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970, :ena ed.. Uni- Chemical characteristics of asbestos and Ex- associated trace elements trial A. MORGAN '4L.J. CRALLEY* t i A I The known varieties of asbestiform minerals can be classified either as serpentine or amphibole ac cording to their crystal structure. Chrysotile is the sole commercial representative of the serpentine class and is by far the most common of the asbestiform minerals, accounting for over 90% of the asbestos produced today. The five amphibole varieties used commcrically are amosite, anthophyllite, crocidolite, actinolite and tremolite. The asbestiform minerals have been the subject of a number of reviews in re cent years (Hodgson, 1965; Speil & Leineweber, 1969). Their physical and chemical properties can be related directly to their crystal structures, which have been well established. In addition to its major structural elements, asbes tos invariably contains trace metal impurities. These may be present as isomorphous substitutes for struc tural elements, as fragjpents of host rock and acces sory minerals; or as fine particles of alloy produced by the abrasive action of the asbestos on processing equipment (Cralley et at., 1967). This paper will review current information concerning the levels of designated trace metals in asbestos with particular reference to the UICC standard reference samples. CHRYSOTILE Composition Chrysotile (MgaSi3Os(OH)4) has a layered struc ture, the basis of which is a sheet ofsilica tetrahedra. Attached to one side of this is a brocite (Mg(OH)a) * Health Physics and Medical Division, Atomic Energy Research Establishment, Harwell, Berkshire, UK. 3 Department of Health Education and Welfare, National Institute for Occupational Safety and Health, Cincinnati, Ohio. USA. layer, in which two out of every three hydroxyls are replaced by the apical oxygens of the silica tetra hedra. A mismatch in the dimensions of the two sheets introduces a strain into the structure; this is relieved by curvature, resulting in the hollow cylin drical morphology of the chrysotile fibril. Fibril diameters vary between about 10 and 80 nm, but the mean values are generally in the range of 30-40 nm (Atkinson et at., 1971). Chrysotile fibres "consist of bundles of fibrils, and it has been suggested that the voids between the bundles are filled with a non crystalline solid. Associated trace elements The minerals most commonly found in association with chrysotile are magnetite (Fe304) and brucite, which may be intergrown with the fibre. Gibbs (1971) has reported that other minerals associated with chrysotile from the asbestos mining areas in Quebec include actinolite, antigorite, awaruite, chlorite, chromite, magnesite, nemalite and talc. Magnetite can be separated magnetically from fully opened fibre. Using neutron-activated samples, Morgan et at. (1971b) separated magnetic fractions from a number of samples of chrysotile, and they found that the fraction of iron present as magnetite varied from 19-58%. Small amounts of chromium (3-13%) and cobalt (7-23%) were separated with the magnetic phase, but in no case was any of the scan dium removed by this method. Neutron activation analysis (NAA) has been used by Holmes et at. (1971) to measure the levels of chromium, cobalt, iron, manganese and scandium in a number of samples of asbestos, including the UICC standard reference samples. These values, together with measurements of nickel by atomic absorption spectrophotometry (AAS) are sum- - 113 - ! HWBUI0007074 OW Hi - {he: tied 4- /. -j t, ft. a abject 114 BIOLOGICAL EFFECTS OF ASBESTOS marised in Table 1. Also included in Table I are measurements made on the eight samples of Cana dian chrysotile which were blended to produce the UICC sample of Canadian chrysotile (Chrysotile B). Cralley et al. (1968) used AAS to measure a similar range of elements in the UICC samples and their values are also included in Table 1. With the excep tion of iron, agreement between the two laboratories is satisfactory. In general, nickel is best determined by AAS and is homogeneous at the 10 mg level, using the repro ducibility of replicate determinations of a number of trace metals as an index of homogeneity. Another advantage of NAA is that it can be used non- destructively. To achieve accurate results with