Document jmgaEeJq6xQw370ZX8Y2eNVQ

R&S 004923 C//lc/ti &(=- Fd Chtm Toxic. Vol. 25. No. 2. pp. 187-202, 1987 Printed in Great Britain All rights reserved 0278-6915/87 53.00 + 0.00 Copyright (0 1987 Pergamon Journals Ltd Review Section VINYL CHLORIDE: AN ASSESSMENT OF THE RISK OF OCCUPATIONAL EXPOSURE* I. F. H. Purchase Central Toxicology Laboratory J. Stafford Plastics and Petrochemicals Division and G. M. Paddle Central Medical Group, Imperial Chemical Industries pic. Alderlev Park, Macclesfield, Cheshire, England {Received 14 December 1983; revisions received 13 January 1986) Introduction Vinyl chloride monomer (VCM), more properly named monochlorethane, is a'colourless gas normally handled under pressure as a liquid which boils at -- 14:C at normal pressure. Discovered around 1835, VCM's commercialization did not begin until the 1930s and did not reach high volume until after 1945. Present manufacture is around 12 x 104 tonnes per annum, nearly ail of which is used to make the polymer polyvinyl chloride (PVC), Until the 1960s, VCM was regarded as a material of low human toxicity and the main concerns were related to the compound's narcotic effect. Indeed there are many reports of employees exposed to VCM monomer in polymer plants becoming dizzy and unconscious. Because VCM was considered to be relatively innocuous, it had a threshold limit value (TLV) of 500 ppm, 8-hr time-weighted average [TWA) for many years (ACGIH, 1974; Lester et al. 1963; Torkelson et al. 1961). Measurements of em ployee exposure were infrequent, since most mea surement and warning systems were designed to ensure that plant atmospheres were beyond the ex plosive limits, fire and explosion being the main hazards of VCM. Retrospective estimates (Barnes, 1976) of typical TWA personal exposures (in ppm) for polymerization workers have been cited as; 1000 in 1945-1955. 400-500 in 1955-1960. 300-400 in 1960-1970. 150 in mid-1973 and 5 in 1975. However in some jobs, particularly in the cleaning of the autoclaves in which VCM is polymerized to PVC, very much higher exposures, in thousands of ppm, were undoubtedly experienced for short/medium pe- "A longer version of this paper has been published in Toxicological Risk Assessment, edited by D. B, Clavson. D. Krewski and 1. Munro and published by CRC Press, Inc., Boca Raton. FL (1985). Abbreviations'. AOL = acro-osteolvsis; ASL = angio sarcoma of the liver; PVC = polyvinyl chloride. TLV = threshold limit value; TWA = time-weighted average; VCM = vinyl chloride monomer. riods. since in some plants operators became faint and unconscious from time to time. The first clear indication of chronic health prob lems associated with VCM arose in the 1960s in men who entered VCM polymerization autoclaves to re move build-up of polymer from the walls. Some of these men developed aero-osteolysis (AOL; Cook et al. 1971; Harris & Adams, 1967; Suciu et al. 1963). Modification of working practices led to a reduction in the incidence of AOL cases in autoclave cleaners. Although AOL is occasionally seen in people not exposed to VCM (Meverson & Meier. 1972; Wilson et al. 1967) it is a rare disease. In the late 1960s, studies in rats involving exposure to high concen trations of VCM for long periods (Viola, 1969) failed to produce AOL but showed an increase in the incidence of tumours at various sites. Further studies (Malioni etal. 1980& 1981; Maltoni & Rondinella, 1980) showed the rare tumour angio sarcoma of the liver (ASL) in exposed rats, and confirmed VCM as an animal carcinogen. Three ASL cases in employees at a PVC polymerization plant (Creech & Johnson. 1974) confirmed VCM as a human carcinogen. Other known aetiological agents for ASL in man were thorium dioxide, arsenic and, possibly, anabolic steroids (Makoni et al. 1980). Since 1974. the health hazards of VCM have been the subject of many investigations, scientific papers, seminars and other presentations (Conference to Reevaluate the Toxicity of Vinyl Chloride Monomer, Poly(vinvl Chloride) and Structural Analogs, 1981; Gauvain, 1976; 1ARC Working Group, 1979; Selikoff. 1975; Szadkowski &. Lehnert, 1982; US DHEW, 1980). The plethora of information (and misinformation) now available suggests that an ob jective historical case study of VCM would be of value. Experimental and human data Experimental studies The principal effect seen in the acute and subacute studies is anaesthesia, which occurs at relatively high 187 188 l F H Pi RC'H\Sh Pf u/ Table t. Lowest concentrations or doses at which a significant excess of various tumour types was observed in rai carcinogenicity studies Tumour Concn tppml Do'ie (mg kg) Foresiomach papilloma Zwnbal-aland carcinoma Neuroblastoma Nephroblastoma Liver angiosarcoma MammarV'gland adenocarcinoma 30,000 10,000 10.000 250 (female) 100 (male) :oo 50 5 (female) 50 (male) 16 65 (female) Data from Maltoni ei al. (1981). doses (7-10%) in both animals and man. The doses responsible for acuie toxicity are about 1000-fold higher than the minimum dose for carcinogenicity and there is frequently no sign of overt organ toxicity prior to the development of the carcinogenic re sponse. VCM is mutagenic in a variety of test systems including Salmonella typhimurium (Rannug et al. 1976), Saccharomyces (Loprieno et at. 1977) and Drosophila (Verburgt & Vogel, 1977), usually with some form of mammalian microsomal metabolizing system to convert VCM into its active metabolites, chloroethylene oxide and chloroacetaldehyde. The data on the mutagenicity of VCM provide useful qualitative information on its mode of action and metabolism, but are not suitable for the quantitative estimation of risk to man. The most useful experimental data are derived from long-term animal carcinogenicity studies. An extensive series of 17 studies (Maltoni et al. 1981) gives a useful database for risk assessment. Other studies (Feron et al. 1981; Lee et al. 1978) tend to confirm the findings of Maltoni. Carcinogenic effects were observed in mice, rats, and hamsters. A complication in the selection of these data for risk assessment is the variety of tumour types observed (Table 1). Some of these occurred at very high exposure levels, but mammary adenocarcinoma in females and ASL in both sexes of both rats and mice occurred at 50 ppm or less, exposures similar to those believed to have occurred on manufacturing plants (Barnes, 1976). Epidemiological studies Several major epidemiological studies on workers exposed to VCM have been reported (Table 2). The mam organs that have been associated with higher incidences of cancer in workers exposed to VCM are the liver, lung and brain. Increases in the standard ized mortality ratios of cancers in the buccal cavity and pharynx, of lymphomas and of cancers of the lymphatic and cardiovascular systems have been re ported in one or two studies. The analysis of cancer of the