A tourist and her daughter visit Tiananmen Square while wearing masks due to dangerous levels of air pollution on Jan. 23, 2013 in Beijing, China. Feng Li/Getty Images

The World Health Organization (WHO) on Thursday classified outdoor air pollution as a leading cause of cancer in humans.

"The air we breathe has become polluted with a mixture of cancer-causing substances," said Kurt Straif of the WHO's International Agency for Research on Cancer (IARC). "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."

The IARC's declaration that air pollution is a carcinogen, alongside dangers such as asbestos, tobacco and ultraviolet radiation, came after a consultation by an expert panel.

The IARC said a panel of top experts had found "sufficient evidence" that exposure to outdoor air pollution caused lung cancer and raised the risk of bladder cancer.

IARC had previously deemed some of the components in air pollution such as diesel fumes to be carcinogens, but this is the first time it has classified air pollution in its entirety as cancer causing.

Although the composition of air pollution and levels of exposure can vary dramatically between locations, the agency said its conclusions applied to all regions of the globe. Air pollution also increases the risk of respiratory and heart diseases.

Air pollution is a complex mixture that includes gases and particulate matter, and IARC said one of its primary risks is the fine particles that can be deposited deep in the lungs of people.

The IARC said pollution exposure levels increased significantly in some parts of the world in recent years, notably in rapidly industrializing nations with large populations.

Straif said there were dramatic differences in air quality between cities around the world and that the most polluted metropolises were in China and India, where people frequently don masks on streets to protect themselves. China recently announced new efforts to curb pollution after experts found the country's thick smog hurts tourism.

The predominant sources of outdoor air pollution were transport, emissions from factories and farms, power generation and residential heating and cooking, the agency said.

The most recent data, from 2010, showed that 223,000 lung cancer deaths worldwide were the result of air pollution, the agency said.

In the past, the IARC had measured the presence of individual chemicals and mixtures of chemicals in the air — including diesel engine exhaust, solvents, metals and dust. But the latest findings were based on overall air quality.

"Our task was to evaluate the air everyone breathes rather than focus on specific air pollutants," said the IARC's Dana Loomis.

"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," he added.

Wire services