your carbon footprint by changing the way you drive. Avoid aggressive speeding and breaking, and don’t carry extra weight in your car.

2. Increase your home’s energy efficiency. Seal drafty areas around your windows and doors, turn off lights when you leave a room for more than 15 minutes and keep your thermostat turned down at least eight hours a day.

3. Eat less meat. More than 50 percent of global greenhouse-gas emissions are caused by animal agriculture, according to a report from the Worldwatch Institute. Also, the United Nations says a shift toward a vegan diet will help protect us from the effects of climate change and curb global warming. In a nutshell, fewer animals raised for slaughter means less waste and less air pollution.

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Angela is a Pulitzer Prize-winning digital editor with more than 15 years of experience delivering news and information to audiences worldwide. Prior to joining SafeBee, she was the features editor for at The Boston Globe, overseeing health, travel, entertainment, business and lifestyle coverage. Before moving to features, she was the news and homepage editor, covering stories such as the Boston Marathon bombing, Red Sox World Series victories, presidential elections, a papal inauguration, and more. Her favorite safety tip: Clean your phone! The average cell phone has 18 times more germs than the toilet handle in a men’s restroom.