AAS. it is essential to ensure that both fibre and accessory minerals are completely dissolved before the solution is aspirated. Some minerals are insoluble in boiling hydrofluoric acid, and it is necessary to use fusion to dissolve all the contaminating minerals. Table 1. Levels of trace metals in samples of asbestos from various regions Source Chrysotilt Rhoden* (UICC A) .. (UICC AR) Cened* (UICC) B Cened* (UICC BR) Cenede (Min* A) ,, (Mine B) ,, (Mine C) ,, (Min* 0} ,, (Min* E) ,, (Min* F) ,, (Min* G) .. (Min* H) Cypru* Ameiitt South Africa (UICC) - CroaMitt South Africa (UICC) , ,, NW Cap* (Min* A) .. (Min* B) Tranavail (Min* A) ,, (Min* B) Anthoplfyllitt Finland (UICC) L*b Ft <%> AERE DHEW DHEW AERE OHEW DHEW AERE AERE AERE AERE AERE AERE AERE AERE AERE 1.7 0.6 0.91 2.6 1.24 1.08 48 35 20 41 3.2 2.9 19 3.2 3.1 AERE OHEW M M AERE DHEW AERE AERE AERE AERE M M M M M M AERE OHEW 4.4 2.0 Cr (ppm) 1390 1378 1170 490 317 317 515 780 1200 930 435 730 4B0 380 340 35 31 16 20 <20 <20 20 <20 870 584 Co (ppm) Mn (ppm) Hi (ppm) Sc (ppm) 55 450 1360 6 54 393 1482 -- 43 231 1284 -- SO 480 820 S 45 444 802 -- 48 419 873 -- 63 540 720 5 110 580 1820 4 60 420 1510 12 78 600 840 7 57 610 540 5 78 450 1790 8 44 420 1520 8 S3 530 330 6 54 720 870 2 7 11800 11 13350 <100 33 5 2 880 <100 0.5 10 833 8 0.6 240 <100 <0.1 0.4 170 <100 03 0.8 140 <100 0.6 0.6 220 <100 0.3 50 1060 1360 24 986 414 5 "" * Atomic Energy Research Establishment. Harwell, UK. * Department of Health Education and Welfare. Cincinnati. USA. * Massive. scandium by NAA, but the other elements can be de termined with adequate sensitivity by either tech nique. NAA is more sensitive than AAS for evaluat ing the levels of the majority of elements, and this is an advantage when small samples are to be analysed. For example, Morgan & Timbrel! (1971) used NAA to determine the amounts of chrysotile and crocidolife in 10 mg samples of blended asbestos and also to show that the UICC sample of Canadian chrysotile Emission spectrography has also been used for the analysis of a wide range of elements in the UICC samples (Timbrel!, 1970). The accuracy of this method is, however, inferior to that which can be achieved with either AAS or NAA. As shown in Table 1, the levels of trace metals in all the chrysotile samples fall within well-defined ranges; thus it does not appear that the source of a sample can be identified by its trace element comple- ' i ' j > 1 1 HWBUI0007075 116 BIOLOGICAL EFFECTS OF ASBESTOS tion. After 2 months, about 30% of the magnesium had dissolved; but after this period, further leaching occurred only very slowly. AMPHIBOLE ASBESTOS Composition Idealised chemical formulae for the three main types of amphibole asbestos are given below. Where cations are written in parentheses without subscripts a variable composition is indicated, with the most abundant species first. Crocidolite Amosite Anthophyllite NasFej * Fef * * Si80M(0H)a (Fe*+Mg)a Sig023(0H)a (MgFe* *}, Si0M(0H)2 and 7%, respectively, being dissolved in 28 days. In neutral phosphate/citrate buffer at the same tem perature, about 3% of the iron dissolved in the same period. The iron of the anthophyllite was consider ably more soluble than was the magnesium, which indicates that these elements may be located at dif ferent sites in the crystal structure of this amphibole. Excretion of 5BFe by rats which had been injected intrapleurally with neutron-irradiated amosite and crocidolite (Holmes & Morgan, 1968) showed that the structural iron dissolves in vivo, but at a much slower rate than does the magnesium of chrysotile. Langer et al. (1972) measured the Mg: Si ratios in amphibole fibres isolated from lungs of asbestos workers and compared them with the corresponding ratio in the appropriate UICC reference sample. The magnesium loss from amphiboles in the lung appeared to follow the sequence: The variability in composition is due to the fact that the crystal structure can accommodate many ions in the spaces between the silica ribbons, and it is the variable nature of host rocks which contributes different ions to this structure. Associated trace elements As shown in Table 1 the levels of chromium, nickel and cobalt in amosite and crocidolite are generally one or two orders of magnitude lower than in chrysotile. The levels of these elements in anthophyllite