Air pollution kills about 7 million people worldwide every year, with more than half of the fatalities due to fumes from indoor stoves, according to a new report from the World Health Organization.

The agency said air pollution is the cause of about one in eight deaths and has now become the single biggest environmental health risk.

"We all have to breathe, which makes pollution very hard to avoid," Frank Kelly, director of the environmental research group at King's College London, who was not part of the WHO report, told the Associated Press news agency.

One of the main risks of pollution is that tiny particles can get deep into the lungs, causing irritation. Scientists also suspect air pollution may be to blame for inflammation in the heart, leading to chronic problems or a heart attack.

WHO estimated that there were about 4.3 million deaths in 2012 caused by indoor air pollution, mostly people cooking inside using wood and coal stoves in Asia. WHO said there were about 3.7 million deaths from outdoor air pollution in 2012, of which nearly 90 percent were in developing countries.

The new estimates are more than double previous figures and based mostly on modeling. The increase is partly due to better information about the health effects of pollution and improved detection methods. Last year, WHO's cancer agency classified air pollution as a carcinogen, linking dirty air to lung and bladder cancer.

WHO's report noted women had higher levels of exposure than men in developing countries.

"Poor women and children pay a heavy price from indoor air pollution since they spend more time at home breathing in smoke and soot from leaky coal and wood cook stoves," Flavia Bustreo, WHO Assistant Director-General for family, women and children's health, said in a statement.

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