The International Agency for Research on Cancer (IARC) has classified outdoor air pollution as a cancer-causing agent (carcinogen). The IARC is part of the World Health Organization, and is one of the primary sources for information on cancer-causing substances for the American Cancer Society and other organizations. In its evaluation, the IARC concluded that outdoor air pollution causes lung cancer and is also linked to an increased risk for bladder cancer.

The IARC has previously classified many components of outdoor air pollution as carcinogens, including diesel engine exhaust, solvents, metals, and dust. But this is the first time it has classified outdoor air pollution – as a whole – as a carcinogen. It also classified another major component of outdoor air pollution – called particulate matter – as a carcinogen on its own. Particulate matter is a combination of extremely small solid particles and liquid droplets that are found in the air. Particulate matter can include things like dust or smoke, as well as chemicals. The IARC evaluation showed an increasing risk of lung cancer with increasing levels of exposure to outdoor air pollution and particulate matter.

“The air we breathe is filled with cancer-causing substances,” said Kurt Straif, PhD, head of the IARC Monographs Section. “Outdoor air pollution is not only a major environmental risk to health in general, it is the most important environmental cancer killer due to the large number of people exposed.”

Air pollution is already known to increase risks for other diseases, especially respiratory and heart diseases. Studies show that levels of exposure to air pollution have increased significantly in some parts of the world, mostly in rapidly industrializing countries with large populations. The most recent data from the Global Burden of Disease Project indicate that in 2010, 3.2 million deaths worldwide resulted from air pollution, including 223,000 from lung cancer.

The risk of lung cancer associated with air pollution is lower in the US, according to Elizabeth Ward, PhD, American Cancer Society National Vice President, Intramural Research, but should not be ignored. "Even though the lung cancer risk associated with air pollution for an individual in the US is relatively low, even a low risk can be important for a large population where many people are exposed. As with many other environmental exposures with a relatively small individual risk, large numbers of people exposed to air pollution may result in a significant number of lung cancers.”

Ward credits policies in the US for helping to lower the levels of exposure. “Air pollution levels in most US cities used to be much higher than they are now, but scientific research and effective policies to reduce pollution levels have made the air safer and continue to be important to protect public health. High levels of air pollution in other parts of the world may present more significant health hazards."

The IARC based its report on a review of more than 1,000 scientific papers from studies on 5 continents. The studies analyzed the cancer risk caused by various pollutants present in outdoor air pollution, especially particulate matter and transportation-related pollution. The findings came from large epidemiologic studies that included millions of people living in Europe, North and South America, and Asia.

According to the IARC, the predominant artificial sources of outdoor air pollution are transportation, stationary power generation, industrial and agricultural emissions, and residential heating and cooking.

“Classifying outdoor air pollution as carcinogenic to humans is an important step,” said IARC Director Christopher Wild, PhD. “Given the scale of the exposure, it sends a strong signal that it is vital to implement efficient policies to reduce exposure to pollution worldwide.”