Document wDbxZ378EbdKgBJNYwK031JwJ

R&S 002390 Environmental Health Perspectives Vol. SI, pp. 195-201, 1931 ; Power Considerations in Studies of i Reproductive Effects of Vinyl Chloride land Some Structural Analogs by Maureen Hatch,* Jennie Kline,1 and Zena Stein1 We review the evidence examining the relation of reproductive function and exposure to vinyl chloride and selected structural analogs. Investigation of these compounds for possible reproductive effects has focused on paternal exposure, a much less well studied route than maternal exposure. Drawing on animal models, we discuss what is known about the possible reproductive consequences of exposure to the father as well as to the mother. In evaluating the studies of reproductive outcome in relation to vinyl chloride or analogs, we consider what biologic model may have been tested and whether there was statistical power to detect moderate increases in risk. Parameters influencing statistical power are reviewed, and recommended sample sizes are set out which would insure sufficient power, in future studies, to detect adverse effects. As a setting for research on the relations be- I lween exposures and adverse reproductive events, workplace has both strengths and limitations. A advantage is that exposures in the occupaunal setting are usually at higher dose levels than those in the general environment. Since higher le''els of exposure are often associated with greater Hsks, studies of occupationally exposed individuals facilitate the detection of modest effects. A ^tond advantage is that it is usually possible to distinguish which parent is exposed, since most Parents do not share a common work environment. . limitation of studies set in the workplace is that frequently the number of exposed subjects is too 'ew to yield a valid test of the association being ^Ught. This problem of small numbers revolves around work was supported bv NICHD Grant No. 5T32-1| 1D;07<U0. I -U ^.'V`s`0n Epidemiology, School of Public Health and ) jAPevsky Center, Columbia University, 630 West 166th St., . i ^ ork, N. Y. 10032. Author to whom reprint requests are to | -^addressed. H h>ew York State Psychiatric Institute, School of Public I lA^rh, and Sergievskv Center. Columbia Universitv, 030 West wall St. the question of power, the statistic that guards against the observer reporting no association, when in fact one does exist. The smaller the study population, the greater the chance that an association between an exposure and an effect will not be detected. False negative results can lead, in turn, to erroneous inferences about the safety of the workplace. In this evaluation of the studies where exposure to vinyl chloride and structural analogs has been examined in relation to adverse reproductive out comes emphasis will be placed on considering whether the statistical power in studies reporting negative results was sufficient to justify strong inferences from the findings. Conversely, we will also evaluate whether results reported as demon strating an association truly support this conclu sion. The paper is divided into three sections. First we briefly consider the types of effects which may follow on either exposure to the mother or to the father. Second, we outline the parameters which influence statistical power. Third, we review the evidence with a view to summarizing current knowl edge of the relation of vinyl chloride exposure to reproduction. ! October 1981 195 Hi ii i Ii I`` !t i|: > >I,M 1{ ` 'I in i i. I-M i I: r !-in Types of Reproductive Effects The birth of a child with -malformations is only one of many outcomes that may follow on exposure to a reproductive hazard. So too. maternal expo sure during pregnancy is just one of the routes through which an agent may affect reproduction (2). We consider below, several outcomes and routes of exposure. Paternal Exposure The route of exposure may be through the father, in which case possible reproductive effects include: sterility, infertility, reduced sperm production or mobility, alterations in sperm morphology and ge netic damage to the germ cell. Work in the labora tory' lays the necessary foundation for our thinking about these processes, but it is unfortunately often less precise, particularly in descriptions of outcomes, than we would now wish. Thus we would ask not only that experimental work distinguish between types of agent, dose, age at administration and duration of exposure, and the supposed action on spermatogonia, spermatocytes, spermatids and sperm: but also that outcomes be distinguished in the offspring, in terms of chromosome structure and function, as well as morphology and morbidity. With few exceptions, such specificity is available on almost