respiratory system is often confounded by smoking, making quantitative analysis of the con tribution of VCM difficult. The excess of liver cancers is due to an excess of ASL in many of the studies. An analysis of the statistical power of various studies for association between VCM exposure and cancer of the lung, liver and brain (Beaumont & Breslow, 1981) concluded that the results for liver were consistent with an aetiological role for VCM. For brain cancer, where three out of five studies had statistically significant findings, the results were more variable, positive findings occurring in the studies with the greatest statistical power. The most reason able interpretation was that the data were consistent with a causal association between VCM exposure and an excess of brain cancer. Infante (1981), in reaching the same conclusion, points out that the relative risk for brain cancer is much lower than that for liver cancer. Only two out of eight studies on lung cancer (Beaumont & Breslow, 1981) yielded statistically significant results and, because studies with a high power were negative, a causal association was consid ered unlikely. ASL is the most suitable endpoint for analysis of the risk of exposure to VCM for a number of reasons. It is a rare cancer in unexposed populations, making attribution to VCM exposure on the basis of work history a reasonable approach, ASL occurs in both animals and humans exposed to VCM and it is unlikely that any other carcinogenic effect of VCM will be found to occur at lower exposures than the lowest exposures that induce ASL, For these reasons, most work on the quantitative risk assessment of chronic exposure to VCM has used ASL as the endpoint to study. Case register The availability of data from a comprehensive case register of ASL cases with a history of occupational exposure to VCM provides an opportunity to identify nsk factors for the induction of ASL. Persons potentially exposed to vinyl chloride Current manufacture and use of VCM and PVC results in the potential exposure of four groups of the population. The highest exposure category covers the workers involved in the manufacture of VCM. its polymerization to PVC and certain other industrial uses of VCM. Within this group, certain occupations, particularly autoclave cleaning, involve higher poten tial exposure than others, although all groups would now be expected to have exposures complying with hygiene standards of 1-5 ppm. The next category covers those exposed as a result of using the PVC. Workers in the compounding and fabrication of PVC products are exposed to residual VCM released from PVC on heating (but PVC does not decompose to VCM when heated). In general the exposure levels for these workers arc very low in comparison to those for PVC polymerization workers (from 10 to 100 times lower). Consumers who eat food and drink beverages that have been packed in PVC may ingest unreacted VCM whic 197to le hum 01. Foe T are max amb ar. rot J3 F 8 W 5b o O :pic fO mer ^ inh* con ihc siur Slue qL ritir for CXIS derr exp Rls; 1: ani; m3i me; tra; Mrr to ` C2k '"2 mic sim sarr \ Z2 is e* spe star c nsk wd. exr of cep exr ma iha the Me \ exit more udies .isonistent e and ching c risk liver ancer ically high nsid- >is of sons, iking work both it is .'CM t the the case anal mify .^VC 'the the . its trial ons, teniuld with suit and iual loes the in ters hat :m Vinyl chloride--risk assessment IS9 which has migrated into the food or beverage. Since 1974, the amount of VCM in PVC has been reduced to less than 1 mg/kg with the result that the maximum human daily intake of VCM in food and drink is 0.1/tg/day (Ministry of Agriculture, Fisheries & Food, 1978). The fourth group with potential exposure to VCM are those who live in the vicinity of VCM or PVC manufacturing or fabricating factories. The levels in ambient air around a factory are very low (in the parts per 10* range) but much larger population groups, which include all age groups, are involved. For the workers in VCM manufacture and PVC polymerization and fabrication, the route of exposure is by inhalation. Much of the animal carcinogenicity data are based on inhalation exposure and the human epidemiology is predominantly of populations ex posed occupationally by inhalation. Thus an assess ment of the risk factors and the quantitative risk of inhalation exposure is the main objective. For the consumer exposed to VCM via food and beverages the route is by ingestion. Relatively few experimental studies have used oral administration and only one study used a comparable exposure pattern (Feron et at. 1981). Similarly there are no specific epi demiological data on oral ingestion. Risk assessment for exposure via the oral route must rely on the existing animal data and on extrapolation from epi demiological and experimental studies of inhalation exposure. Risk assessment from experimental animal data After administration by gavage or inhalation, part of the dose is exhaled unchanged and the remainder is excreted or retained in the carcass. A general scheme Ci> cO Clu Glv ( c= O 1 1 CHCH,SCH,CO,H 1 `' 1 NH | 1 Clu Assumptions In carrying out a risk assessment on the basis of animal data, a number of assumptions have to be made. The first of these relates to the overall dosi metry. Experimental animals are exposed to concen trations of vinyl chloride or dosed with amounts of vinyl chloride that allow an estimate of the amount to which they have been exposed. It is possible to calculate a correction factor for these Quantities so that they are applicable to man. However, rats and mice live for relatively short periods of time (up to 2 years) during which they develop cancers of a type similar to those seen in man. The latent period for the same tumours in man may be between 20 and 40 years. It is therefore assumed that the lifetime of man is equivalent to the lifetime of an experimental animal species even though the chronological time is sub stantially different. Strictly speaking, mathematical extrapolation of risk on the basis of experimental animal data pro vides an estimate of the risk at low doses to the experimental animal under consideration. A variety of factors, particularly inherent biological sus ceptibility and differences in metabolism, render the extrapolation of the data from animals directly to man subject to numerous errors. It is at this point that scientific judgement is required to decide whether these data are applicable to the human situation. Metabolism In rats, VCM has been shown to be metabolized extensively, producing a range of excretion products. COjH 11 CHCH ,SCH ,CH, 1 `| 11 NH(XC) OH (c) C'OjH 1 1 <.HCHjSCH,CO,H 1 1 NH, a) CO,H S(CH,CO,H): f` <81 Fig. 1. Scheme showing the metabolism of vinyl chloride monomer (VCM) in rats to S-containing metabolites. VCM (a) is converted to chloroethylcne oxide (b) which is trans formed spontaneously to chloroacetaldehyde (c). These two metabolites arc mutagenic and hence are considered to be the proximate carcinogens. The urinary excretion products At-acetyl-5-(2-hydroxyethyl)cysteine (c) 5-(carboxymethyl)cysteine (f) and thiodiglycollic acid (g) are derived from these mutagenic metabolites via (d). Gly and Glu are the glvcme and glutamate residues of glutathione. [After Green & Hathway (1977)]. i R&S 004925 -- oX X Reference Monson el a!. (1974) Tabershaw 3l Gulley (1974) Duck el t. (1975) Nicholson et at. (1975) Oil et at. (1975) Hyrert er at. (1976) ORC (1976) Reinl & Weber, [976; Reinl et at. 1978; Weber et at. 198 1 Wiuweiler et at. (Iy76) Po A Collier 0977) Frelzd-Ucymc et til (1978) Bcmrzi et til. (1979) fluffier et ttl (1979) Chiaue &. Ferenee (1981) Chiatzc et at. (1980) lkaumonl Sl Brcslow (1981) Tabic 2. Fpidcmiulogicat studies of cancer associated with exposure to vinyl chloride monomer Sties {or tumours) with changes in SMR No. in study"--------------------------------------------------------------------- -------------------------------------------------------------------- (% follow up) Increase No increase Comments ? 8384(85%) Dram Ltmg Liver, including ASL Buccal cavity and pharynx Respiratory system Unknown site Lymphoma Angiosarcoma 2120 257 (99V.) 594 (99V.) 771 (97V.) 10,173(95%) 11,028(90%) 1151 7909 (99V.) None ASL All tumours? Livcr/pancrcaj Cerebra 1? Cardiovascular Digestive tract Malignant liver Lymphatic system Gl tract Brain Respiratory trad Lymphatic system ASL Primary fiver ASL 1618(95%) 544 J (86%) 4M (1(81%) 3847 C'olon/slomach Rrmlalic hyperplasi; All tumours Respiratory system Digestive system l.iver Brain Genital Digestive organs Urinary tract Leukaemia Drain Stomach Brain Lymphatic and hacniopoiclic system Significant SMR not significant but increases with exposure and lime Some criticism of conduct of study ArsenicaIs involved Significant increase (2 ASL) Increase not significant I'MR study Related to duration of exposure Mixed exposure, not VCM related Nol significant Significant Breast Respiratory tract FMR study of female and male fabricators Increase in PMR not confirmed by case-controllcd study Review of nine studies 926WJ0 SSU Purchase et at. s yi x Heauimml A Brcslow (I9 K I) t.ivcr Drain Respiratory tract by caMT-cunlrolled sludy Review o r nine studies i 2s jSj v Vt O rt K vZ. x_ V h 9w v0 2> 8C 5 c. -Si t3 3 as-n -5 CilH Su--e |C-S * Vinyl chlonde--nsk assessment 191 of VCM metabolism in rats is given in Fig. 1. On the basis of this scheme, the highly reactive intermediates in the metabolic process (particularly chloroethylenc oxide) react with cellular macromolecules, including DNA to produce the critical lesions leading to mu tation or the induction of cancer. Studies on the quantitative aspect of VCM metab olism have shown that there is a dose dependency in the rate of metabolism. After administration of l4C-labelled VCM by gavage at doses between 0.5 and 100 mg/kg to Wistar rats, the amount of |4C excreted in the urine and faeces and retained in the carcass was estimated over 72 hours (Watanabe & Gehring, 1976). As the dose of VCM was increased, the proportion exhaled increased and that excreted in the urine and faeces decreased (Fig. 2), The proportion retained in the carcass also decreased. The same general trend occurred after administration by in halation, although the magnitude of the differences in retention and excretion was less (Watanabe & Gehring, 1976). Studies of the amount of non-volatile material retained in the carcasses of rats exposed to various levels of uC-labelled VCM for 6 hours demonstrated that the metabolism of VCM appeared to be in accordance with Michaelis-Mcnten kinetics (Gehring et at. 1978). The constants for maximum velocity of metabolism (Vm in pg metabolized/6 hr) and the Michaelis constant (K,,, in fig VCM/litre air) accord ing to the formula: VmS V= Km + S (where V = velocity of metabolism in ng/6 hr and S = concentration of VCM being inhaled) were Vm = 8558ug metabolized,/6 hr and Km = 860/jg VCM/litre air. Thus there was a considerable change in the ratio of administered dose to metabolized dose H 8. as the exposure concentration increased (Table 3). At the higher doses a smaller proportion of VCM was metabolized than at low doses. Review of earlier calculations of risk There have been a number of attempts to calculate the risk of ASL development on the basis of extrap olation from experimental data. These have been reviewed by Barr (1982) and an adaptation of his data is presented in Table 4. The introduction of biotransformation data into the estimation of risk increased the level of exposure calculated to cause a 10-6 lifetime risk, from parts per billion to in excess of one part per million. A further refinement of the technique using DNA binding as the measure of dosimetry (Anderson et at. 1980) provided a similar estimate of the exposure. A variety of mathematical models can be used for extrapolating below the experimental dose range, and it is not possible to select from amongst these math ematical models on the basis of goodness of fit to experimental data. Attempts to do so have shown that most of the models fit the data equally well (Gehring et al. 1979). It is equally difficult to select amongst the models on the basis of the assumed mechanism of action of VCM. Thus a comparison of the lifetime risks calculated using the Armitage-Doll R&S 004927 192 I. F. H. Purchase et at. Tabic 3. Vinyl chloride dose and incidence of hepatic angiosarcoma m Sprague-Dawley rats exposed on 5 days wk for 52 wk* Concn (ppm) Amount metabolized Pg'4 hr ug (total) Angiosarcoma incidence (%) Male Female Mean Expmt no. 30.000 10,000 6000 2500 500 250 200 150 100 50 25 10 5 1 0 5647 5521 5403 5030 3413 2435 2129 1761 1309 739 395 169 84 17 0 1 47 * 10* 1.44 x 10* 1 41 x 10* 1.3 x 10* 8 8 x 101 6,3 x 10J 5.5 x 10' 4 6 x I0! 