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What Is Asbestos?

General Definition

Asbestos is the name of a group of highly fibrous minerals with separable, long, and thin fibers. Separated asbestos fibers are strong enough and flexible enough to be spun and woven. Asbestos fibers are heat resistant, making them useful for many industrial purposes. Because of their durability, asbestos fibers that get into lung tissue will remain for long periods of time.

For more information on asbestos, see ATSDR's Toxicological Profile on Asbestos. Other ATSDR resources include the  Public Health Statement on Asbestos, which is the summary chapter from the Toxicological Profile, and the ToxFAQs for Asbestos, which is a shorter question and answer version.

Types of Asbestos

There are two general types of asbestos, amphibole and chrysotile. Some studies show that amphibole fibers stay in the lungs longer than chrysotile, and this tendency may account for their increased toxicity (harmfulness to the body).

Scanning electron micrograph of asbestiform amphibole

Scanning electron micrograph of asbestiform amphibole from a former vermiculite mining site near Libby, Montana. Source: U.S. Geological Survey and U.S. Environmental Protection Agency, Region 8, Denver, Colorado.

See more pictures of asbestos.

Regulatory agencies such as the U.S. Environmental Protection Agency (EPA) and Occupational Safety and Health Administration (OSHA) recognize six asbestos minerals: chrysotile, a serpentine mineral with long and flexible fibers; and five amphibole (with relatively brittle crystalline fibers) minerals, actinolite asbestos, tremolite asbestos, anthophyllite asbestos, crocidolite asbestos, and amosite asbestos.  

How Are People Exposed to Asbestos

We are all exposed to low levels of asbestos in the air. These "ambient" - or typical - air concentrations of asbestos fibers are 0.00001 to 0.0001 fibers per milliliter (fiber/mL). Much more concentrated levels of exposure are known to cause health effects in humans. For more information on asbestos exposure, see the Public Health Statement on Asbestos,  "How might I be exposed to asbestos?"

Asbestos Exposure and Your Work

Many people have come into contact with asbestos fibers through their jobs (occupational exposure). Some of the work environments or occupations in which workers are now or were exposed in the past include:

Work Environments

  • Asbestos product manufacturing (insulation, roofing, building, materials)
  • Automotive repair (brakes & clutches)
  • Construction sites
  • Maritime operations
  • Mining operations
  • Offshore rust removals
  • Oil refineries
  • Power plants
  • Railroads
  • Sand or abrasive manufacturers
  • Shipyards / ships / shipbuilders
  • Steel mills


  • Asbestos removal workers
  • Workers at exfoliation facilities where vermiculite ore from Libby, Montana, was processed
  • Demolition workers
  • Workers at asbestos product manufacturing plants
  • Auto mechanics
  • Boilermakers
  • Bricklayers
  • Building inspectors
  • Carpenters
  • Drywallers
  • Electricians
  • Floor covering manufacturers or installers
  • Furnace workers
  • Glazers
  • Grinders
  • Hod carriers
  • Insulators
  • Iron workers
  • Laborers
  • Longshoremen
  • Maintenance workers
  • Merchant marines
  • Millwrights
  • Operating engineers
  • Painters
  • Plasterers
  • Plumbers
  • Roofers

Individuals who have worked in the above work environments and occupations should consult with a physician with expertise in the evaluation and management of asbestos-related lung disease.

Information on Asbestos from OSHALeaving ATSDR Web site
NIOSH Information on Asbestos Exposure in the WorkplaceLeaving ATSDR Web site
NIOSH Fact Sheet on Contaminated VermiculiteLeaving ATSDR Web site

Chrysotile Asbestos Exposure

The asbestos fibers detected in the samples taken at the World Trade Center sites were chrysotile asbestos.  Click here to link to more World Trade Center information.

Amphibole Asbestos Exposure

  • Mining activities — Exposure to tremolite asbestos (a type of amphibole asbestos) can occur in workers involved in mining, milling, and handling of other ores and rocks containing tremolite asbestos (such as vermiculite or talc).  Residents who live near mining, milling, or manufacturing sites that involve tremolite asbestos-containing material may be exposed to higher levels of airborne asbestos.
A vermiculite mine in Libby, Montana

A vermiculite mine in Libby, Montana. Asbestos has been detected in vermiculite from this mine.

  • Insulation and Building Materials — Amphibole asbestos can be found in a variety of building materials, such as insulation, ceiling or floor tiles, and cement pipes. Amphibole asbestos has been found in some vermiculite sources used as home and building insulation. Much of the asbestos-contaminated vermiculite mined in Libby, Montana, was used to produce attic insulation products.
Bagged asbestos awaits disposal in a landfill

Bagged asbestos awaits disposal in a landfill.