none of the exposures with which we are concerned in the workplace. Given that there are known interspecies variations in tolerance levels which must be considered to act not only in absolute terms, but also in terms of stage of development and tissue affected, it is unlikely in any event that studies in animals can substitute fully for studies in man. When the route to the conceptus is through the father, exposures prior to conception must be con sidered. The interval between exposure and con ception that is relevant in regard to potential ef fects of exposure is not known, and it may vary with the type of exposure and mechanism. The period is sometimes specified as three months, roughly corresponding to the 75-80 days it takes for sperm to regenerate. However, it may be that some agents act not on the spermatids, spermatocytes and sper matozoa, but rather on the spermatogonia, which give rise to the sperm. In that case, judging from the experimental work, all subsequent populations of sperm might be affected, and not simply the generation present at the time of the exposure. Such an example is found in mice, where paternal irradiation exposure is associated with an excess of mutations in all litters conceived after exposure (2)\ we do not know of a similar example in humans. 196 There is some evidence {3-5) that various drugj may be carried in the human semen. If this is indeed the case, then the developing conceptus may also be affected by exposures to the male parent during gestation. Certainly Naeye (5) has now pre sented evidence suggesting that intercourse during late pregnancy can cause amniotic fluid infection and abruptio placentae. Maternal Exposure When the mother is exposed, events occurring during pregnancy as well as prior to conception can influence the outcome. In the female the germ cells are present at birth; thus any postnatal exposure, and possibly exposures encountered when the mother-to-be is still an embryo, may affect the germ cells. There is some evidence that the genu cells are more vulnerable to exposures at some stages (perhaps during follicular development) than at others. Adverse outcomes from maternal exposure be fore conception include infertility, and conception of a zygote with anomalies in either chromosome num ber or structure and/or with a gene mutation. Ex posure to the mother during pregnancy can result in anatomic malformations in the conceptus (a teratogenic effect), it may lead to disability in the conceptus but without patent malformation (a fetotoxic effect), or it may lead to premature expul sion of a normal conceptus (an abortifacient effect). A carcinogenic effect on the offspring is also possi ble (~, S). Experimental models to distinguish these effects in mice are elegantly displayed in the work o* Maudlin and Fraser (5). Tentative though our understanding may be, aj this stage, of the processes involved, we would still argue for researchers to spell out, at the outset oj their investigations, the likely hypothetical mood they are testing. In the discussion of statistic" issues and in the critique of papers that follows, have had in most cases to superimpose the mode that we assumed was the one being investigatedBy so doing, we may sometimes have been less than just to the investigator, and we will point up m' type of problem when it arises. Issues Relating to Statistical Power Although studies evaluating the effects of oc& pational exposure often permit specification ofJ parent exposed and the timing of exposure, tin answers to important questions about effects on t Environmental Health Perspeod'^ fetus ma pnall nu: iffects ti 33 So CO f J the sta the fin octcorr 11% t doublir relativi effect t column Rudy | the doi tailed), tie um is dnjp~ this s tusnrn - pares :ow pre3 during nfection jrius may be impossible to give, because of the snail number of individuals exposed-. Sample size iffects the power of the test. Formally, power can x defined as the probability that, in any study, a ,-jised risk of a specific size will be detected, if it is present (10). Other determinants of power are: the research design, the test statistic, the level of sta tistical significance established, the size of the in- rrease in risk, and the prevalence of the condition aider study in the unexposed population. Some of jese relationships are illustrated in the next two currmg :ion can .m cells posure, en the ret the e germ * some .t) than miles. Table 1 illustrates the relation between the prev- dence of a condition in an unexposed sample and sample size, fixing the relative risk to be detected, die statistical power, and the significance level. In die first column, we set out the frequencies of an outcome among the