Number 9, March 2005

» Chrysotile better than PVA
» Another product to be banned by France?
» Cancer in the kitchen?
» Up in the air (by Sophie Stone)
» Read for you : Solving the asbestos imbroglio in the United States
» The final proof
The final proof

Scientists David M.Bernstein, Rick Rogers and Paul Smith just published their 365-day study regarding the biopersistence of Canadian chrysotile following inhalation. Here is their conclusion.

At one year after cessation of exposure, no long chrysotile fibres (longer than 20 microns) remained in the lung. In contrast, with amosite asbestos there were 4 x 105 long fibres remaining in the lungs at one year after cessation of exposure (Hesterberg et al., 1998).

The results of this and the other chrysotile biopersistence studies clearly differentiate the serpentine chrysotile from amphiboles. Chrysotile fibres clears in a range similar to that of glass and stone wools. It remains less biopersistent than ceramic and special-purpose glasses and considerably less biopersistent than amphibole asbestos.

In another biopersistence study, which compared chrysotile to the amphibole tremolite, histopathological examination of the lungs following cessation of exposure showed no inflammatory response to the chrysotile at doses that did not produce lung overload. This was in marked contrast to the tremolite, which at a similar exposure concentration produced a pronounced inflammatory response that quickly evolved into fibrotic granulomas and interstitial fibrosis (Bernstein et al, 2003b).

Animal exposure

To perform their study, the scientists worked with a group of 56 weanling (approximately 9 weeks old) male Wistar rats (specific pathogen free quality). The rats were exposed by flow-past nose-only exposure to a target fibre aerosol concentration of 200 fibres (about 20 microns) per cubic centimetre, for 6 h/day for a period of five consecutive days. The total number of fibres in the exposure atmosphere was 14,805 fibres/cm3. Please note that the concentration of chrysotile fibres in the Canadian mines environment is about 0.3 fibre/cm3 of air.

This concentration corresponded to two times that required by the EC Biopersistence Protocol in order to assure that there was no question of sufficient long fibre exposure. In addition, a negative control group was exposed in a similar fashion to filtered air. Wistar rats (HanBri:WIST, SPF) obtained from RCC Ltd, Biotechnology and Animal Breeding Division, CH-4414 Füllinsdorf, Switzerland, were used.

Fibre clearance

At one day, two days, seven days, two weeks, one month, three months and twelve months post exposure, the lungs from the groups of animals were digested and reduced to ashes, to be subsequently analyzed by transmission electron microscopy for total chrysotile fibres number in the lungs and chrysotile fibre size (length and diameter) distribution in the lungs.

The clearance half-time and regression coefficients for fibres longer than 20 microns of Canadian chrysotile is 11.4 days. Which means that after 11.4 days, half of the fibres were eliminated from the lungs. For fibres measuring between 5 and 20 microns, clearance half-time is 29.7 days. Finally, for fibres shorter than 5 microns, clearance half-time is 108.4 days. Please keep in mind that the animals were expose to an atmosphere containing 14,805 fibres/cm3.

Please recall that the importance of fibre biopersistance in the lung was subject of a scientific working group (19 experts from 11 countries) under the aegis Centre International de Recherche sur le Cancer (International Research Centre for Cancer). These experts concluded that if a fibre dissolves quickly and disappears from the lung, it does not cause a carcinogenic effect. In 1997 that concept was integrated into the synthetic fibres directive of the European Union, i.e; if fibres longer than 20 microns (recognized has having the strongest carcinogenic potential) disappear with a half-life of less than 10 days, they are then exempted of the “carcinogens” labeling.

And for those still associating chrysotile and immediate death, poison and on and on: neutrophils were not observed in alveolar spaces in any of the data volumes collected. Neutrophil-mediated inflammatory response did not occur in the presence of chrysotile fibres at any of the time points examined.

Canadian, but also Californian and Brazilian

Currently, two other chrysotile samples have also been evaluated in similar inhalation biopersistence studies. These are Calidria chrysotile (Bernstein et al., 2003b) from California and Brazilian chrysotile (Bernstein et al., 2004). Both of the chrysotile samples also cleared rapidly with the fastest reported clearance half-times for fibres longer than 20 microns in length.

These results fully support the differentiation of chrysotile from amphiboles reported in recent evaluations of available epidemiological studies (Hodgson & Darnton, 2000; Berman & Crump, 2004). The contrast between serpentine asbestos and amphibole asbestos is clearly apparent, with the clearance half-time of the amphiboles more than 50 times that of chrysotile.

The authors realize that these results appear to contradict those of many other investigators. Indeed, there is evidence that humans can and do develop lung cancer from exposure to chrysotile asbestos, when the exposure is high and sustained for long periods. The value of this and other similar studies is that it shows that at low exposure to pure chrysotile is probably not hazardous. It also suggests that the hazard may be low if even high exposures were of short duration.

The scientists Bernstein, Rogers and Smith concluded their study with the following: It would be most helpful if future studies on chrysotile and amphiboles, whether in vitro or in vivo, could be performed at doses approaching those to which human have been exposed.
While magnesium is an important part of both chrysotile (about 33%) and amphiboles (6–25%), in chrysotile the magnesium molecule is on the outside of the curled chrysotile structure. This is of particular importance in that magnesium is soluble in the lung fluids and can be readily leached from the surface. With amphiboles, the magnesium is locked within the I-beam type structure, which consists of corner-linked (SiO4)4 tetrahedra linked together in a double-tetrahedral chain that sandwiches a layer with the Ca2Mg5. Simply put, amphibole fibres are solid cylindrical shapes, while chrysotile fibres appear more like a rope made up of tiny fibrils that can unwind into smaller pieces.

Fibrous chrysotile has been shown as well to be acid soluble in contrast to the amphiboles. Hargreaves and Taylor (1946) reported that if fibrous chrysotile is treated with dilute acid the magnesia can be completely removed, and the hydrated silica remaining, though fibrous in form, completely lost the elasticity characteristic of the original chrysotile and gives an x-ray pattern of one or perhaps two diffuse broad bands indicating that the structure is “amorphous” or “glassy” in type. This difference in characteristics is also important in the lung, where the macrophage is capable of generating a milieu at a pH of about 4.5.