are, however, very similar to those in chrysotile. Manganese is found in rather similar concentrations in all the U1CC samples, with the exception of the amosite which contains 1.4% of this element. Solubility of amphiboles The amphiboles are much more resistant to attack by acids than is chrysotile. From unpublished work referred to by Hodgson (1965) it appears that the weight loss in refluxing 4N hydrochloric acid is in the order: amosite crocidolite > anthophyllite. More recent leaching studies (Morgan et a!.1} with the U1CC reference samples show that in 0.1 N hy drochloric add at 2S9C the structural iron of amosite is slightly more soluble than is that of crocidolite, 9 1 Unpublished data. anthophyllite > amosite > crocidolite. DISCUSSION Harington & Roe (1965) suggested that asbestos carcinogenesis could be due to the presence of trace metals such as chromium or nickel, which are known to be carcinogenic under certain circumstances. An alternative hypothesis has been put forward by Dixon et al. (1970), who suggested that trace metals associated with asbestos inhibit the metabolism of benzpyrene, thus increasing the residence time of this carcinogen in the lung. They found that nickel, chromium and beryllium were the most effective inhibitors of benzpyrene hydroxylase. If this theory is correct, then the role of asbestos is purely that of a passive carrier of trace metals. Recently, Cralley (1971) has indicated that alloy metal particulates associated with asbestos fibres could give rise to high concentrations of biologically active cations at localised tissue sites, and that these could be responsible for asbestos carcinogenesis. Metals such as manganese, which are high in the electromotive series, could have the effect of precipitating metals lower in the series when they dissolve. If the role of trace metals in asbestos carcmogenesis is to be elucidated, more information is required on their distribution between fibre and contaminat ing minerals and alloys. Attempts to correlate carcinogenicity with trace metal concentration per se are pointless, since it is clear that they can be present j i ' ; * 1 tg taa i-J i HWBUI0007076 pro:r Bl ither nonvAS, sory ition iling into I i I I i CHEMICAL CHARACTERISTICS OF ASBESTOS AND ASSOCIATED TRACI ELEMENTS 115 ment. With the possible exception of the manganese of Chrysolite A, it appears that the UICC standard reference samples were not significantly contaminated during the milling process which was required to pro duce a high proportion of respirable material from the rough-milled samples (AR and BR in Table I). In addition to making measurements of trace metals in bulk samples of asbestos, Cralley et al. (1968) also determined the same metals in < 10 pm fractions, prepared by the sieving method of Kupel et al. (1968). They showed that, in some cases, the composition of the < 10 pm fractions differed con siderably from that of the bulk material. As this sieving process must have the effect of concentrating the non-fibrous material in the < 10 pm fraction, a change in composition relative to the bulk material would be expected. Gibbs (1971) measured levels of cobalt, iron, manganese and nickel in different grades of chrysotile fibre from a number of mines in Quebec and found that the shorter grades invariably contained the highest concentration of all four metals. Solubility ofchrysotile asbestos The solubility of chrysotile in mineral acids has been studied by a number of workers. Acids attack chrysotile by reacting with the hydroxyl groups on the surface of the fibrils. Electron micrographs of parti ally decomposed fibrils show that they consist of an inner layer of intact chrysotile and an outer layer of silicaceous residue. As decomposition proceeds, the electron diffraction pattern becomes weaker, al though the fibre morphology is retained. Atkinson Sl Rickards (1971) showed that at room temperature, the rate of attack is directly proportional to acid concentration provided this is greater than 1 N; at lower concentrations, however, the rate of attack de creased more slowly, possibly because the diffusion of magnesium becomes the rate-controlling process. The reaction rate with fibrillar chrysotile is indepen dent of acid type, provided the acid is fully ionised. Morgan et al. (1971b) studied the rate