unexposed; these vary from 0.1% to 45%. In the second column we show a doubling in the risk. (Throughout, we define a relative risk of 2, or a doubling, as a nontrivial ire beitlon of 2 n1uUmnr- effect that one would wish to detect.) In the third column we set out the sample size needed in each study group in order to have 80% power to detect lie doubling in relative risk (at a = 0.05, two- re :us; (a in the ion (a expul- r'fect). possi- 'ailed). It is obvious that the rarer the outcome in 'die unexposed population, the larger the sample needed to detect a doubling in risk among the wposed. The need to use large samples when study ing rare outcomes relates to the fact that a doubling m a rare event--which may result from an addi tional handful of cases--is far more likely to arise y chance than a doubling of a more common event. ffects >rk of Table 2 illustrates the relation between statisti cal power and the size of effect (relative risk), fixing die sample size of the two study groups and the oe, at d still set of node! stical s, we node! ated. than j this frequency of the outcome among the unexposed. In toe example, there are 100 individuals in each cooort and a prevalence of 15% for the outcome ^ong the unexposed. It is obvious that statistical Power increases with increases in the relative risk, ytc greater the size of the effect, the smaller the ^ynple required to detect it. Che moral of these two tables is that not all Judies with negative results are equal. Some nega!lve studies are more equal than others. In order to oterpret a negative finding, we need to determine _ e probability that a particular increase in risk ;'uld have been detected, if present. It is with this ln mind that we evaluate the studies which have Examined exposure to vinyl chloride and structural analogs in relation to reproductive outcomes. CCU; We have grouped the studies by outcome. For ' thd each outcome, we consider whether those studies firm ^hich appear to produce conflicting results were the es>gned to detect effects of the same magnitude. ves ctober 198 L Table 1. Relation of prevalence to sample size requirements.* Probability of outcome among une.xposed group Probability of outcome among exposed group Samples sizes-of exposed and unexposed groups*1 0.001 0.01 0.10 0.15 0.25 0.35 0.45 0.002 0.02 0.20 0.30 0.50 0.70 0.90 22.403 2.243 197 123 65 41 28 "Illustrated by the need to detect a doubling in relative risk (RR) with 80% statistical power. bSample sizes were calculated for o = 0.05. two-tailed test. Table 2. Relation of relative risk and statistical power.* Probability of Probability of outcome in un outcome in exposed group exposed group Relative risk Statistical power to detect increase in risk 0.15 0.19 1.3 0.11 0.15 0.23 1.5 0.29 0.15 0.27 1.8 0.56 0.15 0.32 2.1 0.81 0.15 0.37 2.5 0.95 'Illustrated among 100 exposed and 100 unexposed subjects when prevalence among the unexposed is 15%. Power calculated for a = 0.05, two-tailed test. A Review of the Evidence Birth Defects The initial suggestion that vinyl chloride might pose a risk to human reproduction came from a study of birth defects in three Ohio communities housing vinyl chloride production facilities (11). This was an ecological study, comparing malformation rates in the index communities with the statewide rates. Attention focused on the finding of a significant excess of central nervous system malformations, especially prominent in one of the cities, where the risk of neural tube defects relative to the state as a whole was 5.8. Ecological studies always raise knotty statistical issues so that some biostatisticians and epidemiolo gists shun them utterly. It was therefore entirely appropriate that, following this first report, the Center for Disease Control (CDC) undertook a case-control study to see if. individually, the cases in this city could be linked to the vinyl chloride facility (12). Occupation and residence data from hospital records were used to explore, first, whether the parents of cases had had direct occupational exposure to vinyl chloride and. second, whether 197 R&S 002392 their homes were located closer to the plant than the homes of controls. No differences in work place exposure or in proximity to the plant were found between the two groups. In this sample, comprising 15 cases and 30 unaf fected controls, the chance of detecting a doubling in the proportion of residents living close to the plant compared to controls was about 70%. It seems certain that power was ample to detect a sixfold relative risk, even a twofold risk, but not a more modest effect. More recently, CDC reported a second study examining the relation of neural tube defects to parental exposure to vinyl chloride (13). Data from the Birth Defects Monitoring Program were re viewed for other locales with poly(vinyl chloride) facilities, and an intensive investigation was launched in Kanawha County, West Virginia, where rates of CNS defects had also been observed to be significantly higher than in reference populations. In this study, unaffected births (controls) were matched to af fected births (cases) on several factors (seasonality, race, social class and maternal age) which may relate to CNS malformations. Reproductive, resi dential and occupational histories were obtained by telephone interviews with the parents of affected and unaffected births: a matched-pair analysis was performed to test whether the frequency distribu tion of distances from the plant were similar for cases and controls (Table 3). The power of this study of 46 matched pairs was the same as in the earlier CDC study; that is, there was 80% power to detect a 2.3 increase in the proportion of cases living close to the plant com pared with controls, but only 70% power to detect a doubling in this proportion. Once again, no associa tion was found between either working in or living near a poly(vinyl chloride) plant and central ner vous system defects. In fact, the proportion of parents employed at the plant was equivalent (4%) in the case and the control group, and the percent age living close to the plant was similar within a Table 3. Vinyl chloride neural tube defects. Study Study 1 Ecological analysis Study 2 Study 3 Case-control study 15 cases 30 controls Case control study -1G matched pairs RR of CXS defects in com munity with PVC plant = 5.S Power to detect a doubling = 70%: RR detectable with S0% power = 2.3 Power to detect a doubling = 70%; RR detectable with .SO% power = 2.3 radius of either one or three miles. When the ad dresses of the two groups were plotted on a map, the direction of the residences with respect to the plant did differ, with families of affected births living more to the northeast and families of unaf fected births living to the south of the plant. Emis sion and meteorologic data were explored to see if exposure levels varied with direction, but the re sults were ambiguous. In summary, if there is an association between parental exposure to vinyl chloride and CNS de fects in offspring, these two case-control studies suggest it is likely to be smaller than the moderate effect exemplified by an odds ratio of 2.3. The route of exposure was never explicitly specified in these studies of neural tube defects. However, malformations in offspring are often considered to implicate the mother rather than the father as the source of exposure. Hence the study examining effects of vinyl chloride inhalation in pregnant fe male animals, described elsewhere in this volume, is of interest (11>). Since power considerations are as pertinent to the laboratory as to population studies, we calculated, from the published report, the power of the most sensitive test available in this experi ment. That comparison could only achieve S&x power if the effect on the treated animals was large: that is, only an increase of more than 4-fold in the incidence of anomaly was likely to have been de tected. Reporting such findings as negative is t0 disregard considerations of power, which are not species-specific. Spontaneous Abortion A later investigation into the reproductive eifects of vinyl chloride explicitly proposed the father as the route of exposure and fetal loss as the outcome (15). Using fetal loss data obtained interview with the fathers, rates of loss were com* pared in the wives of exposed and unexposed wot ers, in both the time periods before and after exp0" sure to vinyl chloride. In the time period subsequen to exposure, when mean paternal ages were equi^a, lent (and. by inference, maternal age. a known11-', factor for spontaneous abortion), the rate of loss among the wives of exposed workers was ^ and that among wives of unexposed workers "y: S.87c, yielding an unadjusted relative risk for te loss of 1.8 (Table 4). ^or A comparison of age-adjusted rates of lo= ^ exposed and unexposed men was carried out b> authors, taking the number of pregnancies two groups (412 in total) as the sample size- * this chi-square analysis it was concluded that t Tat Study I Study 2 was a stat the two gr there is ev neous abo) which latei neous abor the risk of some of th the wives t to the inen experiencii Thus the s ered indept satisfy the squ der n 30 wi' flo fet in lx o de o w on co dc ef to co HealthP murnn m nnf `j| ive-' Perspeeh Table 4. Vinyl chloride/fetal loss: comparison of fetal loss rates in wives of exposed and unexposed workers. Prevalence in T unexposed :V per group Power Study 1 Study 