Anything that increases your chance of contracting a disease is considered a risk factor. Strong exposure to sunlight is a risk factor for developing skin cancer and smoking is a risk factor in developing lung cancer. In the case of mesothelioma, the primary risk factor is exposure to asbestos. Nevertheless, just as not everyone exposed to sunlight or tobacco smoke gets cancer, the same holds true for exposure to asbestos.
Asbestos, the name for a group of silicate fibers that occur naturally in the environment, was widely used in a variety of industrial products over a period of decades. Up to 8 million Americans may have been exposed at some point in their lives. According to the Environmental Protection Agency (EPA), up to 733,000 schools and other public buildings in the U.S. still contain asbestos insulation, and while most uses of asbestos have now been banned, there are still some asbestos-containing products on the market.
Asbestos has been used commercially since the late 1800s, however, its use was greatly increased during the World War II era when shipyards constructing warships were in full swing. Even today, statistics show that the highest number of malignant mesothelioma cases diagnosed are in coastal areas where shipyards were in operation. Other at-risk trades include construction workers (particularly those who installed insulation), factory workers, mine and mill workers and railroad workers. It has also been proven that family members of exposed workers are at increased risk for developing mesothelioma because of secondary exposure to fiber brought into the home on the clothing, hair or skin of those who worked with or around asbestos products. For those ultimately diagnosed with mesothelioma, it may be 20 to 50 or more years to the onset of symptoms, and risk does not appear to diminish with time B — it is lifelong.
While the vast majority of mesotheliomas diagnosed are linked to asbestos exposure, there is the occasional report of patients with a history of Hodgkin’s disease contracting mesothelioma as a result of their radiation therapy. In these cases, mesothelioma develops on an average of 15 years post-treatment, and is most often located at the site of the radiated areas.
Absolute and relative risk
Absolute risk is the probability that an individual will contract a particular disease within a specified period of time. For instance when epidemiologists say that American men face a 1:4000 chance of developing mesothelioma, what they are describing is an absolute risk, as relevant to the average risk or general population group. This statistic is applicable to the general population on the whole, and cannot be associated with any specific person. Different groups of people have different risk estimates. Military veterans past age 60 face a higher absolute risk of developing mesothelioma in comparison to younger men.
Relative risk compares two different groups, with one group facing a specific risk factor whereas the other group does not face any specific risks. For instance, researchers can study two groups of people, with both the groups sharing common characteristics such as same age and socioeconomic profile. However, one group has people who have specific risk factors such as asbestos exposure. The researchers seek to determine the number of individuals in each group who may develop cancer over a specified period of time. If twice as many individuals develops mesothelioma in the exposed group as the in the non-exposed group, then individuals in the exposed group will be said to have a relative risk of 2.0.
Both absolute and relative risk number can help tell the story about whether an individual faces the chance of contracting mesothelioma and can be useful in diagnosing the disease. The absolue risk of getting mesothelioma is low for all groups. However, repeated studies have shown that people exposed to asbestos fibers have higher relative risk of mesothelioma than those who have not been exposed.
The SV40 Controversy
Over the past several years, some research studies have indicated a possible link between SV40 (the simian virus) and the risk of developing mesothelioma. Based on the assumption that injectable polio vaccines prepared between 1955 and 1963 were contaminated with the virus, it is estimated that 10 to 30 million people could have been exposed.
In early tests with laboratory animals, researchers found that intentional infection with SV40 could cause mesotheliomas to develop, and that asbestos increased the cancer-causing effect of SV40 on these cells. Other researchers studying human biopsy specimens of mesotheliomas had also detected SV40 DNA, however, it was found that the SV40 DNA was also present in non-cancerous tissue, leading them to believe the viruses were contaminants.
Later tissue studies of both mesothelioma patients and healthy people where there did not appear to be contaminants, showed the SV40 virus was not linked to mesothelioma unless the person was also exposed to asbestos. These researchers determined that the SV40 infection was not caused by the polio vaccine, but occurred naturally as do other types of viral infections.
The largest human studies to date have not found any increased risk for mesothelioma among those who received the contaminated vaccines as children, however, some researchers feel the peak age for developing mesothelioma has not yet been reached by many of the subjects involved. Research is ongoing.