at which mag nesium was leached from chrysotile fibres in N hydrochloric acid at 25C Leaching curves were obtained which showed that milling increases the magnesium solubility, but that the origin of the fibre is also an important factor. Intrinsic differences in solubility are attributed mainly to variations in the porosity of fibre bundles, but fibril diameter and the nature of the inter-fibrillar material may also play a part. Using neutron-activated chrysotile, the same workers compared the leaching curves of the associ ated chromium, cobalt, iron and scandium with that of the magnesium; they were thus able to determine the fractions of these elements which were present as isomorphous substitutes for the magnesium. Virtu ally all the scandium appears to be present in this form in all the samples examined, presumably be cause its ionic radius is close to that of magnesium (Whittaker & Muntus, 1970). Generally, most of the non-magnetic iron, chromium and cobalt could be accounted for in the octahedral (brucite) layer of the fibre. In some samples from Quebec, however, a considerable fraction of the chromium is present in an insoluble non-magnetic phase (probably chro mite). Cralley et al. (1968) showed that trace metals associated with asbestos have a finite solubility in bovine serum; and Holmes & Morgan (1967), using neutron-activated chrysotile, found that the associ ated chromium and cobalt dissolved at a significant rate after its administration to rats by intrapleural injection. Evidence is accumulating that chrysotile magne sium dissolves fairly readily in vivo. Langer et al. (1970) used the electron microprobe to analyse the Mg: Si ratio in fibres isolated from the lungs of animals and of workers occupationally exposed to asbestos. In both series they found ratios consistent with magnesium-depleted chrysotile. Recently, Pooley (1972) has examined chrysotile fibres, isolated from human lung, by electron diffraction and has been able to obtain characteristic diffraction pat terns. Work with magnesium-depleted chrysotile, obtained by acid leaching in vitro, showed that such patterns could be obtained from fibres with up to half their magnesium removed; the fibres isolated from lung could therefore have lost up to half their magnesium in vivo. Morgan et al. (1971a) have used a radioactive tracer technique to obtain information on the rate of magnesium dissolution in vivo. They selected samples of chrysotile in which most of the cobalt is present in the octahedral layer as an isomorphous substitute for magnesium and, after neutron irradia tion, injected them intrapleuraiiy into rats. As leached cobalt is excreted quite rapidly, the excretion of *Co was used as an index of magnesium solu bility. The results indicate that the magnesium on the surface of the fibre (which accounts for 5-10% of the total) dissolves within a few days of administra- ! I i HWBUI0007077 I %` rf la 28 days. In I (he tame temMis the same hw consider^ptsram, which hinted at difiibaniphibole. M teen injected Mrt imotite and M thowed that ^ lit it i much fin of chrysotile. M:Si ratios in Up of asbestos Jl corresponding pm sample, t is the lung [asbestos t of trace bare known An 1, forward by t trace metals of (time of this ^that nickel, I effective tn|to theory is r that of a if, Cralley 1 particulates line to high rations at ibercsponi such as otive t metals CHEMICAL CHARACTERISTICS OF ASBESTOS AND ASSOCIATED TRACE ELEMENTS 1 17 in a variety of chemical and physical forms with varying solubility. As evidence accumulates that chrysotile magnesium dissolves at a finite rate in vivo, the effect of magnesium removal on the cytotoxicity and carcinogenicity of chrysotile should be investiga ted. SUMMARY The chemical characteristics of asbestiform minerals are described, with particular reference to the UICC standard reference samples. Measurements of the levels of chromium, cobait, iron, manganese, nickel and scandium in a range of samples are reported, together with information on their distribution between fibre and accessory minerals. The solubility of structural and trace constituents of asbestos, both in vitro and in vivo, is discussed. REFERENCES Atkinson, A. W., Cettins, R. B. & Rickards, A. L. (1971) Morphology of chrysotile. Presented at the 2nd Inter national Conference on the