3.4 x I05 1.9 x I01 1.0 x 101 4.4 x 10* 2.2 x 10* 4.4 x I01 0 16.6 10.0 10.3 20.0 0 3.4 11.7 1.7 0 1.1 1.7 0 0 0 0 43 3 30.0 13.3 11.7 33.3 22.0 23.3 21.7 20.0 10.0 6.7 5.1 8.3 10,0 8.3 5.0 1.7 0.8 7.2 4.2 6.7 4.2 1.7 0.8 00 00 00 BT 6* BT 1 BT 1 BT I BT 1 BT 1 BT 2 BT 2 BT 2 BR 1.9 BT 15 BT 15 BT 15 BT 15 BT 1,2, 9,15 'After Maltoru et at. (1981), tExpenmcnt BT 6 ended after onjy 68 wk. while the rest were all approximately 140 wk: therefore the percentage of tumours in BT 6 is probably low relative to the rest because of the short latency period available. multistage model by the Food Safety Council (1980) and by Gaylor & Kodell (1980) showed that for the same 10"4 lifetime risk, the Food Safety Council estimated the dose as 2 x 10':ppm whereas Gaylor & Kodell estimated the dose as 5 x 10'4ppm. The difference between these two estimates was due to alternative assumptions on the value of the expansion of the exponential term used. In general, calculations based on the amount of material metabolized or on human data have pro duced exposure values of about 1 ppm for a 10~6 lifetime risk. All the other studies have produced exposure values in the ppb range. A large variable appears to be the selection of the mathematical model applied to the experimental data. In the following section two models are used to calculate the exposure for a IQ"4 risk from a variety of experimental animal data applying the correction for metabolism used by Gehring et al. (1979). Calculation of exposure for 10~6 risk A summary of the crude ASL incidence rates for inhalation studies in Sprague-Dawley rats is given in Table 3. Similar data for Wistar rats exposed by Tabic 4. Summary of quantitative risk assessments for vinyl chloride monomer* Reference Species Exposure for 10"* lifetime risk (ppbt) Comments Schneiderman et al. (1975) Rat Ktumack Si McGaughy (1975) Gehnng e: at. (1979) Food Safety Council (1980) Rat. man Rat. man Rat Rat Anderson it at. (1980) Gaylor Si Kodell (1980) Carlborg (1981) BarT (1982) This paper (Table 9) EPA (1980) NAS (1980) Crump & Guess (1980) After Barr (1982). Except where staled otherwise. Rat, man Rat Rat Man Rat Mouse Man Rat Mouse Man Rat Rat Man Rat tA X o By inbaJttKU) 73 119 14 140-1400 > 1000 <I0->I000 20 20 2.1 x 10-* 3.9 x 10-7 > 1000 0.7 0.5 > 100 0.025--9.16 T 2 x 10"l! / 0,63-90 2 x 10-J-2 x I0"1 6 x lO'*1 0.067-8.14 Bv ingestion 4 pg/day 3 x 10_J mg/kg/day 0 7pg/day 0.5 pg/day Probit (slope -- 1, Mantel) Logit (slope 3,45) Logit (slope 2.3. one-hit) Linear through zero Log-probit Biotransformauon data included Linear or log-probit Depends on mathematical model used Onc-hit Armitage-Doll Weibull Multi-hit DNA binding used for dosimetry Upper 97,5% confidence limit of linear model Armitagc-Doll Weibull Derived from Harris negative epidemiology Log-probit Log*probii including biotransformauon data for man Weibull Weibull including biotransformauon for man Food or water Water Applying worker data to water Upper 95% confidence limits I 33 0) oo -pi to N> 00 i I inha and prespose 8. F lized slam appi, derri Thes elude Fc gava: amot exhal on a fracu doses was r used halati 1977) Wista mann v ar. have : (1981! about VCM been l C tx *s $5 V rv riable nodel ed to iriety ction :s for en i^fe dW Vinyl chloride--risk assessment 193 inhalation (Table 5) for rats exposed orally (Table 6) and for mice exposed by inhalation (Table 7) are also presented. Data from experiments with various ex posure periods of short duration are given in Table 8. For calculating the amounts of the dose metabo lized in rats in the inhalation experiments, the con stants calculated (Gehring et al. 1978) have been applied. For Wistar rat's, the Kra and Vra values derived for Sprague-Dawley rats have been used. These estimates of metabolized dose have been in cluded in the tables. For the experiment in which VCM was given by gavage, the data from Fig. 2 were used to estimate the amount of VCM exhaled unchanged. As the t, 2 for exhalation of VCM was 14 minutes, these data based on a 72-hour period give a good estimate of the fraction of VCM exhaled in the 24 hours between doses. It has been assumed that the VCM not exhaled was metabolized, an assumption similar to the one used for estimating metabolized dose in the in halation experiments. Green & Hathway (1975 & 1977) showed that VCM administered by gavage to Wistar rats was exhaled and metabolized in a similar manner to that in the Sprague-Dawley rats, and the Vn and Km values derived for Sprague-Dawley rats have been used. In the experiments by Feron et al. (1981), who used Wistar rats, the same assumptions about Vra and Km have been made. The quantity of VCM administered has been dealt with as if it had been administered by gavage. VCM dos (mg/kg I Fig, 2. Summary of dose-dependent urinary and pulmonary excretion of vinyl chloride monomer (VCM). Urinary excrelion () represents metabolites of VCM. while pulmonary elimination (A) is unchanged VCM. [After Watanabe & Gehring (1976)]. For mice, the data have been combined in Table 7. The estimation of the dose metabolized in mice has been calculated using values for Vn that have been adjusted on the basis that, for a chemical requiring metabolism to its active form, the quantity metabo lized will be proportional to the body surface area and must be expressed in terms of metabolized dose/kg body mass. This technique has also been used by Gehring et al. (1978) for estimating the dose metabolized by man. 43 U* O 4* CD to <0 Tabic 5. Vinyl chloride dose and incidence of hepatic angiosarcoma in male Wistar rats exposed on 5 days/wk for 52 wk Concn (ppm) Amount metabolized rig 4 hr rig (total) Angiosarcoma incidence (%) Expmt no. 10.000 5521 1 4 v 10s 29 6 BT 7 6000 5403 1.4 * 10* 11 5 BT 7 2500 5030 1.3 * 10* 12.0 BT 7 500 34)3 8.8 x 10' 107 BT 7 250 2435 6.3 x I0! 37 BT 7 50 739 1 9 x I0f 0 BT 7 1 17 4.4 x 10` 0 BT 17 0 00 0 BT 7. 17 Tabic 6 Vinyl chloride ACM) dose and incidence of hepatic angiosarcoma in rats given VCM tv gavage or ingestion Dose (mg kg) Amount exhaled* (% of dose) Amount metabolized ug doset rig OoLai) Angiosarcoma incidence (%) Male Female Mean Expmt no. so; 16.65 3.33 1.0 03 0.03 0 300|| 14.Ill 5.0 1.7 0 50 6250 1.6 x 10* 20 22.5 21.2 BT 11 35 2705 7.0 x 10' 10 15.1 12.5 BT II 10 750 2.0 x 10' 0 0 0 BT 11 > 3245 7 26 x 10' 1,3 2,7 2.0 BT 27 1,7 74 2 16 x 10' 0 1 4 0.7 BT 27 14 7 4 2.16 x 10' 0 Q 0 BT 27 -- 00 00 0 BT 11, 27 80 15.000 6.2 x 10* 49 53 51 7 32 2390 1.65 x 10* 49 16 32 Feron 16.5 1040 7.25 x 10' 10 4 7 > el aL 2 420 2.9 x I0! 0 00 (1981) 69 00 0 0 oj Calculated from data derived from Watanabe & Gehring (1976) presented in Fig. 2. tAssummg a 250-g rat. JSpraguc-DawIcy rats dosed by gavage with VCM in com oil 5 limes/wk for 52 wk. BT27 dosed for 59 wk, ll Wistar rats used as controls by Feron et at. (1981) and dosed for 83 wk. vWistar rats receiving a diet containing VCM dissolveo in PVC. 194 1. F H PX-RCHASt es al Table 7. Vinyl chloride dose and incidence of hepatic angiosarcoma in mice ConCrt (ppm) Amount metabolized fi% 4 hr n% (total) Angiosarcoma incidence (%) Male Female Mean no, 10.000 11.245 1.7 x 10* 38 30 17 8 BT4* 6000 11.007 1.7 x 10* 6 7 36.7 21 7 BT 4 2500 10.246 1.5 x 10* 20 7 33.3 27 1 BT 4 1000 8699 3.4 x 10* 39 4 50.0 44,7 Lee et al.f 500 6952 1,0 x 10* 20.0 26.7 23.3 BT 4 250 4959 74 x 10* 30.0 30.0 30.0 BT 4 250 4959' 7.4 x 10* 24 0 47.0 36.5 Lee et al t 50 1506 2.2 x 10! 3.3 0 1.7 BT 4 1 1506 5.9 x I01 10.3 0 5.2 Lee et al,t 0 0 0 0 0 0 BT 4 & Lee et al. 