Mesothelioma and Asbestos
Asbestos is a term that covers a number of different mineral fibers. While they all share many similar properties and attributes, each fiber is individually distinct from the others in very particular ways, including shape, width and length. It has been known for decades that asbestos is carcinogenic and is the only confirmed cause of mesothelioma, but what has not been conclusively established are the carcinogenic properties of the individual fibers. Researchers in Canada and Australia have recently completed a study that attempts to answer some of these questions.
Asbestos Fiber Types
Asbestos fibers can be categorized into one of two main classes: amphibole fibers, which are characterized by long, needle-like fibers, and serpentine fibers, which are curl-shaped. Amphibole asbestos fibers include amosite, crocidolite, tremolite, anthophyllite and actinolite, while chrysotile is the only serpentine asbestos fiber. Even though the fibers are themselves structurally distinct, they are often found in the same mines so the different individual fibers can be mixed together in the mining process. For example, in many cases mining for chrysotile also resulted in tremolite findings, so studies on the health effects of chrysotile would have to include data on pure chrysotile as well as chrysotile contaminated with tremolite.
Asbestos Carcinogenicity by Fiber Type
Amphibole fibers are the most carcinogenic of asbestos fibers, seemingly due to their long, thin structure. Of these fibers, crocidolite carries the greatest risk for developing mesothelioma, as epidemiological surveys of crocidolite mines in South Africa and Australia have shown very high rates of mesothelioma and other cancers in mine workers in those areas. However, amosite and anthophyllite also show a risk of mesothelioma development in people exposed to those fibers. Tremolite, when mined for its own properties, as well as when found as a contamination element, such as the vermiculite mine in Libby, Montana which produced the greatest site-specific public health disaster in U.S. history, has a carcinogenicity similar to crocidolite.
Chrysotile is a serpentine fiber and for reasons still not fully understood, seems to present a reduced individual risk when compared to the other asbestos fibers. However, chrysotile was the most popular asbestos fiber used in the U.S., so its smaller overall carcinogenicity must be weighed against the much larger exposure concentrations that would be associated with its use here. The researchers note that authors of a previous study on asbestos carcinogenicity (Hodgson and Darnton) report a relative causation ratio for mesothelioma for crocidolite, amosite, chrysotile as 500:100:1.
Mesothelioma Risk Factors
As discussed above, the type of asbestos fiber a person is exposed to will have a great influence on his or her likelihood of developing mesothelioma in the future. The amphibole fibers are the most carcinogenic. As compared to them, chrysotile has a markedly lower risk, although it does still pose a risk. However, other factors also play an important role in the development of mesothelioma. Those factors include the duration and total amount of exposure, the time since first exposure and age at which it occurred, as well as the rate of elimination of fibers from the lung.
Of these risk factors, the number of years of regular exposure is a major component to the development of mesothelioma. The researchers state, “It is now reasonably well-established that the risk of mesothelioma increases with time since first exposure to the power of 3–4.”
Asbestos is a known human carcinogen that has claimed countless lives. While certain forms of asbestos carry greater risk for mesothelioma than do other forms, there is no such thing as safe asbestos. Being aware of the known risk factors for mesothelioma can help international workers and their families protect themselves if they are required to work around it.