Asbestos and Malignant Mesothelioma
Why and how does asbestos cause cancer? Although various researchers studied the issue through the early twentieth century, the most well-known studies demonstrating the relationship between exposure to asbestos and malignant and non-malignant lung disease are those conducted by Dr. Irving Selikoff of the Mt. Sinai School of Medicine beginning in the 1960’s and later. The number of asbestos-related diseases was on the rise by the time Dr. Selikoff began his work, and he was able to study a large sample of workers in the shipyard industry instead of looking at a small number of cases, as most earlier researchers had done. It can take twenty years or more for asbestosis or pleural plaques to develop to the extent that they can be diagnosed, and asbestos became widely used in the United States in the 1940’s, twenty years before Dr. Selikoff’s first study in 1964.
At the most basic level, an asbestos disease is caused when asbestos fibers enter the body, usually through the nose and mouth. The non-malignant diseases, asbestosis (also called fibrosis) and pleural disease, develop when the body’s immune system attacks the invading asbestos particles. The human lung contains many small air sacs which expand and contract with breathing and from which oxygen and carbon dioxide are exchanged via blood vessels. When an asbestos fiber penetrates one of the air sacs, the body attempts to destroy the fiber and repair the cell. In this process, the asbestos fiber becomes covered with a coating that contains iron. This is known as an “asbestos body.” The cellular tissue in the air sac is replaced with collagen, a fibrous material. The collagen is stiff and inflexible compared to the original cellular material in the lungs.
When this happens throughout the lungs, they develop a characteristic fibrous appearance. This fibrosis also hardens the lungs, so that they can no longer contract and expand normally. This leads to shortness of breath, chest pain, complaints of easy fatigue, and frequently a dry cough, among others. In the most severe cases, the affected person can no longer inhale enough oxygen on his or her own and needs to receive supplemental oxygen.
Asbestos-related pleural disease, or pleural plaques, occurs when the pleural lining around the lungs thickens and becomes scarred from damage caused by the asbestos fibers that have penetrated it. The extent of the disease can vary widely. Sometimes the entire pleura is affected. The thickness of the plaques on the pleura affects the lung’s ability to expand with intake of oxygen.
Since the symptoms of asbestosis and pleural plaques are similar to many other lung diseases, diagnosis usually needs to be confirmed with radiology or pathology or both. An x-ray or CT scan will reveal the characteristic patterns of the fibrosis in the lungs or the calcifications of the pleura. If a sample of lung tissue is taken in a biopsy, asbestos bodies may be visible under a microscope. Most people are exposed to asbestos at some level, but persons who inhale large quantities of asbestos fibers daily, year after year, will suffer more damage to their lung tissue.
An asbestos-related cancer, however, is much less clearly tied to the duration and extent of the exposure to asbestos. Persons have been diagnosed with malignant mesothelioma whose only known exposure to asbestos was only a few months long. Further, the exposure to asbestos can occur decades before the cancer begins to grow. While it is unknown exactly why the presence of asbestos fibers in the body cause cell growth to go awry, one theory for any cancer is that the genetic material of a cell is somehow damaged by the cancer-causing agent. When the damaged cell reproduces, it reproduces inaccurately. This is repeated on successive reproductions. Like blurred photocopies, the reproductions become more and more illegible, until the cell can no longer “read” its own instructions and becomes cancerous. Some people appear to be more susceptible to malignant mesothelioma, or to other forms of cancer, because they lack a tumor-suppressing gene.
Asbestos-related cancers, other than malignant mesothelioma, such as lung cancer, colon cancer, or kidney cancer, probably come about in the same way. An asbestos-related cancer can occur anywhere an asbestos particle can go: in the lungs, in the gastro-intestinal system, in the mesothelium. The combination of cigarette smoking and asbestos exposure leads to a much greater likelihood of developing lung cancer than either alone. There is no connection between cigarette smoking and malignant mesothelioma.
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