Risk factors are environmental, hereditary, or behavioral conditions that increase a person’s chances of getting a disease. Risk factors do not control whether someone gets a disease or not; they are simply the statistically-associated factors that seem to indicate that the development of a disease is more likely. For example, the primary risk factor for developing lung cancer is whether a person smokes tobacco or not. Other factors such as diet, hereditary predisposition to cancer, and so forth also affect the odds, and are also considered risk factors in assessing the chances that someone will develop the disease.

For malignant mesothelioma, the primary risk factor is absolutely clear: exposure to asbestos. Approximately 90 percent of malignant mesothelioma cases are in individuals who have had major occupational or environmental exposure to asbestos. At least eight million Americans have had sufficient exposure to asbestos, either in their jobs, through exposure to friable asbestos in commercial products or in homes or schools, or through secondary exposure to someone who worked in asbestos-using industries, to be considered at risk for malignant mesothelioma.

There are three primary factors in determining the risk posed to an individual by asbestos exposure: the time of the exposure, the duration of the exposure, and the level of the exposure. Malignant mesothelioma has a very long latency period – from 20 to 50 years in the majority of cases. Exposure to asbestos at young ages is therefore worse than exposure at an older age; asbestos exposure at age 70, for example, is unlikely to develop into malignant mesothelioma since the exposed person is likely to die of other causes long before the mesothelioma has a chance to form.

Exposure to high levels of asbestos is worse than exposure to modest levels; it is better to have gone to school in a building with some crumbling asbestos tiles than to have worked in an asbestos mine. Exposure over a long period of time is worse than exposure for brief periods. One grim fact of asbestos exposure and mesothelioma risk is that the passage of time without exposure does not reduce the risk; the body does not repair or recover from asbestos exposure the way it can from things like tobacco smoking.

Although asbestos exposure is the primary risk factor for mesothelioma, there are some other risk factors which can contribute to the disease.

Radiation, specifically exposure to thorium dioxide, has been linked to at least a few cases of mesothelioma.

Zeolite, a silicate mineral found mainly in Turkey, is similar to asbestos and is thought to have a similar ability to cause malignant mesothelioma.

Tobacco smoking, although not a direct risk factor, has been found to increase the incidence of mesothelioma in patients who also were exposed to asbestos.

The SV40 virus, a simian virus which some 10 to 30 million Americans were exposed to during the early days of the polio vaccine, is thought by some researchers to contribute to the development of malignant mesothelioma.