'Swiss mice, 81-wk experiment, dosed for 30 wk. tCD, mice, 52-wk experiment, 6hriday exposure (Lee el al. 1978). These results have not been included in the calculations for Table 9 because the experimental design incorporated interim kills. Thus: Vm ,(mouse). = .V,n (rat) x 0--.01--1 m:, 0.045 m* = 5706 fig'4 hr x ^ 0.045 = 1395 ptg/4 hr The values of 0.045 m; and 0.011 m: are the bodysurface area of a rat and a mouse, respectively. Since toxicity is a function of the concentration of the toxic metabolite in the tissue, the amount transformed must be normalized for mass to estimate an equiv alent response. Thus Vm must be adjusted on the basis of the body weights of a rat (0.25 kg) and a mouse (0.03 kg) by dividing by 0.03/0.25 = 0.12. The V,,, for the mouse on a mass-equivalent basis is therefore: 1395 = 11625 ft g/4 hr OTJ This value of Vm has been used in calculating the total amount of VCM metabolized (Table 7). From the variety of models (or mathematical ex trapolation techniques) used for low-dose risk extra polation (Table 4). an arbitrary choice of models has been made to test the robustness of the extrapolation from the different animal studies. A log-probit analysis of the dose that would be expected to produce a lifetime risk of ASL of 10"4 is presented in Table 9. This calculation can be carried out on the basis of the concentration inhaled, the daily dose metabolized or the total quantity metabo lized during the whole experiment. There is a wide variation in the estimated dose depending on the database used for the calculation. The largest vari ation between doses derived from the rat experiments is 360-fold (0.025 ppb v. 9.1 ppb) when exposure in ppb is considered, but this decreases to 100-fold for other estimates of dose. The results from mice are substantially lower when expressed in ppb (2 x 10~15 ppb) but the difference is less for other expressions of dose. Similar calculations of the dose expected to give a 10`4 lifetime risk of ASL have been based on a Weibull analysis (Table 9). This is a more `conserv ative' mathematical model and the estimates of dose are accordingly lower. The variation in estimates of dose is, if anything, larger than that observed with the log-probit analysis (for example, a 10~5 difference between the S values derived from Wistar and Sprague-Dawiey rats). The doses for mice are so much low-er than those calculated for rats or man that the assumptions used in their calculation must be suspect. A further calculation to derive the human dose likely to produce a risk of 10"4 is given in Table 9 (S calculated for man). These calculations are based on a Vm for man of 1675 yzg/8 hr based on corrections for bodv surface area and mass. The values are substan- R&S 004930 Table 8. Vinyl chlonde (VCM) dove and hepatic angiosarcoma incidence in Sprague-Dawiey rais exposed io VCM by inhalation Concn (ppm) Schedule! doses Amount metabolized; yg'4 hr ug (total) Anposarcoma incidence (%) Male Female Mean Expmt no. 10,000 I 260 5521 1 4 x 10* 10 13.3 11.7 BT 1 10,000 M 85 5521 4.7 x 10' 0 0 0 BT 3 10.000 in 25 5521 t 4 x 10s 17 0 0,8 BT 10 10.000 IV 100 1379 1 4 x 10s 1,7 0 08 BT 10 10.000 V 25 5521 I 4 x I0! 0 1.7 08 BT 10 6000 i 260 5403 1.4 x 10s 10.3 33.3 22.0 BT 1 6000 ii 85 5403 4.6 x I0! 0 3.3 1.7 BT 3 6000 HI 25 5403 I 4 x I0! 0 0 0 BT 10 6000 IV 100 1350 1.4 x IO1 3.4 1.7 2,5 BT 10 6000 V 25 5403 1 4 x I0! 0 1 7 0.8 BT 10 After Maltom el at. (1981). tSchcduler. I--4 hr/day, 5 days/wk for 52 wk; It.---4 hr/day. 5 days/wk for 17 wk; III IV--1 hr/day, 4 days/wk for 25 wk: V--4 hr/day, 1 day/wk for 25 wk. hr/day, 5 days/wk for 5 wk; 2Amount metabolized (v) in 4 hour derived from the formula. V (ug/hr) Vm * S/KTM t S where V, is 4/6 of the 6 hr value. B 2. : O- n t y 'd. `t "x s 2.&- a. o 5' 5 S' 8 s? h ^ ", _" S.0,, '^*"cr 3<K5'5-K-3?3'3n-no;?n'3&`" Vinyl chloride--risk assessment 'table 0 Quantitative risk estimations derived fium available animal carcinogenicity data anti expressed as the amount of concentration of vinyl chloride __________ calcula ic<l lo give u Jdehmc risk of AS I, of 10 * either on the basis of log-prohit analysis or si WeibuM distribution 1 able no Iix|>criiocii(al data 4 S H rats, inhalation 5 Wtstar rats, iiutc only, inhalation 6 Rats, ingestion--Wistar --S-D --both 7| Mice, inhalation 4, 5 Wistar and S- D rats combined, inhalation 8 S I) rats, short-term inhalation 4 S O rats, inhalation Wistar rats, male only, 5 inhalation 6 Rais, ingestion--Wistar -S-D --both 7! Mice, inhalation 4.5 Wistar and S* D rats combined, inhalation 8 S-D rats, short-term inhalation Exposure for rodents (S pph*) 0.025 Amount metabolized in 6 hr by rodents (V rig/6 hr) (jQg-probtt analysis} 1.23 0.1 6 3 x tO'1 mg/kE 9 x 10 ` mg/kg 6 x I0"*mg/lcg 2 x JO ,J 159 0.69 mg/dose 2.19 mg/dose 1.70 mg/dose 0.60 0.038 -- 2 x 10 ` 1 41 0.004 Weibull distribution} 0.013 2 x 10 9 x 10 '* mg/kg 4 x 10 * mg/kg 2 x 10 * mg/kg 6 x 10 " 15.7 3 x 10"*mg/i!osc 0.33 mg/dose Q.D05 mg/dose 2 x 10 1 6 x 10 ' -- 0 0172 3 x 10 ` Total amount metabolized by rodents (I'M mg) Exposure (ppb) calculated from V (S calculated for manjf 0.305 39.3 2.27 0.2 0 88 0.0063 0.35 2.86 0.(8132 3.68 0.188)2 0.003 0.0015 2 x 10 * 0.0042 0.19 0.63 90 ___ 0.03 0.72 0.067 8.14 1 x 10 * 0.009 * Except where stated otherwise. ASL Angiosarcoma of the liver S-D * Sprague Dawlcy | Exposure calculated from V (in column 3) using the formula: S = V x 860/1675 - V, where {Estimated using maximum likelihood. for Hum is (675 pg/tt Ur. Wistnr and S-D rats cumlwicd. IStmiy nr 4 only. i-B6t?00 ssy 196 I F, H. Purc hase et at tially higher than those calculated for the rat and mouse and there is still a range of over 100-fold in the estimates derived from the different rodent experi ments. When this amount of variability occurs in the extrapolation of the risk of low-dose exposure to VCM based solely on different experiments in the same species, the reliability and hence the utility of these procedures is open to question. The general relationship between the dose adminis tered and the incidence of angiosarcomas derived from 52-week exposure does not apply to exposures of shorter duration (Table 8). In all experiments a total metabolized dose in excess of 5 x 10sfig was required to produce an incidence of angiosarcoma in excess of 1-2%. This relationship was seen in both rats and mice and in experiments in which VCM was administered by gavage or by inhalation. In long term inhalation studies, a total metabolized dose of 5 x 105/cg is equivalent to about 200 ppm adminis tered over 52 weeks and represents a practical thresh old for this series of experiments. In conclusion there is a wide variation in the estimates of dose for a 10~6 lifetime risk. This vari ation is due to the type of mathematical model that is applied, to the assumptions that are made and to the particular experiment that is used to provide data for the extrapolation. A high level of confidence cannot be placed on low-dose extrapolations when variables that would not be expected to alter the expression of risk have a profound effect on the estimated risk. In addition, the interspecies extrapo lation from experimental animals to man is largely intuitive. It is clear that estimates of risk should take into account all available data, including epi demiology, to provide a degree of reliability. Risk assessment from human studies Register of ASL cases Since 1974. lists of reported ASL cases attributable to VCM exposure in the VCM PVC industry have been kept by NIOSH (Spirtas & Kaminski, 1978), by IARC and by the VCM Committee of the Association of Plastics Manufacturers in Europe (APME). Details of 99 cases in the APME register at Tabic 11. Clustering of ASL cases in individual PVC plants Plant* no. Country No. of ASL cases Western Europe 1 West Germany West Germanv 3 West Germany 4 West Germany 1 France 2 France 3 France 1 UK 2 UK 1 Sweden North America 1 Canada 1 USA USA 3 USA Rat of World 1 Japan 1 Yugoslavia 1 Czechoslovakia Tola!... Total... 