Evidence from studies in both people and laboratory animals has shown that asbestos can increase the risk for some types of cancer.
When asbestos fibers in the air are inhaled, they may stick to mucus in the throat, trachea (windpipe), or bronchi (large breathing tubes of the lungs) and may be cleared by being coughed up or swallowed. But some fibers may reach the ends of the small airways in the lungs or penetrate into the outer lining of the lung and chest wall (known as the pleura). These fibers may irritate the cells in the lung or pleura and eventually cause lung cancer or mesothelioma.
Studies in people
Inhalation of asbestos fibers has been linked to an increased risk of lung cancer in many studies of asbestos-exposed workers. In general, the greater the exposure to asbestos, the higher the risk of lung cancer. Most cases of lung cancer in asbestos workers occur at least 15 years after initial exposure to asbestos.
In workers exposed to asbestos who also smoke, the lung cancer risk is much greater than even adding the risks from these exposures separately.
It’s not clear to what extent low-level or short-term exposure to asbestos might raise lung cancer risk.
Mesothelioma is a rare form of cancer that affects the thin membranes lining organs in the chest (pleura) and abdomen (peritoneum). Mesothelioma is closely linked with asbestos; most cases of mesothelioma result from direct exposure to asbestos at work.
Studies have found an increased risk of mesothelioma among workers who are exposed to asbestos, as well as among their family members and people living in the neighborhoods surrounding asbestos factories and mines. Although the risk of developing mesothelioma increases with the amount of asbestos exposure, there is no way to measure the minimum amount of asbestos exposure that can lead to mesothelioma. However, mesothelioma is very rare in the general population of the United States.
Mesotheliomas typically take a long time to develop. The time between first exposure to asbestos and diagnosis of mesothelioma is usually 30 years or more. Unfortunately, the risk of mesothelioma does not drop with time after exposure to asbestos. The risk appears to be lifelong.
Unlike lung cancer, mesothelioma risk is not increased among smokers.
Other types of cancer
Studies have also linked workplace exposure to asbestos with cancers of the larynx (voice box) and ovaries. Some studies have also suggested that workplace asbestos exposure may be linked to other cancers, including cancers of the esophagus, stomach, colon, and kidney. However, researchers aren’t yet sure if all of these cancers are truly related to asbestos exposure. It’s not clear exactly how asbestos might affect risk for these cancers, but swallowed asbestos fibers might somehow contribute to the risk.
Studies done in the lab
Tests on several different rodent species, using different methods of exposure, have confirmed that asbestos causes cancer in animals. All commercial forms of asbestos have produced tumors in animals. The size and shape of the asbestos fibers influence the incidence of tumors: smaller, straighter fibers seem more hazardous, perhaps because they are more likely to reach the deepest parts of the lungs.
What expert agencies say
Several agencies (national and international) study different substances in the environment to determine if they can cause cancer. (A substance that causes cancer or helps cancer grow is called a carcinogen.) The American Cancer Society looks to these organizations to evaluate the risks based on evidence from laboratory, animal, and human research studies.
Based on animal and human evidence like the examples above, several expert agencies have evaluated the cancer-causing nature of asbestos.
The International Agency for Research on Cancer (IARC) is part of the World Health Organization (WHO). Its major goal is to identify causes of cancer. Based on the data available, IARC classifies asbestos as a “known human carcinogen”.
The National Toxicology Program (NTP) is formed from parts of several different US government agencies, including the National Institutes of Health (NIH), the Centers for Disease Control and Prevention (CDC), and the Food and Drug Administration (FDA). The NTP has classified asbestos as “known to be a human carcinogen”.
The US Environmental Protection Agency (EPA) maintains the Integrated Risk Information System (IRIS), an electronic database that contains information on human health effects from exposure to various substances in the environment. The EPA classifies asbestos as a human carcinogen.
(For more information on the classification systems used by these agencies, see our document, Known and Probable Human Carcinogens.)
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