Health risk of chrysotile revisited

Figures & data

Figure 1.  Schematic illustration of the chrysotile fiber. Chrysotile is a rolled sheet or concentrate rings of silicate with the magnesium on the outside of the sheet and the silica on the inside. The chrysotile fiber is acid soluble. Chrysotile has the formula Mg₃Si₂O₅(OH)₄. The fiber consists of magnesium hydroxide layers condensed onto silicon·oxygen tetrahedra. The fiber walls are made up of 11 to 21 such layers in which there is some mechanical interlocking. There is not any chemical bonding as such between the layers, however. Each layer is about 7.3 Å thick. The Mg(OH)₂ part of the molecule layers is closest to the fiber surfaces; the silicon–oxygen tetrahedra are inside. Under the acid conditions associated with the macrophage, the fiber structure is weakened and the long fibers break into short pieces which can be engulfed and cleared by the macrophages.

Figure 2.  With amphiboles, the soluble cations shown as small circles are located between the fibers which are formed with double chain silicate. When the soluble cations dissolve as can happen in the lung, the amphibole fibers in these bundles are released as individual fibers. The double chain silicate amphibole fibers themselves are highly insoluble in both the lung fluids and in the macrophages.

Table 1.  Capabilities and limitations of analytical techniques used for asbestos measurements (reproduced from Berman & Crump, 2003)†.

Parameter	Midget impinger	Phase contrast microscopy	Scanning electron microscopy	Transmission electron microscopy
Range of magnification	100	400	2000–10 000	5000–20 000
Particles counted	All	Fibrous structures‡	Fibrous structures‡	Fibrous Structures‡,§
Minimum diameter (size)	1 µm	0.3 µm	0.1 µm	<0.01 µm
Visible
Resolve internal structure	No	No	Maybe	Yes
Distinguish mineralogy¶	No	No	Yes	Yes

†The capabilities and limitations in this table are based primarily on the physical constraints of the indicated instrumentation. Differences attributable to the associated procedures and practices of methods in common use over the last 25 years are highlighted in Table 2.

‡Fibrous structures are defined here as particles exhibiting aspect ratios (the ratio of length to width) greater than 3 (Walton, 1982).

§TEM counts frequently resolve individual fibrous structures within larger, complex structures. Based on internal structure, several different counting rules have been developed for handling complex structures. See the discussion of methods presented below.

¶Most SEM and TEM instruments are equipped with the capability to record selected area electron diffraction (SAED) spectra and perform energy dispersive X-ray analysis (EDXA), which are used to distinguish the mineralogy of structures observed.

Table 2.  Epidemiological studies characterized as predominately chrysotile exposure by Hodgson & Darnton (2000).

Study	Referred to as:
Dement et al. (1994) and  McDonald et al. (1983)	South Carolina
Piolatto et al. (1990)	Balangero Italian mine and mill
Liddell et al. (1997)	Quebec
Hughes et al. (1987)	New Orleans (plant 2, y)
McDonald et al. (1984)	Connecticut

Table 3.  Studies characterized as predominately chrysotile exposure (Hodgson & Darnton, 2000).

