Asbestos


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Introduction

Asbestos is the name applied to a group of six different fibrous materials (amosite, chrysotile, crocidolite, and the fibrous varieties of tremolite, actinolite, and anthophyllite) that occur naturally in the environment. Nonasbestos (nonfibrous) forms of tremolite, actinolite, and anthophyllite also are found naturally. The most common mineral type is white (chrysotile), but others may be blue (crocidolite), gray (anthophyllite), or brown (amosite). These minerals do not have any detectable odor or taste. Asbestos can be found naturally in soil and rocks in some areas. Asbestos fibers are resistant to heat and most chemicals. Because of this property, asbestos fibers have been mined for use in a wide range of man-made products, mostly in building materials, friction products, and heat-resistant fabrics. Because asbestos fibers may produce adverse health effects in exposed persons, all new uses of asbestos have been banned in the United States by the EPA. Uses for asbestos that were established before July 12, 1989, which include most of the uses listed above, are still allowable.

Chrysotile Asbestos

Structural diagram: National Institutes of Health


Fate & Transport

Asbestos fibers do not evaporate into air or dissolve in water. However, pieces of fibers can enter the air and water from the weathering of natural deposits and the wearing down of man-made asbestos products. Small fibers and fiber-containing particles may remain suspended in the air for a long time and be carried long distances by wind or water currents before settling. Larger fibers and particles tend to settle more quickly. Asbestos fibers are not able to move through soil. Asbestos fibers are not broken down to other compounds in the environment. Therefore, they may remain in the environment for decades or more. Asbestos fibers may build up in animals.

Exposure Pathways

You are most likely to be exposed to asbestos by inhaling asbestos fibers suspended in air. These fibers can come from natural outcroppings of asbestos or from the wearing down of man-made products including insulation, ceiling and floor tiles, roof shingles, cement, and automotive brakes and clutches. However, these products do not always contain asbestos. Low levels of asbestos that are not likely to be harmful to your health can be detected in almost any air sample. For example, in rural areas, an average of around 0.03-3 fibers are usually present in a cubic meter of outdoor air. A cubic meter is about the amount of air you breathe in 1 hour. Higher levels are usually found in cities, where there may be 2-300 fibers per cubic meter.

Close to an asbestos mine or factory, levels could reach 2,000 fibers per cubic meter or higher. Levels could also be above average near a building that contains asbestos products and is being torn down or renovated or near a waste site where asbestos is not properly covered up or stored to protect it from wind erosion.

In indoor air, the concentration of asbestos depends on whether asbestos was used for insulation, ceiling or floor tiles, or other purposes, and whether these asbestos-containing materials are in good condition or are deteriorated and easily crumbled. Concentrations measured in homes, schools, and other buildings that contain asbestos range from 0.7 to 6,000 fibers per cubic meter. People who work with asbestos (for example, miners, insulation workers, automobile brake mechanics) without proper protection are likely to be exposed to much higher levels of asbestos particles in air.

You can also be exposed to asbestos by drinking fibers present in water. Even though asbestos does not dissolve in water, fibers can enter water by being eroded from natural deposits or piles of waste asbestos, from asbestos-containing cement pipes used to carry drinking water, or from filtering through asbestos-containing filters. Most drinking water supplies in the United States have concentrations less than 1 million fibers per liter (MFL). However, in some locations, water samples may contain 10-300 MFL or even higher.

Metabolism

If you breathe asbestos fibers into your lungs, some of the fibers will be deposited in the air passages and on the cells that make up your lungs. Most fibers are removed from your lungs by being carried away or coughed up in a layer of mucus to the throat, where they are swallowed into the stomach. This usually takes place within a few hours. Fibers that are deposited in the deepest parts of the lung are removed more slowly. In fact, some fibers may move through your lungs and can remain in place for many years and may never be removed from your body.

