Researchers use 2 main types of studies to try to figure out if a substance causes cancer.
- Studies in people: One type of study looks at cancer rates in different groups of people. Such a study might compare the cancer rate in a group exposed to a substance to the cancer rate in a group not exposed to it, or compare it to the cancer rate in the general population. But sometimes it can be hard to know what the results of these studies mean, because many other factors might affect the results.
- Lab studies: In studies done in the lab, animals are exposed to a substance (often
in very large doses) to see if it causes tumors or other health problems. Researchers might also expose normal cells in a lab dish to the substance to see if it causes the types of changes that are seen in cancer cells. It’s not always clear if the results from these types of studies will apply to humans, but lab studies are a good way to find out if a substance might possibly cause cancer.
In most cases neither type of study provides enough evidence on its own, so researchers usually look at both human and lab-based studies when trying to figure out if something causes cancer.
Evidence from studies in both people and lab animals has shown that asbestos can increase the risk for some types of cancer.
When asbestos fibers in the air are inhaled, they can stick to mucus in the throat, trachea (windpipe), or bronchi (large breathing tubes of the lungs) and might be cleared by being coughed up or swallowed. But some fibers 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 can 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 cancer1 in many studies of asbestos-exposed workers. This increased risk is seen with all forms of asbestos (there is no “safe” type of asbestos in terms of lung cancer risk). 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 first exposure to asbestos.
In workers exposed to asbestos who also smoke, the lung cancer risk is even greater than adding the risks from these exposures separately.
Mesothelioma2 is a fairly rare form of cancer that most often affects the thin linings of the organs in the chest (pleura) and abdomen (peritoneum).
Mesothelioma is closely linked with asbestos exposure. All forms of asbestos have been linked to mesothelioma, although amphibole asbestos appears to cause this cancer at
lower levels of exposure than chrysotile asbestos.
Most cases of mesothelioma result from exposure to asbestos at work. There is also an increased risk of mesothelioma among family members of workers and people living in neighborhoods near asbestos factories and mines. Although the risk of mesothelioma increases with the amount of asbestos exposure, there is no clear safe level of asbestos exposure in terms of mesothelioma risk.
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 found clear links between workplace exposure to asbestos and cancers of the larynx (voice box)3 and ovaries4.
Some studies have also suggested that workplace asbestos exposure may be linked to other cancers, including cancers of the pharynx (throat), stomach, colon, and rectum.
However, the link between these cancers and asbestos is not as clear as it is for the other cancers discussed here. For cancer of the throat, the link is strongest for the hypopharynx, the part of the throat closest to the larynx (voice box). 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 forms of asbestos have produced tumors in animals, but 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.