Chrysotile and amphibole asbestos have different chemical composition, resulting in different biopersistence and effects on health. Chrysotile is considerably less biopersistent and less potent for diseases than amphibole asbestos. Under present use conditions, there is no evidence of chrysotile causing cancer.
Types of asbestos fibres
The main difference between asbestos fibres is distinguished by the mineralogical structure of fibres.
• Amphibole fibres are single solid cylindrical shapes.
• Serpentine fibres are like ropes and are composed of many smaller fibrils.
Figure 1: Mineralogical structure chemistry of chrysotile and amphiboles: Chemically all asbestos minerals are silicates. But mineralogically andcrystallographically, the serpentine and amphibole groups are quite different. (Deer et al., 1966)
Chrysotile
Chrysotile is a sheet silicate and chemically known as magnesium silicate. A mismatch in spacing between magnesium ions and silica ions causeschrysotile fibres to curl into a form of thin rolled sheet (figure 2).
Figure 2: Structural formation of the sheet silica in chrysotile
The external surface of a chrysotile fibril is made of magnesium mineral brucite. Wypych et al. (2005) examined what happens to natural chrysotile fibreswhen acid-leached fibres are under controlled conditions. Leached products consist of layered hydrated disordered silica with a “distorted” structure resembling the silicate layer that exists in the original minerals. Extensive characterization techniques confirmed the removal of the brucite-like sheets, leaving silica with an eminently amorphous structure.
Removal of magnesium from the brucite layer by acid weakens the chrysotile fibrils and eventually destroys their dimensional stability. The sensitivity ofchrysotile to acid dissolution is particularly important in the lung where macrophage scavenger cells are capable of generating a milieu at a pH of ~4.5. If the fibre is swallowed and ingested, it is attacked by an even stronger acid environment (hydrochloric acid, PH2) in the stomach.
Amphiboles
The chemical composition of amphiboles fibres is more complex. Although their structures are the same, there is variability in composition. Such is a direct consequence of the fact that the silicate framework can accommodate a mixture of many different ions (as determined by the host rock) in the space between the silicate ribbons which form the fibres (Speil and Leineweber, 1969)
Crocidolite. . . . . . . . . (Na2Fe32+Fe23+) Si8O22(OH)2
Amosite. . . . . . . . . . . . . . .(Fe2+, Mg)7 Si8O22(OH)2
Tremolite. . . . . , . . . . . . . . . . Ca2Mg5 Si8O22(OH)2
Anthophyllite. . . . . . . . . . (Mg, Fe2+)7 Si8O22(OH)2
Actinolite. . . . . . . . . . .Ca2(Mg, Fe2+)5 Si8O22(OH)2
The external surface of the amphiboles is a quartz-like crystal structure, and has the chemical resistance of quartz. This structure is illustrated for tremolite in figure 3.
Figure 3: Structural formation of the double chain silica amphibole asbestos, tremolite
Differences between health effects of fibres
The fundamental difference in chemical structure and as a consequence in solubility of fibres in the lung resulted in different potency for inducing asbestos-related diseases, for instance – amosite and crocidolite on the order of 100 and 500 times more potent for causing mesothelioma and between 10 and 50 times more potent for causing lung cancer than chrysotile. This difference is caused by different chemical structure, biopersistence (biodurability) and results of recent epidemiological studies. In general more attention should be paid to this very important fact in the proposed document.
“The expert panelists unanimously agreed that the epidemiology literature provides compelling evidence that amphibole fibers have far greater mesothelioma potency than do chrysotile fibers—a finding reported both in the review document (Berman and Crump, 2001) and a recent re-analysis of 17 cohort studies (Hodgson and Darnton, 2000) that reported at least a 500-fold difference in potency. Two panelists commented further that the epidemiology literature provides no scientific support for chrysotile exposures having a role in causation of mesothelioma—an observation that is generally consistent with the meta-analysis in the proposed protocol, which failed to reject the hypothesis that chrysotile fibers have zero potency for mesothelioma.” (Report on the Peer Consultation Workshop to Discuss a Proposed Protocol to Assess Asbestos-Related Risk, EPA USA, 2003, page 3-13)
Hodgson and Darnton (2000) conducted a comprehensive quantitative review of potency of asbestos for causing lung cancer and mesothelioma in relation to fibre type. They concluded that amosite and crocidolite were, respectively, on the order of 100 and 500 times more potent for causing mesothelioma than chrysotile. They regarded the evidence for lung cancer to be less clear cut, but concluded nevertheless that amphiboles (amosite and crocidolite) were between 10 and 50 times more potent for causing lung cancer than chrysotile. (Final Draft: Technical Support Document For A Protocol To Assess Asbestos-Related Risk, EPA USA, 2003. page 8.5)
As chrysotile is a naturally occurring mined fibre, across the range of mineral fibre solubilities chrysotile lies towards the soluble end of the scale and ranges from the least biopersistent fibre to a fibre with biopersistence in the range of glass and stone wools.