2 0.088 0.15 0.12 62 exoosed 113 unexposed 205 exposed 144 unexposed 205 exposed 144 unexposed Power to detect doubling = 31%; RR detectable with 80% power = 2.9 Power to detect 1.8 RR " 79% RR detectable with S0% power = 1.81 Power to dete-t 1.8 RR *= 65% RR detectable with 80% power = 1.96 was a statistically significant difference between the two groups in the rate of fetal loss. However, there is evidence that women with multiple sponta neous abortions were concentrated in the group which later became exposed (16). Since one sponta neous abortion is associated with a 66% increase in the risk of a subsequent abortion, it is possible that some of the seemingly excessive loss occurring in the wives of men exposed to vinyl chloride is owed to the increased proportion among them of women experiencing previous abortions prior to exposure. Thus the subsequent abortions cannot be consid ered independent events, and the analysis does not satisfy the assumption which underlies the chisquare statistic, that all observations are indepen{Mkt. If the rates are compared basing sample size 62 wives of exposed workers and the 113 drives of unexposed men. then the difference in fetal loss rates is not statistically significant (t 1-43, 173 df). However, the power of this test to detect a doubling in the frequency of abortion is or>Iy 31%. Thus a negative finding in this analysis does not rule out the possibility of a moderate effect. The statistic that is appropriate here depends | essentially on the explicit model that is being testj fd- We have noted the chi-square is incorrect, I because a woman's first pregnancy and her subse- I 3uent pregnancies cannot be considered indepenI dent. It could be argued, however, that, if the i model to be invoked involves an effect of vinyl I diloride on the spermatocyte II layer of the father, I [hen pregnancies that followed within a given pe- | nod after exposure, and only those, would be af- -ected. In such a case, provided that there was I way of controlling for other risk factors for j Portion (like maternal age and previous spontane ous abortion), then the test statistic might legiti| 'tmtelv be based on pregnancies rather than on '*omen (although some statisticians will still balk at lhis procedure). I , A new investigation may shed some light. Fetal is one of the endpoints currently being evalu- as part of a study of vinyl chloride workers i October 1981 conducted at the University of Texas (17). The design of this study has been fully described, though results relating to reproduction have not yet been published. Fetal loss data in this investigation will be based on telephone interviews with the wives of workers: interviewers will be blind to the husband's exposure status. Information will also be collected on potentially confounding variables such as ciga rette smoking and prior reproductive history. The Texas group has thus far interviewed 205 wives of exposed men and 144 wives of unexposed men. Does this sample of 349 wives yield sufficient statistical power to detect an effect of the magni tude suggested by the prior study (RR - 1.8)? The statistical power will depend on the rate of abortion in the unexposed wives. If our suspicion that women are more accurate reporters of reproductive history than their mates is correct, then we can expect that the baseline frequency of abortion in this study may be somewhat higher than that of the earlier study based on reports from male workers only. In Table 4 we have computed pow-er based on two different estimates of abortion frequency in the unexposed sample: 15% and 12%. The rate reported will de pend partly on the definition of fetal loss and partly on the distribution of risk factors in the population observed. Then if the prevalence of spontaneous abortion among the unexposed is 15%, there will be an adequate test of whether or not paternal expo sure to vinyl chloride is associated with a 1.8 rela tive risk of spontaneous abortion. If, on the other hand, the frequency of abortion among the unex posed is 12%, there is only a 65% chance of detect ing this increase in risk. Infertility Also bearing on the question of reproductive risk are investigations of effects on fertility in workers exposed to ethylene dibromide (EDB) and epichlorohydrin (ECH), structural analogs of vinyl chlo ride shown in animal studies to interfere with sper matogenesis. In studying these compounds, the attempt has been made to demonstrate exposure R&S 002394 WWIWMai, PJUI.I ilillJ-WHIJS effects directly in the male, by examining semen and hormone samples, as well as indirectly, using outcome of pregnancy in wives.