Toxicology, 74 (1992) 91-126 Elsevier Scientific Publishers Ireland Ltd.

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A review of the literature on potential reproductive and developmental toxicity of electric and magnetic fields N e i l C h e r n o f f ~, J o h n M. R o g e r s a a n d R o b e r t K a v e t b aHealth Effects Research Laboratory, Environmental Protection Agency, Research Triangle Park, NC 27711 and bElectric Power Research Institute, Environment Division, P.O. Box 10412, Palo Alto, CA 94303 (USA) (Received January 4th, 1992; accepted June 8th, 1992)

Summary The potential of electric and magnetic fields to adversely affect the health of the human population is an issue which continues to receive a great deal of attention in both public and scientific forums. One of the critical issues is the possibility that such fields may adversely affect the reproductive process. Numerous studies investigating the potential of electric and/or magnetic fields to alter reproduction in vertebrates have been conducted. These studies have, in many instances, yielded seemingly contradictory results. A number of epidemiological studies have been conducted as well. This review of the literature examines relevant studies and attempts to draw biologically rational conclusions from them. The studies are ordered in broad categories based upon both classification of the species studied (i.e. submammalian, mammalian exclusive of man and human) and the agent used (i.e. extremely low frequency electric, very low frequency electric, and magnetic fields). From our review we conclude that laboratory experimental and epidemiological results to date have not yielded conclusive data to support the contention that such fields induce adverse reproductive effects under the test or environmental conditions studied. Additional studies may, however, be warranted to clarify some of the experimental results obtained.

Key words: Teratology; Developmental toxicity; Reproductive toxicity; Electric fields; Magnetic fields; Extremely low frequency; Very low frequency

Introduction The transport and use of electricity generate(s) both electric and magnetic fields (E/MF). Several reviews on the biological effects of E/MF have been written in recent years [1-5]. An area which has been studied by a number of workers is that of the potential of these fields to adversely affect reproduction. Reproduction in Correspondence to: N. Chernoff, Health Effects Research Laboratory, Environmental Protection Agency, Research Triangle Park, NC 27711, USA. Disclaimer: The research described in this article has been reviewed by the Health Effects Research Laboratory, U.S. Environmental Protection Agency and approved for publication. Approval does not signify that the contents necessarily reflect the views and policies of the Agency nor does mention of trade names or commercial products constitute endorsement or recommendation for use. 0300-483X/92/$05.00 1992 Elsevier Scientific Publishers Ireland Ltd, Printed and Published in Ireland

92 mammals involves a wide spectrum of biological and behavioral processes including gametogenesis, fertilization, implantation, placentation, embryogenesis, fetal growth, parturition and adaptation to the extrauterine environment, maternal care and lactation and postnatal growth and maturation necessary for the reproductive cycle to continue. The majority of studies which have focused upon the potential reproductive toxicity of E/MF have specifically dealt with the potential of these agents to affect prenatal development. In this paper, we review the laboratory and epidemiologic research that has addressed potential effects of exposure to environmental E/MF on reproduction and development. A critical review by the senior author on the potential of E/MF to affect reproduction was previously published [6]. At that time 21 studies were examined, all published between 1977 and 1983. That review concluded that extremely low frequency E/MF had not been conclusively demonstrated to produce adverse reproductive or developmental effects in either mammalian or avian species. This conclusion was based upon studies reporting no exposure-related biological effects and a lack of dose-related phenomena and replication among those studies that did report positive E/MF-related effects. It was recommended, however, that a small number of studies (especially with magnetic fields) that did suggest agent-related effects needed to be replicated before any definitive conclusions could be reached. A more recent review [7] has tabularized studies involving the determination of potential developmental effects of pulsed magnetic fields on embryogenesis. This effort did not involve a critical analysis of the data presented in the different studies and its conclusions were based upon acceptance of the interpretations reached by the various research scientists responsible for the studies. No other comprehensive reviews of the potential of E/MF to affect reproduction have been published to date. The current review was undertaken to provide a comprehensive, critical update on the literature dealing with the potential reproductive and developmental toxicity associated with E/MF exposures. We first present a brief background on definitions of E/MF terms and units and ambient exposure conditions in various scenarios to provide a context in which to place the reproductive and developmental toxicity research which is reviewed. We then critically review relevant studies with special emphasis on those published since 1983. Wherever possible, papers that have been published in peer-reviewed journals are used, but in some instances data that have only appeared in conference abstracts are included. Environmental electric and magnetic fields

Ambient E/MF span a wide spectrum of frequencies, intensities, and waveforms. The research conducted on possible associations between E/MF exposure and reproductive outcome has, however, focused on two frequency ranges: extremely low frequency (ELF) and very low frequency (VLF). The ELF range includes E/MF with frequencies less than 300 Hz. The power frequency for electric transmission, distribution and domestic service is 60 Hz in North America and 50 Hz in Europe. Accordingly, much of the mammalian reproductive research has focused on these frequencies because of their ubiquitous presence in the environment. The magnitude of environmental 60-Hz electric and magnetic fields varies over a