It is less biopersistent than the ceramic fibres tested or the special-purpose glasses (Hesterberg et al., 1998a) and considerably less biopersistent than amphiboles. To standardise the evaluation of the biopersistence of fibres a protocol has been developed by a working group for the European Commission which involves a 5 day inhalation exposure followed by analysis of the lungs at periodic intervals up to 1 year post exposure (Bernstein & Riego-Sintes, 1999). For mineral fibres, the clearance half-time of fibers longer than 20 um ranges from a few days to less than 100 days.
Table: Comparative clearance half-times of fibers longer than 20 um and fibers between 5-20 um for chrysotile, synthetic vitreous fibers and amphiboles
| Fiber | Type |
Clearance half time (T1/2) (days) Fibres Length |
Clearance half time (T1/2) (days) Fibres Length |
Reference |
| Calldria chrysotile |
Serpentine asbestos | 0.3 | 7 | Bernstein et al.,2005b |
| Bazilian chrysotile |
Serpentine asbestos | 1.3 | 2.4 | Bernstein et al., 2004 |
| Fibre 13 (BO1.9) |
Experimental Glass wool | 2.4 | 11 | Bernstein et al., 1996 |
| Fibre A | Glass wool | 3.5 | 16 | Bernstein et al., 1996 |
| Fibre C | Glass wool | 4.1 | 15 | Bernstein et al., 1996 |
| Fibre G | Stone wool | 5.4 | 23 | Bernstein et al., 1996 |
| MMVF34 (HT) |
Stone wool | 6 | 25* | Hesterberget al, 1998a |
| MMVF22 | Slag wool | 8.1 | 77 | Bernstein et al., 1996 |
| Fibre F | Stone wool | 8.5 | 28 | Bernstein et al., 1996 |
| MMVF11 | Glass wool | 8.7 | 42 | Bernstein et al., 1996 |
| Fibre J (X607) |
Calcium Magnesium silicate |
9.8 | 24 | Bernstein et al., 1996 |
| Canadianchrysotile (Textile grade) |
Serpentine asbestos | 11.4 | 29.7 | Bernstein et al.,2005a |
| MMVF11 | Glass wool | 13 | 32 | Bernstein et al., 1996 |
| Fibre H | Stone wool | 13 | 27 | Bernstein et al., 1996 |
| MMVF 10 | Glass wool | 39 | 80 | Bernstein et al., 1996 |
| Fibre L | Stone wool | 45 | 57 | Bernstein et al., 1996 |
| MMVF21 | Stone wool | 46 | 99 | Bernstein et al., 1996 |
| MMVF33 | Special purpose glass | 49 | 72* | Hesterberget al, 1998a |
| RCF1a | Refractory ceramic | 55 | 59* | Hesterberget al, 1998a |
| MMVF21 | Stone wool | 67 | 70* | Hesterberget al, 1998a |
| MMVF32 | Special purpose glass | 79 | 59* | Hesterberget al, 1998a |
| Amosite | Amphibole asbestos | 418 | 900* | Hesterberget al, 1998a |
| Crocidolite | Amphibole asbestos | 536 | 262 | Bernstein et al., 1996 |
| Tremolite | Amphibole asbestos | ? | ? | Bernstein et al.,2005a |
The clearance half-time (T1/2) for fibres 5-20 um in length was not reported by Hesterberg et al. (1998a); the values shown were calculated from raw data compiled in studies by Bernstein.