Air pollution kills about 7 million people a year and is linked to 1 in 8 deaths worldwide, according to a report released Tuesday by the World Health Organization.
The finding more than doubles previous estimates "and confirms that air pollution is now the world's largest single environmental health risk," the agency said.
An estimated 4.3 million people died in 2012 as a result of indoor air pollution, mostly from cooking inside with coal or wood stoves in developing countries, according to the report by the public health agency of the United Nations. An additional 3.7 million died from outdoor air pollution.
Many people are exposed to both indoor and outdoor air pollution, so deaths attributed to each cannot simply be added together, the agency noted. Because of the overlap, the combined estimate for 2012 is about 7 million deaths.
"The risks from air pollution are now far greater than previously thought or understood, particularly for heart disease and strokes," said Maria Neira, the agency's public health and environment director.
The greatest burden is in Asia, where most deaths related to air pollution occur.
The developing world gets a "double whammy" of outdoor air pollution from vehicles and industrialization and indoor pollution from the continued use of wood, charcoal, dung and biomass-burning cook stoves, said John Balmes, a professor of medicine at UC San Francisco and member of the California Air Resources Board. He did not contribute to the report.
"That generates a lot of smoke that is not healthy to breathe," Balmes said. "It's basically like having a campfire in your kitchen."
About 2.9 billion people across the globe live in homes that use wood, coal or dung as their main cooking fuel, the report estimates.
Women have higher levels of exposure because they do more cooking, but more men die because they have a higher rate of underlying health problems, according to the report.
The higher mortality estimates are the result of new research showing a stronger link between exposure to polluted air and cardiovascular disease, cancer and respiratory illness, the WHO said. They also reflect improved air quality measurements and new mapping that allowed scientists to conduct a more detailed analysis of the exposure in urban and rural communities.
Because air pollution worsens underlying illnesses, most of the premature deaths cited were from heart disease, stroke, chronic obstructive pulmonary disease, lung cancer and respiratory infections in children.
The greatest health risk is posed by fine particles -- also known as PM2.5 -- which include diesel soot, wood smoke and chemical-laden droplets. Experts say those combustion particles, less than 1/30th the width of a human hair, cause most of the fatalities because they penetrate deep into the lungs, inflame the airways and put strain on the heart and other organs.
"People don't die of air pollution alone; they die of other things the pollution tends to exacerbate," said Michael Kleinman, a professor of toxicology at UC Irvine, who was not involved with the report. "It's a contributor to many deaths, and the more we learn, the more effects can be attributed to air pollution."
Last year the WHO's cancer research arm declared air pollution a human carcinogen, saying it increases the risk of lung and bladder cancer.
Reducing pollution levels could save millions of lives, according to the agency.
“The evidence signals the need for concerted action to clean up the air we all breathe,” Neira said.