93 TABLE I ORDER-OF-MAGNITUDE ESTIMATES OF 60-Hz ELECTRIC AND MAGNETIC FIELDS ASSOCIATED WITH DIFFERENT HUMAN EXPOSURES a Exposure

Electric field (V/m)

Magnetic field (mG)

Residential background Near household appliances Edge of transmission right of way Electrical occupation

1-10 10-100 100-1000 3000

0.1-5 10-1000 10-100 ~ 3000

aSee Refs. 10-12.

wide range, as the order-of-magnitude estimates in Table I indicate. It must be noted that the field ranges listed in Table I are rough approximations and are not at all interchangeable with exposure within each scenario. Similar to the situation with various air pollutants, integrated E/MF exposure depends on an individual's activity, which may include proximity to a variety of field sources in and out of the home. Fixed area measurements cannot capture all of the contributions to an individual's exposure [11]. The VLF range includes E/MF with frequencies between 3 and 30 kHz. In the VLF range, the video-display-terminal (VDT) is probably the best characterised field source. During the 1980s, the use of VDTs in the workplace rose sharply and a number of laboratory studies subsequently investigated potential reproductive risks from exposure to the VLF magnetic field. In addition, over 10 studies of pregnancy outcome among VDT operators were reported. Field measurements for VDTs that represent 'operator exposure' are usually recorded, for the sake of convention, 30 cm in front of the screen face, although in most instances, the operator is more distant. At the standard operator position, the VLF electric field is typically below 10 V/m (rms) and the VLF magnetic field rarely exceeds 2 m G (rms) and is on the average below 1 mG (rms). The VLF magnetic field at a VDT has a sawtooth shape, with a fundamental frequency of between 17 and 31 kHz, depending on the individual VDT model. The VLF E/MF environment in front of a VDT is essentially no different from that which is found within a meter of a color television set. E/MF exposures in the VLF range are also associated with radio communications systems, magnetic detection security systems in airports, devices in the home with high speed electronic switching and appliances containing motors that rotate at high speeds. In addition, workers in a number of industrial operations work with equipment that generates VLF fields [12]. An electric field results from an applied voltage and is expressed in units of volts per meter (V/m). Under transmission lines, the fields are expressed as kilovolts per meter (kV/m). A magnetic field results from an electric current. Magnetic fields are expressed either in terms of magnetic flux density, or magnetic field strength. Scientists studying the biological effects of E/MF generally express magnetic fields in terms of flux density, for which the standard unit is the Tesla (T). Environmental flux density,

94 however, has been commonly expressed in terms of milligauss (mG). Conversions of these units may be calculated as follows: 1 m G = 10 -7 T and 1 /~T = 10 mG. Scientists interested in the physics of radiofrequency magnetic fields generally use field strength as the unit of measurement, expressed in units of Amperes/meter (A/m). In free space and biological media 1 A/m = 12.57 mG. For the sake of uniformity, the exposure conditions of all studies reviewed are expressed as flux density in units of Tesla. Environmental E/MFs are frequently expressed in terms of their root-mean-square or rms values. For sinusoidal fields the rms value is 0.707 × amplitude of the field. The use of the rms value often provides a basis for comparing field strengths from sources that do not necessarily produce sinusoidal fields. Another reason for using rms values is that electric and magnetic field exposure guidelines (or standards) have typically been provided in terms of rms field strength [13]. The use of rms values as a descriptor of field strength is not exclusive, as sawtooth fields are often characterized by their peak to peak ( p - p ) values; for a sawtooth waveform, rms field strength is equal to approximately 0.29 x p - p field strength. As described below, some experiments with avian embryos have used rectangular pulsed magnetic fields. In addition to the pulse frequency, these fields are characterized by the pulse height, the rise- and fall-times and the percent of each cycle the pulse is on (i.e. the duty cycle). Time-varying E/MF induce electric currents in exposed subjects. Electricallyinduced currents are due to capacitive coupling between the source and the subject and magnetic currents result from Maxwell-Faraday induction. The estimation of induced current densities (current per unit cross-sectional area of tissue) is one way in which an individual's exposure to E/MF may be characterized and is also used to compare the strength of different field sources. Indeed, the induced current density has been used in some cases as a criterion for health/safety standards. However, induced current densities are not necessarily predictive of biological responses which may depend on mechanisms yet to be elucidated. The amount and direction of current induced within the body of a field-exposed subject depends on field-specific, as well as subject-specific characteristics. For both electric and magnetic fields, induced current and current density within a given subject are proportional to field frequency x field strength. For electric fields, most efforts to estimate induced currents have focused on exposure to 60-Hz fields from overhead sources, specifically transmission lines. The key determinants of induced currents in this case are the exposed subject's size and shape, and the subject's electrical connection to the ground. Kaune and Phillips [14] have shown that, given the same field, the currents within a person are considerably higher than those in a quadruped. An average-sized adult, well grounded in a 60-Hz, 1 kV/m overhead field collects a total of approximately 15/~A that flow through the feet, with a current density at the waist of 0.25 mA/m 2. With non-conductive shoes (if the person were not grounded) current density at the waist would drop to approximately 0.16 mA/m 2 [15]. In contrast, current density in analogous anatomical sites in the rat and pig would be about 0.002 mA/m 2 and 0.02 mA/m ~, respectively. To model current density levels induced in humans from environmental electric fields, experimental electric field levels are typically scaled upwards to appropriate levels.