Study	Exposure estimates	Smoking histories	Fiber specificity	Lung cancer risk (% expected lung cancer per fiber/cm^3 year)† RL (95% CI)	Mesothelioma risk (% total expected mortality per fiber/cm^3 year)† RM age adjusted 95%CI
South Carolina: Dement et al. (1994) and McDonald et al. (1983)	MI measurements 1930–1975 In 1968 and 1971, both impinger and PCM samples were collected (a total of 986 samples)	Based on two surveys conducted by the U S Public Health Service in 1964 and 1971 and on data collected by the company	Chrysotile textile plant. Crocidolite yarn was used in small quantities to make tape or braided packing from 1950s until 1975	Women 6.7 (3.6, 11) Men 4.6 (2.9, 6.7)	Women 0 (0.0, 0.35) Men 0.013 (0.0016, 0.047)
Balangero: Piolatto et al. (1990)	Fiber levels were measured by PCM in 1969. In order to estimate earlier exposures, information on daily production, equipment changes, number of hours worked per day, etc. were used to create conditions at the plant during earlier years. PCM samples were obtained under these simulated conditions and combined with work histories to create individual exposure histories	No information on smoking	Chrysotile mine and mill with presence of Balangeroite fiber	0.03 (−0.11, 0.24)	0.0025 (0.0003, 0.009)
Quebec: Liddell et al. (1997)	MI measurements Conversions between dust levels and PCM concentrations were derived from side-by-side samples	Smoking history was obtained in 1970 by a questionnaire administered to current workers, and to proxies of those who had died after 1950	1. Chrysotile mine and mill at the town of Asbestos 2. Factory at the town of Asbestos that, in addition to processing chrysotile, had also processed some crocidolite 3. Chrysotile mining and milling company complex near Thetford Mines (evidence of greater amounts of tremolite in the ore at Thetford Mines) 4. Number of smaller mines and mills also in the vicinity of Thetford Mines	0.06 (0.042, 0.079)	0.0009 (0.0006, 0.0013)
New Orleans (plant 2): Hughes et al. (1987)	MI measurements initiated in the early 1950s Levels estimated from initial samples in the 1950s were also assumed to hold for all earlier periods because no major dust control measures had been introduced prior to that time In plant 2, the revised estimates tended to be about one-third of the previous estimates through the 1940s and about one-half the previous estimates thereafter	Based upon a cross sectional study of over 95% of workers employed in these plants in 1969. Information concerning the smoking habits of earlier workers in these plants is not available	Plant 1: Some amosite was used from the early 1940s until the late 1960s, constituting about 1% of some products, and crocidolite was used occasionally for approximately 10 years beginning in 1962 Plant 2: Utilized only chrysotile, except that pipe production, which began in 1946 and was housed in a separate building, produced a final product that contained about 3% crocidolite	0.81 (0.21, 1.6)	0 (0, 0.033)
Connecticut: McDonald et al. (1984)	Dust levels from impinger measurements were available for the years 1930, 1935, 1936 and 1939. There was little other exposure information available until the 1970s. No conversion from MPPCF to fiber/cm^3 value was suggested by the authors	No information on smoking	Plant that manufactured asbestos friction products. The plant began operation in 1913 and used only chrysotile until 1957, when a little anthophyllite was used. Also, a small amount of crocidolite (about 400 pounds) was handled experimentally between 1964 and 1972	0.80 (0.029, 1.8)	0 (0, 0.016)

†Risk estimates as determined by Hodgson & Darnton (2000).

Figure 3.  Map of North Charleston showing the location of the Textile plant (GARCO) and the US Navy Yard. The distance from GARCO to the Navy Yard is a few hundred meters. The width of the map is approximately 3.5 km.

Table 4.  Concentrations of fiber and dust for workers in major sections of the Chongqin, China, asbestos plant, by job category, 1999. (Reproduced from Yano et al’s)*.

Job category	Fiber (fibers/ml (range))	Dust (mg/m3 (range))
Raw material (opening)	6.5 (5.8–7.5)	8.8 (6.1–12.3)
Raw material (bagging)	12.6 (5.2–58.4)	18.2 (14.5–22.4)
Rubber plate†	2.8 (2.6–3.1)	237.5 (176.0–320.5)
Textile	4.5 (0.7–17.0)	22.4 (15.8–35.5)
Asbestos cement‡	0.1	22.3

*Geometric mean (range) for three to five workers exposed in each asbestos plant section.

†In the rubber plate section, workers were engaged mainly in dumping mica and various raw materials into a pit in a small room without ventilation.

‡In the asbestos cement section, the number of workers in the dusty environment was small, and only one worker who was engaged in dumping raw materials into the pit was monitored.

Table A1.  Chronic inhalation studies with chrysotile.