If you swallow asbestos fibers (either those present in water or those that are moved to your throat from your lungs), nearly all the fibers pass along your intestines within a few days and are excreted in the feces. A small number of fibers may penetrate into cells that line your stomach or intestines and a few penetrate all the way through and get into your blood. Some of these become trapped in other tissues and some are removed in your urine.

If you get asbestos fibers on your skin, very few of these fibers, if any, pass through the skin into your body.

Health Effects

Information on the health effects of asbestos in people comes mostly from studies of people who were exposed in the past to high levels of asbestos in the workplace. Workers who breathe in asbestos may develop a slow buildup of scar-like tissue in the luns and in the membrane that surrounds the lungs. This scar-like tissue does not expand and contract like normal lung tissue and so breathing becomes difficult. Blood flow to the lung may also be decreased and this causes the heart to enlarge. This disease is called asbestosis. People with asbestosis have shortness of breath, oten accompanied by a cough. This is a serious disease and can eventually lead to disability or death in people exposed to high amounts of asbestos. However, asbestosis is not usually of concern to people exposed to low levels of asbestos. Changes in the membrane surrounding the lung, called pleural plaques, are quite common in people occupationally exposed to asbestos and are sometimes found in people living in areas with high environmental levels of asbestos, but effects on breathing are usually not serious.

Asbestos workers have increased chances of getting two types of cancer: cancer of the lung tissue itself and mesothelioma, a cancer of the thin membrane that surrounds the lung and other internal organs. Lung cancer is usually fatal, while mesothelioma is invariably fatal within a few months of diagnosis. These diseases do not develop immediately, but appear only after a number of years. There is also some evidence from studies of workers that breathing asbestos can increase the chances of getting cancer in other locations (for example, stomach, intestines, esophagus, pancreas, kidneys), but this is less certain. Members of the public who are exposed to lower levels of asbestos may also have increased chances of getting cancer, but the risks are usually small and are difficult to measure directly.

The levels of asbestos in air that lead to lung disease depend on a large number of factors. The most important of these are (1) how long you were exposed, (2) how long it has been since your exposure started, and (3) whether you smoked cigarettes. Interactions between cigarette smoke and asbestos increase your chances of getting lung cancer. Also, there is a scientific debate concerning the differences in the extent of disease caused by different fiber types and sizes. Some of these differences may be due to the physical and chemical properties of different fiber types. For example, several studies suggest that the amphiboles (tremolite, amosite, and especially crocidolite) may be more harmful than chrysotile. However, most data indicate that fiber size (length and diameter) is the most important factor for cancer-causing potential, particularly for mesothelioma. Most studies indicate that long fibers (greater than about 1/5,000th of an inch) are more likely to cause injury than short fibers (less than about 1/10,000th of an inch). Generally, smaller fiber diameters or widths are associated with mesothelioma, while larger widths are associated with lung cancer.

The health effects from swallowing asbestos are unclear. Some groups of people who have been exposed to asbestos fibers in their drinking water have higher-than average death rates from cancer of the esophagus, stomach, and intestines. However, it is very difficult to tell whether this is caused by asbestos or by something else. Animals that were given very high doses of asbestos in food did not get more fatal cancers than usual, although some extra nonfatal tumors did occur in the intestines of rats in one study.

Several government offices and regulatory agencies have considered all the evidence regarding the carcinogenicity of asbestos. The Department of Health and Human Services (DHHS) has determined that asbestos is a known carcinogen. The EPA has determined that asbestos is a human carcinogen. Despite the ongoing debate concerning health effects resulting from different asbestos fiber types, ATSDR considers all the different mineral forms of asbestos to be known human cancer-causing substances with a prolonged latency period of between 10 and 40 years between exposure and the onset of disease. The International Agency for Research on Cancer (IARC) has determined that asbestos is carcinogenic to humans.

Information excerpted from:

Toxicological Profile for Asbestos August 1995 Update

Agency for Toxic Substances and Disease Registry
U.S. Dept. of Health and Human Services