95 Magnetically-induced current flows in loops perpendicular to the field direction. The electric potential that drives the current is equal to the time rate of change of the magnetic flux within the loop (flux = flux density x loop area). Current density is proportional to the product of: field strength x frequency x tissue conductivity x radius of the loop. We may consider, for example, a horizontal body section at the abdominal level with a 15-cm radius (assuming a circular shape) and uniform conductivity of 0.2 Siemens/m (S/m). At the periphery of this section, a uniform 60-Hz magnetic field of 10 mG (rms) would induce a current density of 0.006 mA/m 2. Since magnetically induced current density is proportional to the linear dimension of the exposed subject, experiments with laboratory species must scale up magnetic field strength to achieve the induced current levels equivalent to those that magnetic fields induce in humans. Review of the literature

This review deals with effects of E/MF on reproductive parameters. It is subdivided into laboratory and epidemiology studies of ELF electric and/or magnetic fields with frequencies of 50 or 60 Hz at exposures designed to test for possible effects from electric power facilities; and VLF studies concentrating on magnetic fields similar to those found in the vicinity of video display terminals and other types of equipment to which a large number of people are exposed. Within the laboratory studies categories, the literature is arranged phylogenetically, with sections on submammalian vertebrates (mostly avian) and mammals exclusive of humans. Epidemiology studies are grouped by occupational exposure (VDT and other sources) and residential exposures. Laboratory studies: 50- or 60-Hz electric fields Non-mammalian studies

The embryonic effects of concurrent exposure to electric and magnetic fields in the Medaka fish were studied by Cameron et al. [16]. They exposed early embryos (2-4-cell stage) for 48 h to either a 60-Hz electric field that produced a current density of 300 mA/m 2, a 60-Hz magnetic field of 1.0 G rms, or to combined fields. The authors noted no significant developmental delays immediately after exposure, but found significant delays 36-73 h after removal from magnetic fields alone, or with combined magnetic and electric fields. Electric fields alone had no effects on development. The delays, which averaged 18 h for the magnetic field alone, did not result in developmental abnormalities or decreased survival through hatching. The authors did not discuss whether the delays were transient or remained throughout development. A question concerning possible interactions of electric and magnetic fields remains, since the combination of treatments resulted in a lesser degree of growth retardations (5 h) than the magnetic fields alone. The avian (chicken) embryo has been used extensively as an experimental system to study potential effects of electric and magnetic fields on development. Graves et al. [17] studied embryonic development and posthatching development of White Leghorn and Hubbard White Mountain chickens exposed to 60-Hz electric fields at

96 nine field strengths ranging from 0.1 to 100 kV/m. They did not find any exposurerelated responses in the parameters measured. Blackman et al. [18] exposed chicken eggs during the entire 21-day incubation period to low intensity sinusoidal electric fields of either 50 or 60 Hz at 10 V/m (rms). The authors used this intensity of the electric fields because it is similar to that found in typical dwellings. They calculated that the 60-Hz field produced a current of 0.2 nA within the egg. There was an ambient 60-Hz magnetic field of less than 70 nT also present. After hatching, the chicks remained in the exposure apparatus for 1.5 days before being killed. Brain tissue was then removed, placed in a physiological salt solution containing radioactive calcium, placed in a similar solution without radioactive calcium and subsequently exposed to E/MF of 15.9 V/m rms and 73 nT rms at either 50 or 60 Hz. After a 20-min exposure, the solution was analyzed for radioactivity. Eggs exposed during the incubation period to 60-Hz electric fields produced animals with brain tissue that was affected (in terms of calcium efflux) by 50-Hz but not by 60-Hz fields. Eggs exposed to 50-Hz fields during development were not affected by either 50- or 60-Hz fields after hatching. These results were observed in three replicates. The authors state that the biological significance, if any, of these data cannot be ascertained. Mammalian studies Laboratory studies on the potential developmental effects of ELF fields on mammals have been conducted with a number of laboratory species, primarily the mouse and rat (although some studies have used the guinea pig and swine). The perinatal effects of 50-Hz electric fields at 5 kV/m on rats (strain not given) was studied by Adrienko [19]. The author reported numerous effects on both male and female reproductive processes and in utero development after animals were exposed to these fields from 1.5 to 4.5 months. There appeared to be a slight reduction in the weight of the newborn pups and in their survival to 21 days for offspring of both treated males and females. There was no apparent relationship of response to exposure, however. Lack of experimental and statistical methodologies render this study extremely difficult to interpret. Cerretelli et al. [20] examined the effects of the exposure of male rats to a 50-Hz electric field of 100 kV/m on their fertility and the fetal outcome of their matings to control females. Groups of males were exposed for either 30 min/day or 8 h/day for 2-7 weeks. The biological significance of effects on fertility are difficult to interpret. Male reproductive function was considered to be unaffected although the methodologies used to determine a number of the parameters tested such as number of copulations and 'vitality' of sperm are not clear. Examination of litters resulting from matings of exposed males and control females did not show any morphological effects. They did report a significant fetal weight reduction in litters sired by males exposed to the fields for 8 h/day, but no numerical data are given. Marino et al. [21] conducted a three-generation study of male and female Ha/ICR mice exposed to either horizontal or vertical electric fields of 3.5 kV/m. Postnatal weight gain was similar in exposed and unexposed mice. They stated that the electric field caused an excess postnatal mortality among the field-exposed groups. In a previous study [22] similar results with 10 and 15 kV/m were reported, but the

