السبت، 7 يوليو، 2012

Asbestos

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Asbestos (pronounced play /æsˈbɛstəs/ or /æzˈbɛstəs/) is a set of six naturally occurring silicate minerals used commercially for their desirable physical properties.[1] They all have in common their eponymousasbestiform habit: long (ca. 1:20 aspect ratio), thin fibrous crystals. The prolonged inhalation ofasbestos fibers can cause serious illnesses, including malignant lung cancer, mesothelioma (a formerly rare cancer strongly associated with exposure to amphibole asbestos), and asbestosis (a type of pneumoconiosis). Long exposure to high concentrations of asbestos fibers will cause health problems. This is most common among the miners of asbestos, since they have the longest exposure to it.[citation needed] The European Union has banned all use of asbestos[2] and extraction, manufacture and processing of asbestos products.[3]
Asbestos became increasingly popular among manufacturers and builders in the late 19th century because of its sound absorption, average tensile strength, its resistance to fire, heat, electrical and chemical damage, and affordability. It was used in such applications as electrical insulation for hotplate wiring and in building insulation. When asbestos is used for its resistance to fire or heat, the fibers are often mixed with cement (resulting infiber cement) or woven into fabric or mats.
Asbestos mining began more than 4,000 years ago, but didn't start large scale until the end of the 19th century. The world's asbestos mining peaked around 1975, when asbestos was being mined in some 25 countries, but is today less than half of what it was in the mid 1970s.[citation needed] For a long time, the world's largest asbestos mine was the Jeffrey mine in the town of Asbestos, Quebec.[4]

hazardous asbestos fiber types, chrysotile asbestos has produced tumors in animals and is a recognized cause of asbestosis and malignant mesothelioma in humans.[37]
Mesotheliomas have been observed in people who were occupationally exposed to chrysotile, family members of the occupationally exposed, and residents who lived close to asbestos factories and mines.[38] According to the NCI, "A history of asbestos exposure at work is reported in about 70 percent to 80 percent of all cases. However, mesothelioma has been reported in some individuals without any known exposure to asbestos."[39] The most common diseases associated with chronic exposure to asbestos include: asbestosis and pleural abnormalities (mesothelioma, lung cancer).[40]Asbestosis has been reported primarily in asbestos workers, and appears to require long-term exposure, high concentration for the development of the clinical disease. There is also a long latency period (incubation period of an infectious disease, before symptoms appear) of about 12 to 20 years.[41]
Studies have shown an increased risk of lung cancer among smokers who are exposed to asbestos compared to nonsmokers.[42]
Asbestos exposure becomes a health concern when high concentrations of asbestos fibers are inhaled over a long time period.[43] People who become ill from inhaling asbestos are often those who are exposed on a day-to-day basis in a job where they worked directly with the material. As a person's exposure to fibers increases, because of being exposed to higher concentrations of fibers and/or by being exposed for a longer time, then that person's risk of disease also increases. Disease is very unlikely to result from a single, high-level exposure, or from a short period of exposure to lower levels.[43]
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[edit]Mechanisms which might be triggering cancer development

Stanton and Layard hypothesized in 1977–78 that toxicity of fibrous materials is not initiated by chemical effects,[44] that is any trigger-effects of asbestos must presumably be physical, such as (A) mechanical damage or (B) unwanted signal channels (a plausible property for slender transparent fibres) which might disrupt normal cell activity—especially mitosis.
(A) Mechanical Damage. There is experimental evidence that very slim fibers (<60 nm, <0.06 μm in breadth) do tangle destructively with chromosomes (being of comparable size).[45][46] This is likely to cause the sort of mitosis disruption expected in cancer.
(B) Unwanted Signal channels. This has recently been explored theoretically, but not yet experimentally. The theory argues that this effect would only be feasible for asbestos fibers >100 nm in breadth (>150 nm in the case of chrysotile), which suggests that we should be on the look-out for a possible mixture of different mechanisms for the different fiber-diameter-ranges.[47] [48]
One popular idea of the causal chain is (1) Asbestos fiber → → (3) inflammation → (4) other pathology. While that may be true, it does not explain "(2), the actual trigger":
"What is the physical property of asbestos which initiates any such inflammation?" (After all, inflammation is usually seen as caused by chemical-based processes: immunological &/or bacterial). So inflammation (&/or oxidation etc.) may well be part of the causal chain, but not the crucial first step.[47]

Other asbestos-related diseases

  • It is important to consult a doctor, particularly if the follow symptoms develop: shortness of breath, wheezing, or hoarseness, persistent cough that worsens over time, blood in fluid coughed up, pain or tightening in chest, difficulty swallowing, swelling of neck or face, decreased appetite, weight loss, fatigue or anemia.[49]
  • Asbestosis: Progressive fibrosis of the lungs of varying severity, progressing to bilateral fibrosis, honeycombing of the lungs on radiological view with symptoms including rales and wheezing. Individuals who have been exposed to asbestos via home, environment, work should notify their doctors about exposure history.
  • Asbestos warts: caused when the sharp fibers lodge in the skin and are overgrown causing benign callus-like growths.
  • Pleural plaques: discrete fibrous or partially calcified thickened area which can be seen on X-rays of individuals exposed to asbestos. Although pleural plaques are themselves asymptomatic, in some patients this develops into pleural thickening.
  • Diffuse pleural thickening: similar to above and can sometimes be associated with asbestosis. Usually no symptoms shown but if exposure is extensive, it can cause lung impairment.
  • Pneumothorax: Some reports have also linked the condition of pneumothorax to asbestos related diseases



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