Fiber type	ExposureTime (h/d, d/wk, total months)	Fiber concentration (f/cm^3)	Fiber mass concentration (mg/m^3)	Equivalent fiber concentration/ cm^3 TEM	Type, total number of rats	Number of pulmonary tumors	% of Pulmonary tumors	Number of mesotheliomas	References
Chrysotile Canadian (nickel, cobalt, chromium and lead contamination)	6, 5, 14	Nd	86 mg/m^3 42–106 mg/m^3 first year67–146 mg/m^3 second year	8 600 000	NS,41	10	24	1	Gross et al. (1967)
Chrysotile UICC Canadian	7, 5, 24	Nd	9.7 mg/m^3	970 000	W, 21	10	48	1	Wagner et al., (1974)
Chrysotile UICC Rhodesian	7, 5, 24	Nd	14.7 mg/m^3	1 470 000	W, 17	11	65	0	Wagner et al., (1974)
Chrysotile Canadian 714-7D (friction linings)	5, 5, 24	1.7 × 10^5 SEM 9978 > 5 µm	15 mg/m^3 Total 5.2 mg/m^3 response	1 500 000	W, 45	9	20	0	Le Bouffant et al. (1987)
SFA chrysotile	7, 5, 24	430 > 5 µm PCOM 669 particles PCOM	10.8 mg/m^3	1 080 000	W, 22	8	36	1	Wagner et al., (1980)
Grade 7 chrysotile	7, 5, 24	1020 > 5 µm PCOM 745 particles PCOM	10.8 mg/m^3	1 080 000	W, 24	3	13	0	Wagner et al., (1980)
UICC chrysotile	7, 5, 24	3750 > 5 µm PCOM 338 particles PCOM	10.8 mg/m^3	1 080 000	W, 23	5	22	0	Wagner et al., (1980)
Chrysotile Calidria	5, 5, 12	241 by SEM 131>5 µm reported as “thick bundles”	6 mg/m^3	600 000	W, 50	0	0	0	Muhle et al. (1987)
Chrysotile long	7, 5, 12	1170 > 5 µm PCOM 33 > 20 µm PCOM 12% > 5 µm SEM	10 mg/m^3 0.5% > 20 µm SEM	1 000 000	W, 40	20	50	2	Davis et al. (1988)
Chrysotile short	7, 5, 12	5510 > 5 µm PCOM 670 > 20 µm PCOM 7% > 5 µm SEM	10 mg/m^3 0.2% > 20 µm SEM	1 000 000	W, 40	7	17	1	Davis et al. (1988)
Chrysotile UICC A	7, 5, 12	2560 > 5 µm PCOM	10 mg/m^3	1 000 000	Included for comparative fiber numbers  without animal exposure				Davis et al. (1988)
Chrysotile NIEHS	6, 5, 24	1.02 × 10^5 SEM 1.06 × 10^4 > 5 µm	10 mg/m^3	1 000 000	F, 69	13	18	1	Mast et al. (1995) and Hesterberg et al. (1993)
Chrysotile	7, 5, 12	1950 > 5 µm PCOM 360 > 20 µm PCOM	10 mg/m^3	1 000 000	W, 40	15	38	0	Davis et al. (1978)
Chrysotile	7, 5, 12	390 > 5 µm PCOM 72 > 20 µm PCOM	2 mg/m^3	200 000	W, 42	8	19	1	Davis et al. (1978)
Author’s reported “With a 2 mg/m^3 cloud the percentage retention of chrysotile is almost double that for a 10 mg/m^3 cloud”.
Chrysotile Calidria	7, 5, 12	Nd	7.78 mg/m^3	778 000	F, 51	2	4	0	Ilgren & Chatfield (1997, 1998) and Pinkerton et al. (1983)
Chrysotile Jeffrey	7, 5, 12	Nd	11.36 mg/m^3	1 136 000	F, 49	11	22	0	Ilgren & Chatfield (1997, 1998) and Pinkerton et al. (1983)
Chrysotile UICC/B	7, 5, 12	Nd	10.99 mg/m^3	1 099 000	F, 54	13	24	0	Ilgren & Chatfield (1997, 1998) and Pinkerton et al. (1983)

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