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authors attributed the effects to microshocks induced by the field. Although the authors [21] claim to have rectified that problem, the field-related lethality they report has never been confirmed by research in other laboratories. Fam [23] exposed male and female ICR-SW mice to 240 kV/m, 60-Hz electric fields throughout the entire developmental period. Animals were bred and their litters monitored for growth, blood histology and biochemistry and histology of critical organs. No significant changes were noted in any of the parameters examined. Free et al. [24] examined the effects of 60-Hz electric fields at 64 kV/m (effective field strength) on a wide spectrum of hormones which impact the reproductive cycle (testosterone, FSH, LH, corticosterone, prolactin, TSH, GSH and thyroxin). Sprague-Dawley rats were exposed for 7 weeks starting at 20 days of age. Results were not consistent from month to month and no treatment-related effects were reported. The effects of prenatal exposure to a 60-Hz, 80 kV/m electric field on postnatal development of the Sprague-Dawley rat were studied by Burack et al. [25]. They exposed pregnant rats during the latter period of gestation (days 14-21). The number of litters exposed was relatively small (a total of 17 treated and 12 controls in two experiments). After birth, litters were examined for viability, growth and maturation as measured by body weight and developmental landmarks including ear flap separation, eye opening, anogenital distance at birth, age at testes descent, age at vaginal opening and male sexual response in testosterone-treated gonadectomized animals. No significant changes were detected in litter size or viability. The authors did find a tendency towards reduced postnatal development as measured by slight reductions in body weight and delays in ear flap separation and eye opening but these differences were not statistically significant. No significant differences were noted in anogenital distances or age at vaginal opening. There was a significant reduction in the percentage of exposed males that displayed copulatory behavior when compared to controls. The small numbers of animals used in these studies precludes definitive conclusions and the authors state that the experiments 'were intended as pilot studies'. The presence of general stress as a confounder in these experiments must also be considered in attempting any conclusions, since the effects noted by the authors resemble those seen after general stress in rats [26,27]. Margonato and Viola [28] studied the offspring of male rats exposed at 50 Hz, 100 kV/m for short (30 rain/day) or long (8 h/day) exposure periods for up to 48 days. No treatment-related effects on male reproductive parameters (fertility, sperm viability and morphology) or offspring (number of implantations, percent live/litter, or incidence of malformations) of treated animals were reported. Seto et al. [29] exposed rats to a 60-Hz electric field at an intensity of 80 kV/m. Animals were exposed for 21 h daily over four generations. A teratology study was performed on the fetuses of fourth generation animals. No significant effects were noted in fertility measures or postnatal growth of offspring of the first three generations. No intrauterine effects were seen on the frequency of resorptions and/or fetal deaths, or fetal malformations. The authors concluded that exposures used in this study produced no effects on the reproduction or in utero development of the rat. Sasser et al. [30] studied the effects of electric field exposure on the prenatal