10 4 2 5 5 2 5 2 5 42 10 11 9 4 34 2 4 Total,,, i For the purposes of this case study, tt is not necessary to identify the predsc ownership and location of these plarus. the end of 1982 have been analysed by country and by manufacturing company and plant. The cases have been recorded from all major VCM/PVC manu facturing countries (Table 10), but the incidence has not necessarily been in proportion to the PVC pro duction capacity now or prior to 1962. In the absence of data on the number of workers employed, pro duction capacity is the only available indication of the numbers of people potentially exposed. The majority of the ASL cases are PVC autoclave cleaners or men who have worked in or around autoclaves. There are ASL cases among men who manufactured VCM and a few cases were involved both with monomer and with polymer production. Only one case suffered from both acro-osteolvsis and ASL. The ASL cases tended to occur in larger numbers in some plants than in others (Table 11), Of the total of 39 ASL cases recorded in North America, 34 have occurred at four PVC plants, while over 40 Table 10. Distribution of ASL cases by country Country No. of ASL cases PVC production nameplate capacity (kilotonnes/yr) 1952 1962 1972 USA West Germany France Canada UK Sweden Yugoslavia Italy Czechoslovakia Japan Belgium Norway Total... Western Europe North America Rest of World Total ,, 29 21 14 10 7 5 4 3 2 2 1 1 99 52 39 8 99 193 704 2090 22 260 1155 ii 176 627 5 22 88 27 177 502 3 20 105 3 8 60 9 212 778 1 25 48 t2 384 1699 3 25 195 2 20 65 82 951 3950 198 726 2178 51 709 3334 331 2386 9462 ASL = Angiosarcoma of the liver 8 co oo > to CO to t i North ASL . The an oc from Swede Germ; predic amom cases Weste: to dec high i: On possib tors a; cases l in oth. manui cause, the typ differe1 cases autocl. or V' auther. i inis of .ascs identify y and cases nanue has prosence proon of cla ound who olved -lion. ; and arger ). Of :rica. ;r 40 Vinyl chloride--risk assessment Table 12. ASL case numbers by year of death and geographical location (excluding ITOI*) Year of death Western Europe ASL casest in: North America Rest of world Key publications 1955 Cl 7 8 9 I960 1 2 4 - C2 USE C3 US5 Cz2 6 7 8 9 1970 1 2 3 4 j 6 7 8 9 1980 1 FI G1 Swl G2 Nl. Sw2, UK1. H2 G3t G4. G5. UK3 F2, F3, G6. G7. G8. It3 Bl. F4, FJ. F6. F7, Sw3 F8. F9. GI0. G1I. GI2. Sw4 FI0, FI 1, G9. G13. G15, GI6. G17 FI2. FI3. UK4, UK5. GI8 UK6, UK7, G19. Sw5, G20. G21 Ii4. FI4, UK8, G22 C4. C5, US4. US7. US 10 US 12. US 16 US II C6, US2 C7 C8. US1. US3. US23 C9, US13 US6. US9. US 18. US26 US 19. US20. US22 CIO. US2I, US24 US27. US2S US 17, US29. US30. US32 Yl. Y2. Czl Japl Jap2, Y3 Y4 Viola Maltoni Creech & Johnson Total.. 52 38,1 8 ASL Angiosarcoma of the liver `Italian case 01 was not a typical ASL: his primary tumour was probably of the pericardium. This man was engaged in extrusion of PVC sacks. tB -- Belgium, G * W, Germany; $w Sweden; C = Canada: It -- Italy; UK = United Kingdom: Cz Czechoslovakia: Jap Japan; Y = Yugoslavia: F * France; N = Norway; US * USA. Thus G9 " case no. 9 in West Germany. Cases UK2, GI4, US 14, US 15 and US25 were shown not to be associated with VCM exposure and hence withdrawn from the list. I Aerosol can filler. fCholangiosarcoma. IDoes not include US3I (still alive). 197 North American PVC plants have not recorded an ASL case so far. The average latent period between starting work in an occupation involving VCM exposure and death from ASL for the 99 cases is 21.9 years (in France, Sweden and the USA between 24 and 25 years, in Germany about 18 years). It is still loo early to predict whether the annual number of ASL cases amongst VCM workers has reached a peak. ASL cases appeared earlier in North America than in Western Europe and while the occurrence is tending to decrease in North America (Table 12). it is still high in Western Europe, On the basis of the data in this case register, it is possible to draw certain conclusions about risk fac tors associated with ASL, The large number of ASL cases in some factories and the absence of ASL cases in others of similar age indicates that variations in manufacturing practices between factories may be the cause. These variations may reflect both differences in the types of job carried out by individual workers and differences in engineering practices. The bulk of the cases have occurred, however, in highly exposed autoclave cleaners, with relatively few in other PVC or VCM production jobs. So far no wellauthenticated cases have occurred in PVC com pounding or fabrication where many more people have been exposed but to a much lower dose. Prediction of future ASL cases as a consequence of pre-1974 exposure The causal relationship between VCM and ASL is proved beyond doubt by the specificity of the tu mour. the high relative incidence of that tumour in highly exposed workers, the consistency of the excess in different parts of the world, the time relationship between exposure and diagnosis and the doseresponse relationship. An intensive analysis of the pre-1974 cohorts should establish the dose-response curve for ASL after VCM exposure and predict the likely outcome for the future. It will be impossible to collect a complete data set on which to calculate risks of ASL for the whole world, but within a single company there may be closer definition of the cohort, the number of cases and the pattern of exposure. Using these data and averaging across the worldwide population exposed to VCM, it is possible to calculate the future inci dence of ASL using relatively crude assumptions which can only be tested in time when the prediction can be judged against the final outcome. R&S 004933 198 1. F, H, Purchase et aL Table 13. ASL case numbers by year of first exposure and geographical location (excluding IT01") Year of first exposure Western Europe ASL casest in: North America Rest of world Key events 1939 40 1 2 3 4 5 6 Frl 1 Frl4 UK 1 Sw2 Frl, Fr3, Sw4 7 8 9 1950 1 2 3 4 5 6 7 8 9 1960 1 2 3 4 5 6 7 8 9 