Physics and Chemistry of Asbestos Minerals, Louvain, 1971 Atkinson, A. W. A Rickards, A. L. (1971) Acid decom position of highly opened chrysotile. Presented at the 2nd International Conference on the Physics and Chemistry of Asbestos Minerals, Louvain, 1971 of chrysotile asbestos in vivo. Nature (London), 215, 441-442 Holmes, A. A Morgan, A. (1968) Studies with radio active crocidolite and amosite asbestos injected intrapleurally and intratracheaily into rats. United Kingdom Atomic Energy Authority Unclassified Report AERE-R 5636, Harwell, Atomic Energy Research Establish ment Cralley, L. 1. (1971) Electromotive phenomenon in metal Holmes, A., Morgan, A. A Sandalts, F. J. (1971) Deter and mineral particulate exposures: relevance to ex mination of iron, chromium, cobalt, nickel and scan posure to asbestos and occurrence ofcancer. American dium in asbestos by neutron activation analysis. Industrial Hygiene Association Journal, 32, 653-661 American Industrial Hygiene Association Hmmal, 32, Cralley, L. J. Keenan, R. G., Kupel, R. E., Kinser, R. E. 281-286 t St Lynch, J. R. (1961) Characterisation and solubility Kupel, R. E,, Kinser, R. E. A Mauer, P. A. (1968) of metals associated with asbestos fibres. American In Separation and analysis of the less than 10 micron dustrial Hygiene Association Journal, 29, 569-573 fractions of industrial dusts. American Industrial Cralley, L. J., Keenan, R. G. A Lynch, 1. R. (1967) Ex Hygiene Association Journal, 29, 364-367 posure to metals in the manufacture of asbestos textile products. American Industrial Hygiene Association Journal, 28,452-461 Langer, A. M., Rubin, I. A Selikoff, I. J. (1970) Electron microprobe analysis of asbestos bodies. In: Shapiro, H. A., ed., Pneumoconiosis. Proceedings of the Inter Dixon, 1. R,, Lowe, D. B.. Richards, D. E., Cralley, L. J. A Stokinger, H. E. (1970) The role of trace metals in national Conference, Johannesburg, 1969, Cape Town, Oxford University Press, pp. 57-69 chemical carcinogenesis: asbestos cancers. Cancer Re Langer, A. M., Rubin, I. B., Selikoff, I. J. A Pooley, F. D. search, 30. 1068-1074 (1972) Chemical characteristics of uncoated asbestos Gibbs, G. W. (1971) Qualitative aspects of dust exposure fibres from the lungs of asbestos workers by electron in the Quebec asbestos mining and milling industry. microprobe analysis. Journal of Histochemistry and In: Walton, W. H., ed., Inhaled Particles III. Proceed Cytochemistry, 20, 735-740 ings of the British Occupational Hygiene Society Symposium, London, 1970, Old Woking,- Unwin, pp. 783-799 Harington, I. S. A Roe, F. J. C. (1965) Studies ofcarcino Morgan, A., Holmes, A. A Gold, C. (1971a) Studies of the solubility of constituents of chrysotile asbestos in ,oioo using radioactive tracer techniques. Environ mental Research, 4 558-570 i genesis of asbestos fibres and their natural oils. Annals of the New York Academy of Sciences, 132,439-450 Morgan, A., Holmes, A. A Lally, A. E. (1971b) Solu bility of chrysotile asbestos and associated trace metals Hodgson, A. A. (1965) Fibrous silicates. Royal Institute in N hydrochloric acid at 25*C. Presented at the 2nd of Chemistry Lecture Series, No. 4 International Conference on the Physics and Chemistry Holmes, A. A Morgan, A. (1967) Leaching ofconstituents of Asbestos Minerals, Louvain, 1971 HWBUI0007078 118 BIOLOGICAL EFFECTS OF ASBESTOS Morgan, A. & Timbrcll, V. (1971) The use of neutron activation analysis to determine the composition of blended samples of asbestos. International Journal of Applied Radiation and Isotopes, 22, 745-751 Pooley, F. D. (1972) Electron microscope characteristics of inhaled chrysotile asbestos fibres. British Journal of Industrial Medicine, 29, 146-153 Spcil. S. & Lcineweber, J. P. (1969) Asbestos minerals in modern technology. Environmental Research, 2, 166208 Timbrell, V. (1970) Characteristics of the International Union Against Cancer standard reference samples of asbestos. In: Shapiro, H. A., ed.. Pneumoconiosis. Proceedings of the International Conference, Johannes burg, 1969, Cape Town, Oxford University Press, pp. 28-36 Whittaker, E. J. W. & Muntus, R. (1970) Ionic radii for use in geochemistry. Ceochimica et Cosntochiniica Acta, 34, 945-956 Ic T hyd ;I Sch; 3 Wal TA1 Amt Qro. Min Che F Ref' Mo' F tc C( tt D Anti Gro Min Che Res Mo t P Chr Gro a Min Che M Refr V S