Workers or homeowners involved in demolition work, maintenance, repair, or remodeling of buildings containing these products can be exposed to higher airborne fibrous amphibole levels than levels in ambient air. However, exposure can occur only when materials containing asbestos are disturbed in some way to release fibers into the air. When asbestos-containing materials are solidly embedded or contained, exposure risk will be minimal.

ATSDR, in conjunction with the EPA recently published a brochureLeaving ATSDR Web site which identifies potential health hazards posed by asbestos in certain building insulation products. The brochureLeaving ATSDR Web site provides information on how to identify these products and steps individuals can take to reduce exposure. Also, see ATSDR's Vermiculite Consumer Products Fact Sheet.

  • Consumer Products — Vermiculite was also commonly sold in gardening and hardware stores. It was used as a soil amendment (conditioner to improve soil quality) or fertilizer carrier, and it was an ingredient in many potting soil mixtures. Learn More

In addition, small amounts of amphibole asbestos have been found in some talc-containing crayons. The Consumer Product Safety Commission (CPSC) concluded that the risk is extremely low that children would be exposed to asbestos fibers in crayons. The U.S. manufacturers of these crayons, however, have agreed to eliminate talc from their products.

Detecting Asbestos

The combined use of detection methods called light microscopy, electron microscopy, and energy dispersive X-ray analysis offer the most accurate approach to identify asbestos and to estimate concentrations that may become airborne upon disturbance. For the purposes of counting asbestos fibers in these samples, regulatory agencies commonly count as fibers those particles of asbestos minerals at least 5 micrometers in length and with length:width ratios of 3:1. For other purposes, such as detecting fibers in bulk building materials, asbestos particles with length:width ratios of 5:1 are counted. Air concentrations of asbestos fibers in ambient (typical) air are 0.00001 to 0.0001 fibers per milliliter (fiber/mL). The recently established exposure limit for U.S. workplaces is 0.1 fiber/mL.

Vermiculite Overview

Vermiculite is a group of minerals with a flaky, mica-like structure. It is mined for its uses in insulation and gardening.

While vermiculite's toxicity has not been studied completely, to date no research has linked serious health effects with exposure to this mineral.

Tremolite Asbestos

Tremolite Asbestos

Sample of vermiculite ore mined in Libby, Montana

Sample of vermiculite ore mined in Libby, Montana

Raw (right hand) and popped (left hand) vermiculite

Raw (right hand) and popped (left hand) vermiculite.
(Photo source: U.S. Geological Survey and U.S. Environmental Protection Agency,   Region 8,  Denver, Colorado)

When heated, vermiculite exfoliates (or "pops"), forming a lightweight material ideal for packing, insulation, and as a soil additive. If the vermiculite is contaminated with asbestos, the exfoliation process releases asbestos fibers into the air.

 Most vermiculite is not contaminated with asbestos. However, the vermiculite ore taken from a mining operation in Libby, Montana, is contaminated with asbestiform (asbestos-like) amphibole minerals, including the regulated forms tremolite and actinolite. It also contains forms that are not currently regulated, including winchite, richterite, and ferro-edenite. Research has linked all of these forms to asbestos-related diseases.

Vermiculite was mined from Zonolite Mountain in Libby for more than 65 years (until 1990). The mine itself is approximately six miles from the town. A transfer facility was located approximately three miles from Libby. From the transfer facility, vermiculite was loaded on trains or trucks. Two expansion ("popping") facilities operated at different times within the town; these plants heated vermiculite to approximately 600 degrees Fahrenheit to expand the crystals. One of these facilities was next to a baseball field, which was readily accessible to the community's children.

The exfoliation process released asbestos fibers from the vermiculite ore into the air, where they could be inhaled. Inhalation of asbestos fibers suspended in air can result in lung diseases such as  asbestosis, mesothelioma and lung cancer. The risk of developing any of these diseases depends on many factors, including the type of fiber, the level and duration of exposure, and the smoking history of the exposed individual. (See Health Effects of Exposure for more information.)

While current airborne asbestos levels in Libby now appear to be low, levels were certainly much higher during the many decades that vermiculite was actively mined, processed, and shipped. In fact, air concentrations up to 15 times current occupational limits were once reported for downtown Libby in the past.

Studies by the NIOSHLeaving ATSDR Web site and McGill UniversityLeaving ATSDR Web site investigators found that former employees of the mine had substantial occupational exposure to these asbestiform minerals. These investigators documented pulmonary abnormalities and disease (asbestosis and lung cancer and mesothelioma) among employees. Cases of asbestos-related lung diseases have also been reported among people who lived with mine employees as well as others in the community.

This page last updated on October 30, 2008