1970 1 2 3 Total.. Sw3 Fr9 Frl2. Fr4 Fr7. Nl, UK8 Swl, UK5 G3 GI5, Ii3 G7, G8. UK4, G19 Gil, G16. GI8 FrlO, G1 FrS, G4. Il2, G2 Fr6, B1 Fr2. H4 G5, G13 G9, G10, G12. GI7, G20, G22 G6. UK6, G21 Frl3, UK7 Sw5 Fr5 UK3 52 US24J C3. US27 US 13. US29 C2, US 19 Cl. C5. US5, US7, US28 C4. US3. US9 C7. C9, US8. USU. US2I. US31 C6, US22, US26 US1 US12 US16 US 10, US32 US4 CIO US18 US2. US 17, US20 Y2. Cz2 Japl. Y1 Y3 US23 Cal Jap2. Y4 CS US6 US30 39 8 Viola Maltoni ASL = Angiosarcoma of the liver MtOl is not consistent with other ASL cases; the primary tumour may have been of the pericardium. The man extruded PVC sacks. tFor explanatory key, see Table t2. tCholangiosarcoma. US31 is still alive. !;Aerosol can tiller. The data required are: (1) Annual populations of employees classified by age; (2) Annual exposure estimates for each person in (1); (3) An exposure-response latency model for ASL induced by VCM. The data under item (1) are available in the UK as a result of the data extracted from the relevant occupational records (Fox & Collier, 1977). Exposure data for item (2) are more difficult to obtain, but can be gleaned from the records that are used to define the occupational population. The problem of oc cupation changing, which occurred frequently, has been dealt with by using the principal employment category or the highest exposed employment cate gory. The estimation of time-weighted average ex posures for the ieast exposed employees is straight forward, as the exposures were essentially continuous and constant, but for autoclave cleaners, mainte nance workers and laboratory workers, exposures could vary from zero to near narcotic levels. In the calculations described below, it has been possible to avoid using the exposure data directly by relying on the similarity in exposure levels in differing locations. The exposure/response/latency data indicated under item (3) can be derived from established cases. The key data for these procedures are the set of cases worldwide, together with the descriptive data (Tables 12-14), It has been possible to calculate an incidence rate for each latency period for each ex posure level for each age group (on the basis of the UK data and assuming that it is representative of the worldwide population) and to use these rates to derive a simple model of dose-response latency that can be applied to the population data. The broad conclusions are that most cases have a latency of about 20 years and cases will continue to occur for the next 10 years. In the calculation used to estimate the future number of ASL cases (Table 15) an assumption has been made that when exposures were reduced to low levels, the future risk of ASL became negligible. Two dates at which the negligible risk levels were attained have been selected: 1964, when levels were reduced to hundreds of ppm and 1974 when the levels were reduced to below 10 ppm following the discovery of the as.1 hypotf i* used, estima i ical `tc based i already latent ; or mor The nt calcula accoun 33 aopula. .'ear fo C/> lave be o :olum; o "he inc > (O :ars) . CO tencie -o '0.5 a iable If. Laienc- (yr) 1-5 1m 6-10 11-15 16-20 21-25 26-30 31-35 36--40 41--45 46-50 51- 16- For details .`lying on ocarions. id under ;ses. ne set of `.ive data :ulate an each ex.is of the ve of the rates to mcy that te broad tency of recur for e future >tion has d to low b!e. Two attained reduced els ove Vinyl chloride--risk assessment Table 14. Annual incidence of ASL cases (date of death) by geographical area No. of ASL cases dying in: Year Western North Europe America Rest of world Annual total Cumulative total Key events 1955 i 1 i 7i 12 1961 1 1 3 2- 1 i4 4 i I26 71 17 ' 8 5 5 12 9 12 3 15 1970 1i 2 17 Viola 1 I2 3 20 2 41 5 25 3 i 4 3 8 33 Maltonj 4 32 5 38 Goodrich 5 64 1 H 49 6 63 2 ii 60 7 63 9 69 8 72 1 10 79 95 5 84 1980 1 64 4 10 94 4 98 2+ 0 0 0 0 Total... 52: 38* 8 98* 98* ASL - Angiosarcoma of the liver Does not include US3I (still alive in 1982). tAt time of compilation. JIndudes 003 (aerosol can filler) but omits ItOI (bag extruder). 199 the association between ASL and VCM exposure. A hypothetical exposed population of 100,000 has been used, but this is unimportant (see (a) below). An estimate of the age distribution within the hypothet ical `total' exposed population of 100,000 has been based on UK data (Fox & Collier, 1977). For persons already exposed during the whole of the various latent periods, the numbers with a latency of 30 years or more form only a small proportion of the total. The numbers of persons at risk in the future are calculated by advancing time in 5-year periods taking account of the age-dependent death rates in the population at large. Death rates for an intermediate year for the male population of England and Wales have been used in this calculation and the future cases (column 10) have been obtained by multiplicaiion. The incidence figures for long latent periods (>25 years) are unreliable or non-existent but those for latencies of 15-25 years are fairly constant and values of 0.5 and 0.8 cases/1000 persons have been used for all latency periods over 15 years to calculate the expected number of cases for the 1964 and 1974 assumptions. The calculation is unrealistic in many respects but the simplifications are unlikely to affect the estimate of future cases by more than a small factor. For example: (a) The population size used for the calculation is probably larger than the exposed population, but the calculation depends on the ratio of "person-years to come" and "person-years ex perienced" and this ratio is the same for any population size. (b) Exposure level has been ignored. The calcu lations are based on the overall risk to the cohort and although the incidence figures for sub-cohorts could be higher, the estimate of future cases will change very little. Similarly duration of exposure has been ignored. Tabic 15- Hypothetical calculation of future ASL cases using two different assumptions about the date at *hich the levels became free of nsk Calculations assuming no risk after 1964 Calculations assuming no nsk after 1974 Latency (yr) Cases to date Persons at 5-yr nsk to date incidence Future persons at risk Future cases Persons at risk to date 5-yr incidence Future persons at nsk Future cases 1-5 6-10 11-15 16-20 21-25 26-30 31-35 36--40 41--45 46-50 5116- 0 100.000 0.00 00 1 98.250 0.01 00 n 95.500 0.12 00 28 84,750 0.33 6750 2 28 61,400 0.46 24.550 I! 18 36,750 0.49 41.750 20 6 21.250 0.28 48,100 13 6 6750 0.89 51.750 46 0 600 7 45.750 *) 0 0 7 34,500 7 0 0 7 47,600 7 0.50 300,750 150 100.000 94.500 78.000 46.500 28.750 18.750 10.850 3500 310 0 0 0 00 0 0,01 3750 0.14 17,500 0.60 45,000 0.97 57.200 0.96 59,750 0.55 58.500 1.71 55.000 7 46,350 7 34.500 7 47.600 0.80 403.900 0 0 2 27 57 57 32 94 1 7 n 323 For details of the assumptions and methods see text (pp. 197 & 198). R&S 004935 200 I. F. H. Purchase ei at. (c) The UK is not typical of the worldwide between VCM exposure and ASL in man. ASL and growth in the exposed population. neoplasms of a number of other organs have been (d) No account has been taken of plant im induced in laboratory rodents by VCM. Estimation provements occurring prior to 1964 and hence of the exposure levels likely to cause a lifetime risk of fewer cases may occur in. for example, the ASL of 10-6 on the basis of these data give extremely 1980-2000 period than are estimated from the low levels (down to 3.9 x 10'7 ppb) which appear to 1940-1980 experience. be unrealistic estimates for man. Part of the reason An assumption that the risk of ASL ceased in 1964 rather than in 1974 results in a considerable reduction in the estimate of future cases. For either assumption, the number of new cases observed annually should soon begin to decline and the rate of decline will indicate which assumption is nearer to the truth. There have been two other predictions of the number of cases of ASL likely to result from previous exposure to VCM. Nicholson et al. (1984) suggest that there will be a further 1500 cases of ASL, while Forman et al. (1986) conclude that a further 150-200 deaths might be expected over the next 30 years. Our estimates rely on a more sophisticated model than the latter estimate and on a larger data set than the former. Nevertheless, the conclusions of Forman et al. (1986) are similar to ours. Only the experience of the next few years will show which is the best estimate. for this is that laboratory studies have shown that VCM is metabolized in the liver (and elsewhere in the body) to the reactive metabolites chloroethylene ox ide and chloroacetaldehyde. The rate of conversion is limited at high levels of exposure giving inaccurate estimates of the slope of the dose-response re lationship. It has not been possible to estimate the rate of conversion in man, and hence extrapolation of these low-risk dose estimates is conjectural. The second part of the problem of extrapolation at low risk is the selection of the most suitable mathematical model for extrapolation. Using Maltoni's data from rats (Maltoni et al. 1981), there is a substantial range (up to 10`) of low-risk dose estimates, depending on the mathematical model and the assumptions used in applying the models. Using the same (probit and log-dose) model and different sub-sets of experi mental data, a large range of estimates is again obtained, even after correction for the non-linear kinetics of metabolism at high dose (which reduces this range to about 1(F). Larger differences are Summary and conclusions obtained with calculations using the Weibull analysis as a basis of low-dose estimation, suggesting that this There is little doubt that exposure to high levels of is a problem with the use of mathematical models VCM as a consequence of occupation can result in an rather than one associated with the log-probit anal increased incidence of ASL. A review of 20 epi ysis. Although there was considerable variability in demiological studies involving about 45,000 workers the dose-response relationship in the different experi occupationally exposed to VCM showed that neo ments reported, in all cases a total metabolized dose plasms of the liver showed an increase in incidence in of 5 x 10J/tg (equivalent to inhalation of 200 ppm) the majority of studies. For brain cancer the associ was required to produce an elevation in ASL inci ation between exposure to VCM and an increased dence. This dose represents a practical threshold in incidence was less clear because of the lower relative rodents. At this stage in their development, mathe risk. Neoplasms of the respiratory tract, digestive matical models for low-risk dose estimates are not system, lymphatic md haemopoietic system, buccal sufficiently reliable or reproducible to engender cavity and pharynx, cardiovascular system and confidence in their use. colon/stomach were reported to show an increased Using negative epidemiological studies of popu incidence in one or more studies, but to show no lations living in the vicinity of VCM production increase, or in some cases a decrease, in incidence in facilities, an estimate of the dose for a I0~* lifetime other studies. In view of the increased incidence of risk in man may be made (Barr, 1982). The value breast neoplasms in rodents exposed to VCM. the (100 ppb) is similar to the highest estimates derived studies of Chaizze et al. (1980), who did not confirm from animal data and taking biotransformation these findings in humans, are of importance. data into account, is substantially larger than the The register of ASL cases now contains records of lowest estimates, which are up to 10' lower 99 persons with confirmed ASL and occupational (3.9 x 10'7 ppb using a multi-hit model). The higher exposure to VCM. The average latent period between estimates are compatible with occupational experi first exposure to VCM and death from ASL is 21.9 ence and suggest that the current hygiene standard of years. The majority of cases occurred in autoclave around 1 ppm is sufficiently low to protect the health workers, who are recognized as having been exposed of VCM/PVC workers. The estimates also give a to extremely high levels. Although precise estimates considerable safety factor for the general public of exposure are not available for the periods of most consuming PVC-packed food and drink or living interest, the pattern of cases roughly suggests that near VCM/PVC facilities. extremely high exposures were necessary for the It has been possible to provide a crude estimate of induction of ASL. For example, ASL cases tended to the number of cases of ASL that may occur in the occur in larger numbers in some plants than in others, future from exposure to VCM prior to 1974. Using a finding that can be explained most easily by the age structure of employees in one company, the differences in exposure patterns. total number of cases of ASL reported to date and There is an extensive series of animal studies on the the mortality pattern expected from a normal popu carcinogenicity of VCM. Some of these precede the lation, the possible future number of ASL cases has epidemiological studies confirming the association been estimated as in the region of 150-300. 31 &> CO o o c* CO CO O) I J i ! Act win witr AC< tr ic. Ei C And ge in ex Barr P\ Barr f* Ai Ju Beau err ers Bern M. (1? po Bufik (19 mo Bvrer. Me VC 17. Carlb gen 255 Chia2 PVr Chiaz. Brc. Me. Confe Mo(195 at 5 19S. Cook H. J trial Coope ride Enn Creech liver Crump Cane Duck E studs Lane EPA ( Chlo. EPA Feron \ Spit chlor. Filatova Fedoi T, V.. blasto and e 26(1) SL and e been maiion reason in that c in the _-ne oxrsion is jcurate sse re late the ition of il. 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