The cancer inducing property of asbestos occurs as the result of its physical properties rather than from any chemical properties (Ref 1). Some subtype of asbestos may be associated with relatively low cancer generating potential compared with other subtypes. Asbestos fibers exist in two major forms: curly pliable serpentine asbestos (chrysolite) and rod like amphiboles (crocidolite, amosite, anthophylite, tremolite, and actinolyte). Chrysolite, crocidolite, and amosite are usually mined for commercial and industrial use. Asbestos fibers tend to separate readily from each other and can exist as single strand of fiber. These fibers are microscopic in size. The long rod like fibers associated with amphiboles (crocidolite, amosite, anthophylite, tremolite, and actinolyte) in general have more cancer inducing potential compared to the serpentine fiber types (chrysolite). Long rod like fibers of amphiboles can readily be deposited in the deeper parts of the respiratory tract where they may remain for a long time or may be transported to the pleura or peritoneum.
Not all asbestos fibers inhaled through the respiratory system remain in the respiratory tract. Most of these inhaled fibers are coughed out or swallowed and subsequently eliminated through feces. The remaining fibers are cleared by various clearing mechanisms of the body and respiratory tree. Short fibers are often cleared more efficiently through these mechanisms than long fibers. Those fibers remaining after all these clearing mechanisms may accumulate in the lower part of the lung close to pleural surface of the lung.
Continued presence of asbestos fiber in the lung initiates an inflammatory and scarring process mediated by various inflammatory chemical substances released by the lung tissue. Some growth control genes are turned on by this process, which may result in mesothelial cell proliferation. Asbestos can also cause alteration in the DNA, which is the main center of genetic code. Alterations in DNA and genetic code can lead to misreading of genetic code, which may ultimately result in loss of control mechanisms for the cell. Chrysolite fibers have been shown to be especially potent in causing DNA damage and mutation. There is some evidence to suggest that asbestos can induce the production super reactive oxygen, which in turn can cause damage to DNA.
Reference:
1. Timbrell V. Physical factors as etiological mechanisms, in biological effects of asbestos. Int Agency Res Cancer (Lyon) 1973.
|
