A group of highly fibrous, naturally occurring minerals with separable, long and thin fibers. Asbestos fibers are strong and flexible enough to be spun and woven as well as resistant to heat and chemicals. Historically, asbestos was used for fire proofing, insulating, and fiber reinforcing in tiles, plastics, cements, and many other purposes.
Asbestos is toxic. Because of their durability, asbestos fibers that get into lung tissue will remain there for a long time. Inhaled microscopic fibers are known to cause lung cancer, asbestosis, and mesothelioma. More than 50 countries have banned the use of asbestos. While not banned in the United States, its use is regulated and it is no longer mined (Fig. 1). Because of its widespread use in the twentieth century, millions of metric tons of asbestos are still found in buildings, posing a health risk if disturbed. See also: Buildings; Lung; Mutagens and carcinogens; Oncology; Toxicology
Major deposits of chrysotile asbestos are found in massive serpentinite bodies formed by alteration of magnesium- and iron-rich igneous rocks such as dunite, peridotite, and iherzolite. The world's largest deposits of this type are found in Québec Province in Canada, in the Ural Mountains of the Russia Federation, in the Troodos Mountains of Cyprus, and in the Diablo Range of California. The asbestos in these serpentinite deposits were usually mined in large open pits (Fig. 1). Minor amounts of fibrous and nonfibrous anthophyllite, tremolite, and talc are also found in serpentinites. Serpentinized limestones also contain minor deposits of chrysotile asbestos. Deposits of grunerite and riebeckite asbestos occur in Precambrian banded iron formations located in three areas: in the Cape and Transvaal provinces of the Republic of South Africa and in Western Australia. Anthophyllite asbestos is found within serpentinized periodotites of the Karelian Mountains, East Finland. See also: Chrysotile; Dunite; Peridotite; Serpentinite
There are two general types of asbestos: amphibole and serpentine. Some studies show that amphibole fibers stay in the lungs longer than the serpentine mineral, chrysotile, and this tendency may account for their increased toxicity. See also: Amphibole; Serpentine
Generally, health regulations classify asbestos into six mineral types: chrysotile (the only asbestos member of serpentine group), which has long and flexible fibers, and five amphiboles, which have brittle crystalline fibers. The amphiboles include actinolite asbestos, tremolite asbestos, anthophyllite asbestos, riebeckite asbestos, and grunerite asbestos. Each of the amphiboles includes the term asbestos because, to geologists, amphiboles can also have nonasbestiform habits (that is, the host rock is an amphibole that may contain a mixture of nonfibrous particles, fibers, and cleavage fragments).
Chrysotile has the ideal chemical formula Mg3[Si2O5](OH)4 [Mg = magnesium, Si = silicon, O = oxygen, and OH = hydroxy]. Small amounts of aluminum (Al), iron (Fe), manganese (Mn), calcium (Ca), potassium (K), and sodium (Na) may appear in the bulk chemical analyses of this mineral. The chrysotile fibers consist of long hollow tubes, and each fiber is approximately 25 nanometers in diameter, with lengths varying from well under 1 micrometer to well over 10 cm. The fibers are characteristically curly and ropelike. Chrysotile is identified by a combination of quantitative elemental composition; distinctive shape, size, and hollow-tube morphology of the fibers (Fig. 2); and characteristic x-ray or electron diffraction pattern.
The five forms of amphibole asbestos and their ideal chemical formulas are grunerite asbestos, (Fe2+,Mg)7[Si8O22](OH)2; riebeckite asbestos, Na2Fe23+(Fe2+,Mg)3[Si8O22](OH)2; anthophyllite asbestos, Mg7[Si8O22](OH)2; tremolite asbestos, Ca2Mg5[Si8O22](OH)2; and actinolite asbestos, Ca2(Mg,Fe2+)5[Si8O22](OH)2. A considerable amount of chemical substitution takes place in these asbestos minerals. The amphibole asbestos minerals are characterized by their various elemental compositions and distinctive diffraction patterns, and generally vary in width from 0.1 to over 1 μm and in length from 1 μm to over 10 cm (Fig. 3).
The most important risk factors for asbestos-related diseases are length of exposure, air concentration of asbestos during the exposure, and smoking. Cigarette smoking and asbestos exposure are synergistic; that is, the risk of disease is multiplied if a person exposed to asbestos is a smoker. There is an ongoing scientific debate about the differences in the extent of disease caused by different fiber types and sizes. Some differences may be due to the physical and chemical properties of the different fiber types. For example, several studies suggest that amphibole asbestos types (tremolite, amosite, and especially crocidolite) may be more harmful than chrysotile, particularly for mesothelioma. Other data indicate that fiber size (length and diameter) are important factors for cancer-causing potential. Some data indicate that fibers with lengths greater than 5.0 μm are more likely to cause injury than those less than 2.5 μm in length. Additional data indicate that short fibers can contribute to injury. This appears to be true for mesothelioma, lung cancer, and asbestosis. However, fibers thicker than 3.0 μm are of lesser concern, because they have little chance of reaching the lower regions of the lung.
Some groups of people who have been exposed to asbestos fibers in 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 other causes.
Routes of exposure
Ambient air concentrations of asbestos fibers are about 10−5–10−4 fibers per milliliter (fibers mL−1), depending on the location. Prolonged human exposure to concentrations much higher than 10−4 fibers mL−1 is suspected of causing adverse health effects. Asbestos fibers are very persistent and resist chemical degradation (that is, they are inert under most environmental conditions) so their vapor pressures are nearly zero (they do not evaporate), and they do not dissolve in water. However, segments of fibers do enter the air and water when asbestos-containing rocks and minerals are weathered naturally or extracted during mining operations. One of the most serious exposures occurs when manufactured products, such as pipe wrapping and fire-resistant materials, begin to degrade or are improperly handled during renovation or removal activities. Small-diameter asbestos fibers may remain suspended in the air for a long time and be transported advectively by wind or water, before sedimentation. Like spherical particles, heavier asbestos fibers settle more quickly. Asbestos seldom moves substantially via soil. The fibers are generally not broken down to other compounds in the environment and will remain virtually unchanged over long periods.
Although most asbestos is highly persistent, chrysotile, the most commonly encountered form, may break down slowly in acidic environments. Asbestos fibers may break into shorter strands, and therefore into increased number of fibers by mechanical processes (such as grinding and pulverization). Inhaled fibers may get trapped in the lungs and with chronic exposures build up over time. Some fibers, especially chrysotile, can be removed from or degraded in the lung with time.
Because of its toxicity, it is necessary to have effective monitoring techniques and analytical methods to detect, quantify, and control asbestos in the environment. A number of techniques and methods have been developed to detect and quantify asbestos in bulk samples, air, water, settled dust, and soil.
Numerous methods are used to measure asbestos. The most widely used techniques are based primarily upon observing the fiber by either optical or electron microscopy. These techniques include phase-contrast microscopy (PCM), polarized light microscopy (PLM), scanning electron microscopy, and transmission electron microscopy. Some methods also use x-ray diffraction spectrometry (XRD) for identifying mineral phases and quantitative analysis. See also: Electron microscope; Phase-contrast microscope; Polarized light microscope; Quantitative chemical analysis; Scanning electron microscope; X-ray diffraction
[Disclaimer: The U.S. Environmental Protection Agency through the Office of Research and Development funded and managed some of the research described here. Content in the present article has been subjected to the Agency's administrative review and has been approved for publication.]