-Wong and colleagues assessed the fertility of male married workers ex posed to EDB, by comparing the number of livebirths to their wives with age-paritv-race-calendar yearspecific birth probabilities for all U.S. women (IS). Effects on single workers were not evaluated, nor was this method able to control for regional differ ences in fertility rates. In reporting their negative findings, the authors claimed to have power of 90% to detect a 20% increase in infertility. However, this computation used the number of person-years observed rather than the number of persons as the sample size. Again, whether "men" or "person years" is the risk of 47-XYY offspring, this outcome has not been demonstrated. Milbv and Whorton. in a recent paper-(22), have summarized results of studies they have conducted on epichlorohvdrin in two occupational cohorts. Also Venable and colleagues have recently reported a study of glycerine workers with multiple chlori nated hydrocarbon exposures, including epichlorohydrin (22). Sperm count distribution has been the major focus in these studies comparing semen qual ity in exposed male volunteers and unexposed con trols. although it has been argued that sperm mor phology provides a more stable and predictable parameter (23). These studies vary in the detail with which they have been reported, and in methodology'. The focus the pro! 20 milli< In the there u increase the cont S0% cha Venable sperm c to detec None detect a ffequen, effect, t fears to sperm c correct number to use in the computation depends in the present paper is on issues relating to statisti on the explicit model that is being tested, which is not here spelled out. cal power; however, several of the methodologic problems in the studies of sperm are important and Cone I t'` ' i ,' Sperm Counts call into question the value of considering the data in this way. (For instance, in the Milby and Whorton ECH studies, the participation rate among eligible What present ouslv to The relationship of semen quality to infertility and/or outcome of pregnancy is still imperfectly w'orkers was 36% in one cohort and 45% in the l and rep; other, raising the possibility that the samples were J are sev understood. Men with sperm counts less than ten not representative of the exposed populations.) where million have, for instance, been shown capable of Negative findings for an association between ex issociat impregnating (19), but follow-up studies have not been done to see whether the frequency of adverse posure to ECH and decreased sperm count have | should 1 been reported for all three cohorts, although the / to be ca pregnancy outcome is greater than among men Venable study does note a suggestive reduction in `-here is with normal counts. Kapp et al. have found in sperm concentration in a subgroup of those exposed greater 1 = creased aneuploidy in the sperm of 18 dibromo- (Table 5). For each study, we calculated the size oi exposur chloropropane-exposed workers investigated (20). the relative risk which could be excluded, with a to the i Although an increased number of sperm with tw'o Y 20% probability of falsely concluding that there was studies. chromosomes suggests there might be an increased no association between exposure and sperm count; reprodu ve set i would b Table 5. ECH/sperm concentration: semen analyses of exposed and unexposed volunteers. study a . 'i i For cliff I t i. i i .! 'I" ' it j! Study 1 Study 2 Study 3 Outcome Neural tube defects Spontaneous abortions Sperm counts < 20 million Prevalence in unexposed A' per group Power / sample rve 0.055 0.055 0.095 44 exposed 90 unexposed S4 exposed 90 unexposed 64 exposed 63 unexposed Power to delect doubling = 1954 RR detectable with 805c oower = 4.1 Power to detect doubling = 3354 RR detectable with 8054 power = 3.5 Power to detect doubling = 3554 RR detectable with 3054 power = 3.0 Table 6. Recommended sample sizes for future studies. ZD B C/5 O o ------ ro CO CO Oi er t 1 ch des se e: Stroi Prevalence in unexposed population Relative risk to be detected with 8054 power' Number required in each study group expo offsn - Russ O.OOl.'livebirlhs 0.15, pregnancies 0.12/pregnancies 0.07/males 6.0 1.8 1.8 1362 livebirths 174 mothers 240 mothers 2.0 N'uci ' -Man. agon Heal lb,SO Tower calculated for a = 0.05. two-tailed test. 200 Environmental Health Perspective- ! Octohe R&S 002396 the proportion of men with sperm counts less than 20 million was the index evaluated. 