MAIN TITLE Asbestos
Article Title Health Risks Associated with Asbestos Exposure
To understand the health risks associated with a substance such as asbestos, scientists evaluate data compiled from clinical, epidemiological , and laboratory studies. Clinical data ordinarily provide the first indication that a substance may have adverse effects on the body. Physicians observe a pattern of symptoms, or the presence of a disease, which appears to be linked with a particular activity, or exposure to a particular substance. Such observations lead to a hypothesis that the activity or substance caused the symptoms or the observed disease. Epidemiologists will then undertake an investigation to attempt to confirm the hypothesis. If an association between the symptoms, or disease, and the activity or substance appears to be substantiated, laboratory studies are undertaken. Should animal response to the substance produce effects similar to that observed in humans, the case for an association is strengthened.
Most of the information on the health effects of exposure to asbestos, have been derived from studies of workers exposed to asbestos in the course of their occupation. Asbestos fibre concentrations for such workers are many times higher than those encountered, by the general public, or by most workers in buildings with ACM. Because their exposure w as much higher, asbestos workers w ill have a much higher incidence of asbestos-related diseases than people who live or work in buildings with ACM. This is know n as the dose-response effect. How ever, people in buildings with ACM are still likely to experience higher risks than the public at large. Unfortunately, the available data does not allow us to reliably estimate the actual risk. Because asbestos fibres appear to be ubiquitous, virtually everyone is exposed to some extent. During autopsy, asbestos fibres have been detected in the lungs of most urban residents. Exposure of the general public is troublesome because w e are talking about a large population, which includes unhealthy as w ell as healthy persons. Moreover, exposure may begin during childhood, leaving a long period for the manifestation of asbestos-related disease. Furthermore, asbestos may enhance the carcinogenic effects of other materials. Any additional exposure to asbestos caused by living or working in buildings with ACM should thus be avoided.
Despite epidemiological studies of workers and laboratory studies of animals, questions remain about which properties of asbestos are responsible for the adverse health effects. It is not know whether the particular properties which produce one disease, for example, lung cancer, are the same from those which produce another disease such as asbestosis . Which conditions of exposure are most likely to lead to adverse health effects have not been positively identified. Some characteristics that appear to be important are; the physical size of fibres (long, thin fibres seem to be the most toxic) and their durability. The variation in chemical composition among different types of asbestos does not appear to be an important as differences in physical properties. How ever, the EPA believes that current evidence is not sufficient to say that one type is any more toxic than another.
Some natural substances other than asbestos seem to have health effects similar to those of asbestos. For example, erionite, a fibrous form of a mineral called zeolite, induces mesothelioma in test animals. Persons living in central Turkey, w here this substance is found are reported to have a higher than expected incidence of several diseases associated with asbestos. These include mesothelioma, previously believed to be, caused exclusively by asbestos. Exposure to man-made mineral fibres, such as fibrous glass and ceramic materials is relatively recent. Occupational exposure levels have not been as high as asbestos exposure. However, some epidemiological data do suggest that diseases of the respiratory tract, such a pulmonary fibrosis and lung cancer, may result from long-term exposure to these fibres if the fibres are thin. Fibrous glass used for thermal insulation does not appear to be a problem.
The Respiratory System
To be a significant health concern, asbestos fibres must be inhaled. An understanding of the mechanics of the respiratory system w ill aid in appreciating the potential for exposure and the resulting health effects. Every cell in the body needs a constant supply of oxygen. The respiratory system meets this need by bringing oxygen to the bloodstream, which delivers it to each cell and carries away carbon dioxide. The lungs are the focal point of the respiratory system, which also includes the respiratory tract , the channel by which air flows into and out of the lungs. Inhaled air passes through the nose, w here moisture and tiny hairs filter dust. It then passes down the throat where air is also humidified. Air continues into the trachea , a tube supported by rings of cartilage. Just above the heart, the trachea divides into two bronchi . Each bronchus leads into a lung w here it subdivides into smaller air tubes giving the appearance of an upside down tree. The tiniest tubes end in globular air sacs, or alveoli which at one cell thick, allow oxygen and carbon dioxide to trade places. The carbon dioxide is exhaled backup the respiratory tract. The blood picks up fresh oxygen and transports it throughout the body. The lungs, cone-shaped, balloon-like, elasticised tissue are located on either side of the chest. Each lung is encased by a double layer of membrane or pleura . One layer is attached to the lung, the other to the rib cage. Space and fluid between the two layers enable the lungs to expand and contract in the chest cavity without friction. To visualize this association, think of two panes of
glass with a drop of water between them. The pieces of glass, like the linings, slide easily across each other, but are difficult to pull apart. When we breathe in, the diaphragm stretches out flat and muscles between the ribs, contract with it, pulling the ribs in and out. This expands the chest cavity, creating a vacuum between the linings that expands the lungs and sucks in air. When breathing out, the diaphragm and rib cage muscles relax, the ribs fall in and down, and the lungs contract and push out the carbon dioxide and unused oxygen. The respiratory system is sensitive to bacteria, viruses, and many airborne particles that can be inhaled. Reactions to these irritants can disrupt the functioning of the system, resulting in many ailments including the following; the common cold, hay fever, sinusitis, sore throat, acute or chronic bronchitis, emphysema, and lung cancer.
The body has several mechanisms by which it filters the air it breathes. The tiny hairs in the nose filter out dust and airborne particles. Like the nose, the trachea and the bronchi air lined with small cilia. Together with mucus secreted by cells lining the airways, cilia trap particles and help prevent respiratory infections. The cilia beat in an upward direction sweeping foreign particles up to the back of the mouth from w here they are expelled or swallow ed. Viruses and bacteria are also attacked by enzymes called lysozyme in the mucus cells. Microbes that slip through are usually handled by white blood cells called phagocytes that envelop and eat these invaders in the lung.
Cigarette smoking temporarily paralyses the cilia. If smoking continues long enough, the cilia will wither and the affected party will develop a cough, which attempts to rid the respiratory tract of foreign particles and excess mucus. Dirty, contaminated air presents the greatest challenge to the respiratory system. Some of the particles entering the airways reach the alveoli. When this occurs, white blood cells called macrophages attempt to engulf and digest the particles. In the case of asbestos, we are dealing with a mineral fibre, a substance which macrophages can often not successfully attack. As a means of secondary defence, the macrophages deposit a coating on the fibres, which are then deposited in the smaller passages. Here they clog and actually scar the tissues.
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