Asbestos is a naturally occurring group of minerals that can be identified only under a microscope. It is a flexible, highly fire-resistant fiber with high tensile strength. It has been mined for its useful properties related to thermal insulation, chemical and thermal stability and high tensile strength.
There are two types of asbestos, based on the crystalline structure. Amphibole are long thin fibers that form a chain-like crystal structure. The serpentine asbestos is shorter, thicker and curlier. It has a sheet or layered crystalline structure. The serpentine type of asbestos has only one member and it is chrysotile. Chrysotile makes up approximately 90 to 95 percent of the asbestos contained in buildings of the United States.
In the amphibole group, there are five members:
Amosite is the most common form of asbestos found in buildings in the U.S., next to the chrysotile. Amosite is also known as brown asbestos. Blue asbestos, or crocidolite, is the third most common form of this mineral contained in buildings. Anthophyllite, tremolite and actinolite are quite rare and are typically found in the compounds of the other more common types of asbestos, but rarely on their own.
Asbestos is made up of microscopic bundles that become airborne when disturbed. These fibers get into the air and then can be inhaled by persons exposed to that air. These fibers can cause significant health problems because they become embedded in lung tissue and cannot be removed by the body's natural immune system.
When Is Asbestos a Hazard?
This mineral may not always be an immediate hazard if it is not airborne. If asbestos can be maintained in good condition, it should be left alone, with periodic inspections. It is only when these asbestos-containing materials get disturbed that they can become airborne and can be a problem.
In the asbestos industry, the term "friable asbestos" is used to describe asbestos that can be reduced to dust by hand pressure. Asbestos that is too hard to be reduced to dust by hand pressure is called "non-friable asbestos." Non-friable asbestos can be turned into the friable version by machine grinding, sanding or dry-buffing. Non-friable asbestos materials such as flooring and siding are not regulated, provided they do not become friable.
Some researchers in the asbestos industry have made the claim that chrysotile asbestos is not dangerous because it is short and curly, and thus when inhaled, it is less likely to become lodged in the deepest parts of the lungs. The amphibole are long and thin fibers, and these were thought to be the most dangerous.
However, a number of researchers have investigated this assertion and found it to be false. The results show that chrysotile is a dangerous form of asbestos. Epidemiological research leaves no doubt that the scientific evidence supports the carcinogenicity of chrysotile and that it alone can cause malignant mesothelioma. Also, animal studies have shown that when chrysotile is inhaled, it does become lodged in the deepest parts of the lungs and is not easily removed by the immune system. Today, chrysotile is the only type of asbestos mined in any great quantity.
[Page updated January 2006]