There is "sufficient evidence" that exposure to outdoor air pollution causes lung cancer, leading experts convened by WHO's International Agency for Research on Cancer (IARC) Monographs Programme have concluded.
The experts, who reviewed the latest available scientific literature, also found that exposure to outdoor air pollution increased the risk of bladder cancer.
"The air we breathe has become polluted with a mixture of cancer-causing substances," said Dr Kurt Straif, Head of the IARC Monographs Section.
"We now know that outdoor air pollution is not only a major risk to health in general, but also a leading environmental cause of cancer deaths," Straif said in a statement.
Particulate matter, a major component of outdoor air pollution, was evaluated separately by the scientists and was also classified as carcinogenic to humans.
The scientists found a higher risk of lung cancer was associated with increasing levels of exposure to particulate matter and air pollution.
While the composition of air pollution and levels of exposure can vary dramatically between locations, the conclusions apply to all regions of the world, the agency said.
According to most recent data from 2010, around 223,000 deaths from lung cancer worldwide resulted from air pollution.
In the past, many individual chemicals and specific mixtures that occur in outdoor air pollution have been evaluated. These included diesel engine exhaust, solvents, metals, and dusts. But this is the first time that experts have classified outdoor air pollution as a cause of cancer.
"Our task was to evaluate the air everyone breathes rather than focus on specific air pollutants," said Dr Dana Loomis, Deputy Head of the Monographs Section.
"The results from the reviewed studies point in the same direction: the risk of developing lung cancer is significantly increased in people exposed to air pollution," Loomis said.
The evaluation is driven by findings from large epidemiologic studies that included millions of people living in Europe, North and South America, and Asia.
The predominant sources of outdoor air pollution are transportation, stationary power generation, industrial and agricultural emissions, and residential heating and cooking. Some air pollutants have natural sources, as well.     

"Classifying outdoor air pollution as carcinogenic to humans is an important step," said IARC Director Dr Christopher Wild.
"There are effective ways to reduce air pollution and, given the scale of the exposure affecting people worldwide, this report should send a strong signal to the international community to take action without further delay," Wild said.


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