Air pollution deaths are most commonly from heart disease, strokes or chronic obstructive pulmonary disease. One of the main risks of pollution is that tiny particles can get deep into the lungs, causing irritation.

"Excessive air pollution is often a byproduct of unsustainable policies in sectors such as transport , energy, waste management and industry," he said. "In most cases, healthier strategies will also be more economical in the long term due to health care cost savings as well as climate gains."

Carlos Dora, a WHO public health expert, called on governments and health agencies to devise policies to reduce air pollution, which would improve health and reduce humans' impact on climate change.

The toll, a doubling of previous estimates, means 1 of every 8 global deaths in 2012 was linked to polluted air and shows how reducing pollution inside and outside people's homes could save millions of lives in the future, the United Nations health agency said.

"The evidence signals the need for concerted action to clean up the air we all breathe," said Maria Neira, head of the WHO's environmental and social public health department. "The risks from air pollution are now far greater than previously thought or understood, particularly for heart disease and strokes."

Air pollution kills about 7 million people worldwide every year and is now the single biggest environmental health risk, with more than half the fatalities due to fumes from indoor stoves, according to a new report from the World Health Organization (WHO) published Tuesday.

The real problem is that wearing masks sends out the message we can live with polluted air. We need to change our way of life entirely to reduce pollution.

The WHO estimated that there were about 4.3 million deaths in 2012 caused by indoor air pollution, from using wood, coal and biomass stoves for cooking and heating. It 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 were based mostly on modeling. The increase is partly due to better information about the health effects of pollution and improved detection methods. Last year the WHO's cancer agency classified air pollution as a carcinogen, linking dirty air to lung and bladder cancer.

Poor and middle-income countries in Southeast Asia and the western Pacific region had the largest air pollution-related burden in 2012, with 3.3 million deaths linked to indoor air pollution and 2.6 million deaths to outdoor air pollution.

The WHO estimates that about 2.9 billion people worldwide live in homes using wood, coal or dung as their primary cooking fuel.

Flavia Bustreo, a WHO family health expert, said women and children — especially those living in poor countries — often bear the brunt of the risks from indoor pollution "since they spend more time at home breathing in smoke and soot from leaky coal and wood cooking stoves."

Other experts said more research was needed to identify the deadliest components of pollution in order to target control measures more effectively.

Outdoors, air is mainly polluted by vehicles, power generation, industrial and agricultural emissions and residential heating and cooking.

Research suggests outdoor air pollution exposure levels have risen significantly in some parts of the world, particularly in countries with large populations going through rapid industrialization, such as China and India.

"We don't know if dust from the Sahara is as bad as diesel fuel or burning coal," said Majid Ezzati, chairman in global environmental health at Imperial College London.

Frank Kelly, director of the environmental research group at King's College London, who was not part of the WHO report, said it was mostly up to governments to curb pollution through measures like legislation, moving power stations away from big cities and providing cheap alternatives to indoor wood and coal stoves.

He said people could reduce their exposure to harmful fumes by avoiding traveling at rush hour or by taking smaller roads. Despite the increasing use of masks in heavily polluted cities such as Beijing and Tokyo, Kelly said there was little evidence that they work.

"The real problem is that wearing masks sends out the message we can live with polluted air," he said. "We need to change our way of life entirely to reduce pollution."

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