98 development of the guinea pig (Duncan-Hartley strain). Females were exposed to 60-Hz electric fields (100 kV/m) for 40 days, bred and allowed to litter. There were no statistically significant adverse effects on pregnancy rate, abortion rate, litter size, or birth weight. Benz et al. [31] examined reproductive outcome in two strains of mice tested in various protocols. In all experiments, the electric and magnetic fields were presented simultaneously (3000 mG with 15 kV/m or 10 000 mG with 50 kV/m). One experiment tested effects of pre-breeding field exposure of male mice on fertility and on the health and viability of their offspring in utero. This experiment was designed to determine if field exposure conferred a dominant lethal mutation (DLM) to the fetus. Approximately 13 000 litters were examined in this experiment. In addition to the finding that exposure had no effect on DLM, there were no field-related effects on pregnancy, number of corpora lutea or implants, or litter size. In the only other DLM study conducted with E/MF exposure, Kowalczuk and Saunders [32] reported no effect of 2-week exposure of males to a 20 kV/m electric field on in utero death and/or litter size and viability. In this experiment, the females were not exposed. In a second experiment by Benz et al. [31] two consecutive generations were bred with both males and females receiving exposure (or sham exposure). No exposure-related differences were observed for either fertility or litter size. A series of studies by Sikov et al. [33] investigated the effects of 60-Hz electric fields (65 kV/m effective field strength) on perinatal development in SpragueDawley rats. Experiments examined reproductive performance and prenatal development, postnatal effects of exposure throughout gestation and the first week postpartum and postnatal effects of exposure late in gestation and for 25 days postnatally. There were no consistent treatment-related effects noted in any of the parameters studied, including fetal development and postnatal growth and viability. A study on the potential of 60-Hz electric fields to affect reproduction and development in swine was reported by Sikov et al. [34]. Hanford swine were exposed to a uniform, vertical, 60 Hz, 30 kV/m electric field for 20 h/day, 7 days/week. Sows were exposed for 4 months prior to breeding and pregnant animals were either sacrificed before term for litter evaluation (Teratology I) or allowed to farrow. Those allowed to farrow were bred again and killed before term for fetal examination (Teratology II). The F1 generation was exposed to the fields for 18 months and then bred. These animals were allowed to farrow (creating an F2 generation which was killed 10 months later) and subsequently bred again and killed before term for teratological evaluation (Teratology III). The mean number of live fetuses/litter was increased in the exposed groups in all three teratology studies, but the increase was statistically significant only in Teratology I. This finding was coupled with a general trend for reduced fetal death in the exposed groups in all experiments (again, significant only in Teratology I). Statistically significant reductions in several measures of fetal size appeared only in Teratology II. A statistically significant decrease in fetal malformations was noted in exposed litters in Teratology I, a statistically significant increase in the incidence of defects was seen in exposed animals in Teratology II and no effects were noted in Teratology III. No significant differences were noted in fertility, weight, or perinatal mortality in litters of either the F 0 or F~ swine. A trend for an increased incidence in malformations in both F0 and F l offspring was noted

99 and this trend was significant in the F 1 litters. While these studies did show a number of statistically significant differences, it must be noted that, as the authors state, 'there were also striking differences among the patterns of results obtained by the three teratology studies; and between the litters of the two generations that produced offspring'. The relationship between exposure to electric fields and the differences noted in teratogenicity within these studies remains difficult to interpret and the authors noted that 'it is difficult to advance an explanation to reconcile the differences among these results and to explain the lack of effects in Teratology III'. Finally, the presence of a disease outbreak during the course of the second breeding of the F 0 animals is another factor of unknown significance. As a follow-up to the study on swine reviewed above, Rommereim et al. [35] exposed female Sprague-Dawley (CD) animals to 60-Hz electric fields of 65 kV/m. The study involved exposure to the fields 19-20 h/day, beginning at 3 months of age and continuing through two breedings and a teratological examination was done near term in selected rats during the first and second gestational periods. Female offspring of the first breeding (F1) were continuously exposed, bred at 3 months of age and killed near term for teratologic examination. The complete study design was carried out in two replicates. There were a number of reproductive and developmental measures for which exposed vs. unexposed differences approached or reached statistical significance in one or both of the experiments, but few differences were replicated across studies. In the second pregnancy of the F 0 rats of the first experiment there was a statistically significant decrease in the percentage of litters with resorptions among exposed litters and this tendency remained throughout the studies, although statistical significance was not again reached. There was a consistent decrease in prenatal mortality in litters of F 1 exposed females in both studies. A statistically significant divergent sex ratio was seen in the second pregnancy of the F 0 rats in the first experiment, but this effect was not seen again. A statistically significant increase in the degree of ossification of the skull was seen in exposed litters of the first pregnancy of the F 0 generation, but this effect was not repeated. In the exposed group of the second breeding of F 0 females, the incidence of reduced sternebral ossification was increased. The incidence of litters with reduced ossification of the phalanges was statistically decreased in the exposed litters of the Fi dams, but this effect was opposite to the trend in the other groups. The incidence of malformations of all types (major malformations as well as minor variations) was increased (P < 0.12) in exposed animals in the second breeding of the F0 generation, but this effect was not seen in the first breedings of any experiment, nor in the second breeding of the second study. Growth and viability did not differ between treated and control groups. These studies as a whole are extremely difficult to interpret given the large number of variables coupled with multiple breedings and fetal examinations. The authors state that the effects seen on incidences of prenatal mortality and malformations may be due to random variation rather than a biologically significant effect. The authors suggested that the failure to replicate effects across experiments may have been due to the fact that the 65 kV/m electric field may be a threshold for effects in the developing rat. Accordingly, Rommereim et al. [36] initiated a series of studies to test the ability of rats to reproduce and rear litters at higher levels of electric fields. Male and female

