Asbestos is a commercial term given to 6 naturally occurring minerals that are incombustible and separable into filaments: chrysotile, amosite, crocidolite, anthophyllite, tremolite and actinolite. Only the first three have wide-spread commercial use. Chrysotile is a member of the serpentine group of minerals; crocidolite, amosite and the others belong to the amphibole group.
Chrysotile, or white asbestos, currently accounts for more than 98% of world asbestos consumption. Its fibres are characterized by high tensile strength, resistance to alkalies, high flexibility and good spinnability.
In some countries, and for some special applications, the amphiboles continue to be used. Crocidolite or blue asbestos is the strongest of the asbestos fibres. It has high tensile strength and acid resistance. Amosite or brown asbestos, is highly resistant to heat and quite flexible, but may be susceptible to strong acids and alkalies. It has less tensile strength than chrysotile or crocidolite and has only fair spinnability.
| Back to chapter index | Back to Manual index |
1.2 - OTHER NATURAL AND MAN-MADE FIBRES
Chrysotile asbestos is but one of many natural and man-made fibres (see Table 1). Other naturally occurring mineral fibres include wollastonite and attapulgite. Man-made mineral fibres include glass, rock and slag wool, refractory ceramic fibres, whiskers made of silicon carbide. Important organic fibres of interest are the para-aramids and polyacrylicnitrate fibres.
| Back to chapter index | Back to manual index |
1.3 - USE OF FIBRES AND PRODUCTS
Chrysotile asbestos is found in a wide range of practical applications, including:
€ As a reinforcing agent in the asbestos-cement industry, whose products include pipe, shingles and sheets;
€ As a fire retardant in textiles and paper products;
€ As a friction product in brakes and clutch linings;
€ As an agent to improve wear in vinyl floor tiles;
€ As a cohesive agent in asphalt surfaced roadways;
€ As a filler in resins, plastics, caulking and sealants;
€ As a resisting agent to acids and alkalies and battery boxes, packings, acid pumps, valves and gaskets; and,
€ As a filtering material in the chemical, drug and food industries.
Chrysotile asbestos was also used as an insulation material in various fire resistant, thermal and acoustic insulation applications, including pipe and boiler lagging. These applications of chrysotile asbestos are no longer allowed in most countries of the world today, because of the risk to workers during installation and removal.
Man-made fibres are widely used in insulation applications, which represent over 80% of current day uses. However, they also have a wide variety of applications in friction products, textiles, acoustic, ceiling tiles, high efficiency filtration, etc.
| Back to chapter index | Back to Manual index |
1.4 - ASBESTOS-RELATED DISEASES
For many years, dust concentrations of up to 200 f/cc could be found in certain asbestos industry sectors, particularly those involving the use of friable (spray-on) applications. In many workplace environments, dust clouds were so thick that it was difficult for workers to see each other even at short distances. Construction and maintenance workers were subject to high intermittent exposure on work sites. This resulted in a number of asbestos-related diseases among workers, including:
€ Asbestosis: the scarring of the lung tissue which can impede normal respiratory function and possibly result in death due to heart failure, respiratory infections; or lung cancer;
€ Lung cancer: malignant, invasive growth or tumour in the lung; and,
€ Mesothelioma: malignant tumour of the lining of the chest or abdominal cavities.
Evidence regarding the association of asbestos exposure to other various diseases is unclear, particularly in the case of laryngeal cancer and ovarian cancer. In the case of gastrointestinal cancer, laboratory experiments have failed to produce gastrointestinal tumours in animals exposed to asbestos. In addition, epidemiological studies conducted to date provide little convincing evidence of an association between asbestos in public water supplies and cancer. Therefore there is broad scientific consensus that asbestos does not present a significant risk of cancer by the route of ingestion.
| Back to chapter index | Back to Manual index |
The greatest cause of lung cancer is undoubtedly smoking tobacco. However, lung cancer also occurs among asbestos workers exposed to high levels of dust. This risk is significantly increased if the asbestos worker also smokes.
Many studies have shown a synergism of action between asbestos exposure and smoking. Therefore, all asbestos workers should refrain from smoking on the work site and be encouraged to stop smoking altogether. While there is still some controversy, it is believed by many scientists that asbestosis is a necessary precondition for the development of lung cancer associated with asbestos.
| Back to chapter index | Back to Manual index |
1.6 - THE THREE D'S OF FIBRE-RELATED DISEASES
The 1990's have seen significant advances in the understanding of mechanisms of fibre-induced disease. Today it is known that the risk of fibre related disease is determined by essentially 3 factors:
€ Dose - the quantity of dust inhaled over time;
€ Dimension - whether a material generates respirable dust; that is dust that contains fibres which are longer than 5 microns in length and less than 3 microns in diameter with an aspect ratio greater than 3 to 1; and,
€ Durability - or biopersistence of such dust in the human lung.
In general, long, thin, durable fibres are the most hazardous to health. Fibres shorter than 5 microns in length are not considered to present a health risk because of the ability of the body's natural defense mechanisms to deal effectively with contaminants of this size and nature.
Based on both animal and human studies, all forms of asbestos have been classified as Class I "known human carcinogens" by the International Agency for Research on Cancer. However, there is also a clear international scientific consensus that chrysotile is less hazardous than the amphibole varieties of asbestos. Not only is chrysotile a less dusty material because of its physical properties, it also has a tendency to break down and dissolve in body fluid and thus is more easily cleared from the lungs by natural defense mechanisms.
Amphibole forms, on the other hand, are more dusty materials as well as more durable and persist in the lung once inhaled. The increased potency of amphibole forms of asbestos to chrysotile is corroborated by both animal and human studies.
| Back to chapter index | Back to Manual index |