Asbestos Health Effects : Exposure Pathways

by CDC

(Page 2 of 9)

Asbestos exposure occurs primarily through inhalation of fibrous dust.

Asbestos is a generic term for a group of six naturally occurring fibrous minerals. The basic unit of asbestos-class minerals is the silicate combined in varying proportions with magnesium, iron, calcium, aluminum, and sodium or trace elements.

There are two major classes of asbestos: serpentine, which contains a magnesium silicate called chrysotile, and amphiboles, which represent a small portion of the world's commercial asbestos use and include crocidolite, amosite, anthophyllite, and tremolite. Chrysotiles are curly stranded fibers, and amphiboles are straight, rodlike fibers. Chrysotile, the sole member of the serpentine group, accounts for 93% of the world's commercial, purposeful asbestos use. The different classes and types of asbestos also appear to differ in toxicity, the basis of which might depend on fiber size, shape, and persistence in the lung (e.g., ability to clear the fiber, solubility, and durability). Contamination of other products such as vermiculite and talc from asbestos fibers is a larger problem than once thought.

Asbestos has been used in >3,000 products because of its high tensile strength, relative resistance to acid and temperature, and varying textures and degrees of flexibility. It does not evaporate, dissolve, burn, or undergo significant reactions with other chemicals, which makes asbestos non-biodegradable and environmentally cumulative.

Although many applications have been phased out of production, uses of asbestos have included the following:

Commercial

• Boilers and heating vessels
• Cement pipe
• Clutch, brake, and transmission components
• Conduits for electrical wire
• Corrosive chemical containers
• Electric motor components
• Heat-protective pads
• Laboratory furniture
• Paper products
• Pipe covering
• Roofing products
• Sealants and coatings
• Textiles (including curtains).

Homes and Buildings

• Duct and home insulation
• Fire protection panels
• Fireplace artificial logs or ashes
• Furnace insulating pads
• Fuse box liners
• Heater register tape and insulation
• Joint compounds
• Patching plaster
• Pipe or boiler insulation
• Sheet vinyl or floor tiles
• Shingles
• Textured acoustical ceiling
• Underlayment for sheet flooring.

Asbestos can also be a contaminant in other products such as vermiculite, which is used in gardens, landscape products; and home insulation; and talc, which is used in cosmetics.

Asbestos fibers can result from mining, milling, and weathering of asbestos-bearing rock, and from the manufacture, wear, and disposal of asbestos-containing products. Because of the widespread use of asbestos, its fibers are ubiquitous in the environment.

Insulating materials produced before 1975 commonly contain asbestos.

In industrialized countries, commercial use of asbestos peaked in the 1970s. Although bans and voluntary phaseouts have contributed to declining production of asbestos since the early 1970s, it is still used in construction materials-mostly asbestos-cement products. Building insulation materials manufactured since 1975 may no longer contain asbestos; however, products made or stockpiled before the ban remain in many homes. Vermiculite-contaminated asbestos was produced as late as 1990.

Indoor air can become contaminated with fibers released from building materials, especially if they are damaged or crumbling. Common sources of asbestos in homes are sprayed asbestos ("cottage cheese") ceilings, pipe insulation, boiler coverings, wallboard, and floor and ceiling tiles. Although it is important to repair damaged asbestos as soon as possible, homeowners should not undertake repair or removal of asbestos-containing materials without professional guidance or services.

Although measurable asbestos levels in schools are usually 100 to 1,000 times below the permissible exposure limit for work environments (0.1 fibers/cc [8-hour time-weighted average, or TWA]-see Standards and Regulations section), public concern has led to widespread removal and abatement programs. However, some facilities have higher levels of airborne asbestos after removal than before, indicating that it is essential that any removal of asbestos be done properly.

Street dust can contain fibers from brake linings or crushed asbestos-containing rock used in road construction. Fibrous tremolite, the asbestos commonly found in talc, has also been found in play sand.

The air pathway is the most important route of exposure, but ingestion is possible, both from the water supply and other sources, and from swallowing what is cleared from the lungs. Drinking-water supplies might become contaminated with asbestos from erosion of natural land sources, discarded mine and mill tailings, asbestos cement pipe, and disintegration of other asbestos-containing materials transported via rain. Most water supply concentrations are <1 million fibers per liter, but in some cases have exceeded 100 million fibers per liter. The maximum contaminant level proposed by the U.S. Environmental Protection Agency (EPA) for asbestos in drinking water is 7 million fibers (>10 microns in length) per liter.

About the Author

www.cdc.gov
The Centers for Disease Control and Prevention (CDC) is one of the 13 major operating components of the Department of Health and Human Services (HHS), which is the principal agency in the United States government for protecting the health and safety of all Americans and for providing essential human services, especially for those people who are least able to help themselves.