When any natural or man-made fibrous respirable substances are inhaled most fibres are expelled, but some can become lodged in the lungs and remain there throughout life. Fibres can accumulate and cause scarring and inflammation. Severe scarring and inflammation can affect breathing and increase the risk of lung cancer. Fast removal of a fibre from lungs decreases the risk of fibre induced health effects. The ability of a substance to persist in the lung, in spite of the lung’s physiological clearance mechanisms and environmental conditions, is known as biopersistence. It is generally agreed that the durability of a respirable fibre is a major factor for the characterization of potential adverse health effects. For example, the European Union specifies in Directive 97/69/EC on the Classification, Packaging and Labelling of Dangerous Substances, that a biopersistence test must be undertaken to evaluate a fibre classification as a carcinogen.
Chrysotile is cleared rapidly from the lung. Recent animal experimentations (2003 to 2006, in Brazil, Canada and the USA), performed according to the most stringent protocols recognized by the European Union, show that soon after chrysotile fibres are inhaled, they are quickly cleared from the lungs – in a matter of about 10 days. However, amphiboles, which resist the acidic environment of the lungs, are not cleared as rapidly. The amphibole fibres may remain in the lungs for periods up to a year or more.
To that effect, the animal experimentations thus bring robust support to many epidemiological observations published in the past. They also support the more recent benchmark publication by Hodgson and Damton (2000), showing that amphiboles are 100 times more potent than chrysolile. In fact, chrysolile has a much lower biopersistency than most of the other industrial fibres (some celluloses, ceramic fibres, aramid, rock wool and glass wool).
Evidence from morbidity, mortality and lung burden studies supports the concept of a much lower pathogenic potential for chrysotile compared to amphiboles. There are no less than 25 reports, from human studies, published in the last 25 years, pointing to the definite differences in biological effects and potencies of chrysotile and amphibole asbestos varieties. One of the most important studies in terms of cohort dimensions was done by Liddell, McDonald & McDonald in 1997. The results have shown no evidence of increased cancer risk from chrysotile exposure at presently regulated occupational exposure levels (~1 f/ml, 8-hour time weighted average), as recommended by the Group of Experts convened by the WHO in Oxford (1989).
Biopersistence of Several Respirable Fibres
Source: Undeniable Facts about chrysotile – Chrysotile Institute