100 animals were exposed to electric fields of 112 and 150 kV/m for 19 h/day for a period beginning 1 month prior to breeding and continuing through breeding, completion of gestation and the rearing of offspring until weaning. No differences were noted in breeding success, pregnancy rate, litter size, or postnatal growth and development. Utilizing this study as a range-finding exercise, they studied rats exposed to effective field strengths of 0, 10, 65, or 130 kV/m. Exposed F0 animals were bred, allowed to litter and exposure continued through the breeding and entire pregnancy of the FI generation [37]. Standard evaluation of near-term fetuses was done at the termination of the study. There was a non-statistically significant decrease in gestational weight gain in the F 0 groups exposed to the two highest field strengths. This result is difficult to interpret since a similar reduction did not occur in the previous study which used a higher field strength and it was not seen in the subsequent gestational weight gain of the FI animals. Reproductive outcome was unaffected by exposures; no effects being noted in pregnancy course, pup weight at term and postnatal growth and viability. The teratology study of litters of the Fl animals indicated no effects on fetal weight, viability, or the occurrence of malformations. It should be noted that the exposure levels in this study were shown to be capable of resulting in biological effects, as there was a field strength-related incidence of chromodacryorrhea in both the F 0 and F 1 females. Chromodacryorrhea is a secretion around the eyes which is thought to originate in the Harderian gland and has been shown to be inducible by a variety of stressors [38].

Laboratory studies: magnetic fields Non-mammalian studies Delgado et al. [39] exposed small groups (3-9 embryos per group) of White Leghorn chicken eggs for 48 h immediately after laying to pulsed rectangular shaped magnetic fields of 0.5 ms duration. Three pulse heights were used, 0.12 tzT, 1.2/zT and 12/~T; each at three frequencies, 10 Hz, 100 Hz and 1000 Hz. Examination of the embryos after the exposure period indicated some degree of growth retardation in the 100- and 1000-Hz groups at all intensities tested. Another series of experiments was reported by these workers [40] in which eggs were exposed to 100 Hz, 0.5 ms wide rectangular pulses ranging in amplitude from 0.4/zT to 104/zT. The rise- and fall-times on the fields ranged from 2 #s to 100/zs. No consistent patterns of effect were seen in terms of either pulse shapes or intensity levels and there was a great deal of variation between replicates in which embryos were exposed to a pulsed field with a rise time of 100/~s and an intensity of 1.0/~T. Experimental groups were both growth-retarded and advanced after exposure to different magnetic field shapes. Given the variability of response and lack of a dose-response, the significance of these effects remains difficult to assess. Leal et al. [41] continued these efforts and attempted to relate the difficulty this group experienced in replicating results within their laboratory to the earth's magnetic field. Eggs were exposed for the first 48 h of development to 100 Hz 0.5 ms wide magnetic fields of either 0.4 or 1.0 ~T amplitude, with 2/zs rise time. The authors selected 13 experiments from a large number done in their laboratory based upon differences in the results. They found correlations between the abnor-

101 mality ratio (percent abnormal in exposed/percent abnormal in control) and the horizontal component of the earth's magnetic field. It is not clear from the paper whether changes in the earth's magnetic field were correlated with seasonal differences which might account for alterations in the background incidence of malformations. An additional factor which makes interpretation of their studies difficult is the high incidence of abnormal embryos these investigators find in control eggs. The percentage of abnormal embryos in controls ranged from 7 to 70% with numerous experiments resulting in control incidences of over 30%. The percentages of abnormal control embryos reported by these workers leads these reviewers to question whether other confounding factors which affected both control and treated eggs were not present. In a subsequent study by the above group [42], the authors attempted to define a relationship between the orientation of the embryos and the effects induced by 100 Hz, 0.5 ms wide pulsed magnetic field with 85-100 #s pulse rise time and either a 1.0 or 24.9 #T pulse amplitude. At 1.0/zT there was a significant increase in abnormal embryos in the exposed group and this increase was significant only in southwest and southeast oriented embryos (but not south oriented embryos). No increase in abnormalities was noted in the embryos exposed to 24.9/zT but the authors did find a significantly higher proportion of exposed embryos oriented to the south when compared with controls. This paper is somewhat difficult to interpret since significant effects were scattered among the exposed groups. Given the lack of pattern, the possibility of these data reflecting random variation cannot be excluded. Additionally, the exposure conditions were sufficiently different to preclude direct comparison between the different studies conducted by these researchers. Maffeo et al. [43] exposed White Leghorn chick embryos for the initial 48 h of development to 0.5 ms rectangular pulsed magnetic fields of 1.2 or 12.0/~T at both 100 and 1000 Hz. These four combinations of field parameters were associated with developmental effects in the study by Delgado et al. [39]. The pulse Maffeo et al. utilized had a rise time of 10/~s. The authors state that there did not appear to be any consistent exposure-related effects and they concluded that there were no differences between exposed, sham-exposed and control embryo development. A subsequent study [44] failed to replicate the conclusions of Ubeda et al. [40] that some combinations of magnetic waveforms and intensities adversely influenced chick development. These studies by Maffeo et al. utilized larger numbers of eggs and identical pulse shapes and magnetic field strengths (100 Hz pulsed magnetic fields of 1 /~T amplitude with 0.5 ms duration and a rise time of 42/~s, similar to Ubeda et al.) and found no adverse effects on the development of chick embryos. All of the above studies suffer from a highly variable rate of abnormalities in the control eggs, making the detection of significant field-induced adverse effects and interpretation of these effects, difficult. Juutilainen and coworkers have published a series of studies on the effects of magnetic fields on chick egg development. Juutilainen et al. [45] exposed Makela 16 breed (a variety of White Leghorn) embryos during the initial 52 h of development to 100-Hz magnetic fields of four different waveshapes (a) pulses of 0.5 ms pulsewidth, with amplitudes of 0.25/zT (rise time 0.15 tzs), 2.0/~T and 20 #T (20/~s rise time); (b) sinusoids with rms amplitudes of 0.13 #T, 1.3/zT, 10/~T, and 100/~T;