1), have nducted :ts. Also In the first cohort observed by Milby and Whorton, there was an 80% chance of detecting a 4-fold increase over the 5.5% baseline rate observed in )orted a die control group; in the second cohort there was an i chlorinchloro- S0% chance of detecting a relative risk of 3.5. The Venable study which observed a 9.5% rate of low ieen the sperm count in its control group had power of 80% en qual- io detect a threefold increase in risk. sed eonrm mordietable None of these studies had sufficient power to detect a doubling. If we agree that a doubling in the frequency of sperm count depression is not a trivial effect, then these studies are not sufficient to lay ich they he focus fears to rest concerning a possible effect of ECH on sperm count in exposed males. statist!odologic .ant and he data Conclusions What, then, is it possible to infer at this time? At Vhorton eligible in the es were ns.) present, there are no data which point unambigu ously to a relation between vinyl chloride or analogs md reproductive outcome. On the other hand, there iire several studies which report no association vhere the statistical power to detect a modest een ex it have association between exposure and outcome, if it should be present, is either insufficient or not able igh the :tii xps size of with a re was count: -o be calculated from the published data. Certainly -here is no evidence to indicate that the fetus is at greater risk from maternal exposure than from wposure to the father. Altogether, one must point the need for carefully designed and executed studies, where the association of vinyl chloride with ^productive outcome can be examined. In Table 6, *e set out some estimates of the sample sizes that i'ould be needed, depending on the outcome under >tudy and its prevalence in an unexposed sample. I different outcomes, we have required that the j -'fhiple be sufficient to detect effects of the size 1 ^served in previous studies. Certainly, we stand a | ^tter chance of understanding whether and how 'inyl chloride affects reproduction if future studies I ze designed with the ability to detect these ad- I -*rse effects. REFERENCES 1 " Strobino, B., Kline, J., and Stein, Z. Chemical and physical exposure of parents: effects on human reproduction and , ^spring. Early Human Dev.. 1: .'!71-399 (I97S). Russel. W. L. Studies in mammalian radiation genetics. , Nucleonics, 23: 53-G2 (19651. Manson. J, M.. and Simons. R. Influence of environmental yrents on male reproductive failure. In: Work and the wealth of Women. V. R. Hunt. Ed., CRC Press. Florida, 1980. Lutwak-Mann, C., Schmid, K., and Keberle. H. Thalido mide in rabbit semen. Nature. 214: 1018-1020 (1967). Ericsson. R. J., and Baker, V. F. Transport of cestrogerts in semer. to the female rat during mating and its effect on fertility. J. Reprod. Fertil, 12: 381-384 (1966), N'aeye, R. L. 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Sci., 271: 49-57 (1976). Edmonds. L. D., Falk, H., and Nissim, J. E, Congenital malformations and vinyl chloride. Lancet, ii: 1098 (1975). Edmonds. L. D., Anderson. C. E., Flvnt, J. W., Jr., and James. L. M. Congenital central nervous system malforma tions and vinyl chloride monomer exposure: a community study. Teratology, 17: 137-149 (1978), 14. John. J. A.. Smith, F. A., Leong. B. K. J., and Schwetz, B. A. The effects of maternally inhaled vinyl chloride on embryonal and fetal development in mice, rats, and rabbits. Toxicol. Appl. Pharmacol., 39: 497-513 (1977). 15. Infante. P. F., Wagoner. J. K.. McMichael, A. J., Waxweiler. R. J.. and Falk, H. Genetic risks of vinyl chloride. Lancet, i: 734*735 (1976). 16. Infante, P. F., Wagoner, J. K,, and Waxweiler, R. J. Carcinogenic, mutagenic and teratogenic risks associated with vinyl chloride. Mutat. Res.. 41: 131-142 (1976). Buffler, P. A. Some problems involved in recognizing teratogens used in industry. Contr. Epidem. Biostatist., 1: 118-137 (1979). Wong, O., Litidjian, M. D., and Kanen, V, S. Retrospective evaluation of reproductive performance of workers exposed to ethvlene dibromide (EDB). J. Occup. Med., 21: 98-102 (1979)1 19. Sherins, R. J., Brightwell. D.. and Stemthal, P, M. Longitudinal analysis of semen of fertile and infertile men. In: The Testis in Normal and Infertile Men. P. Troen and H. R. Nankin. Eds. Raven Press. New York, 1977. 20. Kapp. R. W., Jr. Picciano, D. J.. and Jacobson, C. B. X*Chromosomal nondisjunction in dibromochloropropaneexposed workmen. Mutat. Res.. 64: 47-51 (1979). 21. Milby, T. H., and Whorton, D. Epidemiological assessment of occupationally related, chemically induced sperm count suppression. J. Occup. Med. 22: 77-82 (1980). Venable, J. R., McClimans. C. D.. Flake. R. E.. and Dimick. D. B. A fertility study of male employees engaged in the manufacture of glycerine. .J. Occup. Med. 22: 37-91 (1980). 23. Wvrobeck. A. J., and Bruce, W. R, The induction of sperm-shape abnormalities in mice and humans. In: Chemi cal Mutagens, Vol. 5. A. Hollaender and F. J. deSerres. Eds., Plenum Press. New York, 1978. iP'fl 0riober 1981 201