Asbestos fibres and human health

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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
>20 um

Clearance half time (T1/2) (days)

Fibres Length
5-20 um

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.

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