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(c) square-shaped bipolar pulses with rms amplitudes of 0.13 /zT and 1.3/zT (0.15 t~s rise time for both) and 10 #T and 100 I~T (15 t~s rise time for both); and (d) squareshaped unipolar pulses with rms amplitudes of 0.13/zT and 1.3/zT (0.15/zs rise time for both) and 10/~T and 100 #T (15/zs rise time for both). The authors report adverse effects (anomalies or developmental retardation) with all waveforms at some field intensity. Their work failed to replicate that of Delgado et al. [39] since no adverse effects were noted at 0.13/zT, which corresponded to the teratogenic dose of 0.12 #T. In the Delgado study the pulse rise times were not stated. Also in contrast to the study by Delgado et al., these authors found no evidence of an intensity window. Effects reported by Juutilainen et al. occurred at all intensities above 1.3 tzT. Juutilainen and Saali [46] exposed eggs for 48 h to sinusoidally oscillating magnetic fields at frequencies ranging from 1 Hz to 100 kHz and field strengths ranging from 0.13 to 125.7 t~T (rms). They reported increased incidences of anomalies at various frequencies and/or magnetic field intensities. No dose-related response was noted in these studies either in terms of frequencies or field strengths. The authors suggest that at higher frequencies 1.3/zT may be a threshold for induction of adverse effects, above which increasing field strength does not increase the effect. The authors also suggest the existence of 'intensity windows' at lower frequencies since effects occurring at 16.7 Hz and below were seen at 0.13 and 1.3 t~T but not at higher intensities. In a subsequent paper, Juutilainen [47] investigated the variation in response between his previous work and that of Maffeo et al. [43] by altering incubation temperatures and the pre-incubation storage period. The eggs were exposed to a sinusoidally oscillating magnetic field with an rms strength of 1.3/~T at a frequency of 100 Hz during the entire incubation period. Neither varying the temperature between 36.3 and 38.5°C, nor varying the pre-incubation time between 1 h and 4 days, produced clearly synergistic effects in terms of embryonic development, but the percentage of abnormal control embryos increased with increasing temperature or pre-incubation storage for more than 1 day. The authors concluded that the increased incidence of defects in control eggs stored for longer periods of time before incubation or incubated at higher temperatures obscured the effects of the magnetic fields. The rationale for these studies remains obscure since no evidence was ever presented that would implicate either temperature or storage time as factors that differed between the Maffeo and Juutilainen studies. It should be noted that the temperature which Juutilainen finds to be embryotoxic (38.5°C) is extremely close to that routinely used by other workers (38°C in the laboratories of Maffeo, Ubeda/Leal, Sandstrom, and Martin). The classification of defects by the authors as 'mild', 'severe' and 'strongly retarded' is not the standard way in which developmental effects are grouped. Nevertheless, in studies that used incubation temperature as a variable, the groups exposed to magnetic fields exhibited a greater percentage of abnormal embryos in each instance and these experiments replicated those done earlier [45]. The relationship(s) of field strength and abnormal development was investigated by Juutilainen et al. [48] using sinusoidal 50-Hz magnetic fields ranging from 0.13 to 12.6 I~T strength. Two separate studies were done using different field strengths and the authors concluded that normal development occurred up to approximately

