"Asbestos" is a term used to describe any of several naturally occurring fibrous silicate minerals of the amphibole or serpentine groups (see Figure 1). Asbestos fibers may be straight (amphibole asbestos) or curled (serpentine asbestos), and have no detectable odor or taste. There are six minerals that are generally described as asbestos: chrysotile, which is a serpentine mineral; and crocidolite, amosite, tremolite, anthophyllite, and actinolite, which are all amphibole minerals. Asbestos fibers vary in length (usually greater than 5 microns), and width (usually less than 0.5 microns).
Almost 95 percent of the world's mined asbestos is chrysotile asbestos. The world has 200 million tons of identified asbestos resources, and an estimated 45 million tons of additional asbestos resources. Because asbestos fibers are resistant to heat and chemicals, they have been used in the production of building materials (e.g., floor tiles, roof shingles, cement), friction products (e.g., automotive brake pads), and heat-resistant fabrics. However, many countries, including the United States, have banned new uses of asbestos because of its adverse health effects. Worldwide use of asbestos has declined, but certain areas of the world (particularly Southeast Asia, South America, and Eastern Europe) continue to use it, in part because asbestos is an economical and long-lasting building material.
Humans can be exposed to asbestos through inhalation of asbestos fibers, as well as through ingestion (e.g., drinking water from cement pipes
that have been manufactured with asbestos). Asbestos-related diseases commonly occur after a fifteen- to forty-year latency period following initial asbestos exposure, and are primarily associated with occupational inhalation exposure. Nonoccupational exposure to asbestos occurs primarily through exposure to asbestos that is "friable," meaning it can be reduced to dust by hand pressure. Asbestos fibers are long, thin fibers that can be inhaled deep into the lungs and are able to penetrate the lung's walls. The immune system is helpless against these fibers, because they are unable to be engulfed (phagocytised) by alveolar macrophages, and therefore remain in the lung for an extended period.
Diseases associated with asbestos exposure primarily involve the respiratory system and include progressive pulmonary fibrosis (asbestosis), pleural disease (the pleura are the membranes that cover the lungs), and cancer of the bronchi (bronchogenic carcinoma) and pleura (malignant mesothelioma). Cigarette smoking along with asbestos exposure increases the risk of lung cancer. There is disagreement within the scientific community as to the difference in the extent of toxicity between serpentine and amphibole asbestos fibers, although studies in humans and animals have demonstrated that both types of fibers increase the risk of asbestosis, malignant mesothelioma, and lung cancer.
The primary public health approach to asbestos is to ban or severely limit its use. In order to further reduce occupational disease from asbestos exposure, environmental controls should be implemented in the workplace, including ventilation systems, full-face respirators, and changing clothes before and after asbestos exposure.
MARGARET H. WHITAKER
BRUCE A. FOWLER
Agency for Toxic Substances Disease Registry (1995). Toxicological Profile for Asbestos (Update). Washington, DC: U.S. Department of Health and Human Services.
International Agency for Research on Cancer (1987). "Asbestos and Certain Asbestos Compounds." In IARC Monographs on the Evaluation of the Carcinogenic Risk of Chemicals to Humans. IARC Monographs Supplement 7. Lyon: IARC.
International Programme on Chemical Safety (1986). Asbestos and Other Natural Mineral Fibers. Environmental Health Criteria 53. Geneva: World Health Organization.
—— (1998). Chrysotile Asbestos. Environmental Health Criteria 203. Geneva: World Health Organization.
United States Geological Survey (2000). Minerals Commodity Summaries: Asbestos. Available at http://minerals.usgs.gov/minerals/pubs/commodity/asbestos/index.html.