103

1.3/~T, with higher levels resulting in increased incidences of abnormal embryos. All of the studies by Juutilainen and coworkers indicate that sinusoidally oscillating E/MF induce adverse developmental effects in the avian embryo and as such, appear to be internally consistent. The studies reported by these workers consistently indicate a threshold phenomenon in which adverse effects are not noted at < 1.3 ~T. No dose-relationship is apparent, however, at exposures above these levels. The effects of magnetic fields consisting of an asymmetrical, sawtooth waveform on early (42 or 47 h) chick development were studied by Sandstrom et al. [49]. Triangular pulses were used with a frequency of 20 kHz, a rise time of 45/~s and a fall time of 5/~s. The p - p values of the magnetic fields were 0.1 #T, 1.5 ~T and 16 /~T in three different experiments. The authors chose these field conditions because they simulate the exposures of VDT operators. No significant effects in either incidence or type of abnormality were noted in any exposed group. The authors point out the difficulty in comparing various studies of chick development in pulsed magnetic fields because of the variation in the physical parameters of the experiments. They cite orientation of the egg in the fields as one of the variables and note that in Juutilainen et al. [45], the eggs were placed in a vertical magnetic field blunt end up; in Delgado et al. [39], Ubeda et al. [40] and in their study eggs were placed in a horizontal magnetic field on their side; and in Maffeo et al. [43] several eggs were placed in a long coil with the long axis of the eggs perpendicular to the magnetic field. Additional experiments are needed before any conclusions may be reached concerning the importance of the orientation of the egg to the exposure. Berman et al. [50] presented the results of a collaborative study (the 'Henhouse Project') on the effects of pulsed magnetic fields on early chick development (White Leghorn breed). This study was intended to address the validity of previous avian research. Eggs were exposed to similar fields in six different laboratories (unipolar 100 Hz pulsed magnetic fields of 1 #T amplitude, 0.5 #s duration and 2/zs rise and fall time). Eggs were exposed for the initial 48 h of incubation and then examined for fertility, developmental stage and morphology. The authors reported that two of the six laboratories showed a significant treatment effect, seen as a decrease in the percentage of normal embryos as a function of both fertile eggs and live embryos. There was also a trend (P < 0.08) towards increased anomalies in one of the four laboratories where no statistically significant effects were noted. Further, this effect on the proportion of normal embryos among either fertile eggs or live embryos was statistically significant when results from all laboratories were combined. The data remain difficult to interpret since laboratory site (which laboratory performed the experiment) had the strongest effect (statistically significant at P < 0.01) for all primary variables measured. The authors concluded that this study demonstrated a biologically significant relationship between magnetic fields and chick embryo development. Using similar exposure parameters, one of the participants in this study subsequently showed that effects on the embryo were induced during the initial 24 h of incubation. Exposure from 24 to 48 h failed to induce significant effects [51]. Three additional studies have been reported only in abstract form and have not, as yet, been published as full papers. Saha et al. [52] tested the effects of pulsed magnetic fields differing in shapes, amplitudes and repetition frequencies in chick embryos. The length of exposure is not specified, nor are the days on which the

104 embryos were examined. The authors stated that some waveforms increased developmental rate, others decreased growth rate and one type of field produced different effects on different days of development. Based upon these findings, the authors concluded that the rate of embryo growth was affected by E/MF stimulation. Martucci et al. [53] failed to find adverse effects in chicks exposed to weak pulsed magnetic fields. They exposed chick embryos for the initial 48 h of development to a 100 Hz pulsed magnetic field with an amplitude of 1.2/~T, 0.5 ms pulse width and 1.5 t~s rise time. There were no statistically significant differences noted between exposed and sham embryos. Martin [54] stated that significant effects seen in White Leghorn eggs exposed to a unipolar field, 100 Hz, 1 #T pulsed (similar to that used in the Berman study [50]) were not seen in another breed of chicken, the Arbor Acre. This author also reported that no adverse developmental effects were seen in White Leghorn embryos exposed to various 60-Hz sinusoidal fields of 3 ~T, p-p intensity (bipolar, unipolar, or split pulse produced by a light dimmer switch). Sisken et al. [55] exposed chick embryos (White Leghorn breed) for either the first 24 h or 7 days of development to repetitive bipolar pulse bursts. The first burst was a 3.8 kHz pulsed wavelength (approximately 256/zs cycle time) activated for 50 ms at a 2 Hz repetition rate. Power was applied to the coils for 250/~s (mean polarity segment) of the 256 #s cycle, during which the magnetic field averaged 10 izT, reached a peak of 250 #T and increased at a rate of 1 #T//~s. For the 6 ~s the coil was de-energized the field collapsed at a rate of 40 ~T/t~s. The second burst was a 4.4 kHz pulsed wavelength (approx. 220 ~s cycle time) activated for 5 ms at a 15 Hz repetition rate. During the main polarity portion of the pulse (200/~s), the magnetic field averaged 50/zT and reached a peak of 1.6 mT at a rate of 3 #T//~s; during the remainder of the cycle the field collapsed at an 80 izT/~ts rate. Embryos were collected at 7 days and subjected to a standard avian teratology examination. The authors detected a small but apparently consistent increase in malformation incidence in exposed embryos in all groups. The incidence of abnormal embryos was not statistically significant, and no abnormality occurred in exposed embryos that did not also occur in controls.

Mammalian studies Persinger et al. [56] exposed pregnant Wistar rats to one of three treatments (0.5 Hz rotating magnetic field at 5, 100, or 1000 ~T) from day 19 of gestation to 3 days postpartum. Group sizes were small (three animals) and differences noted appeared to be random. Grissett [57] exposed rhesus monkeys (ages unspecified) to a magnetic field of 0.2 mT and simultaneously to an electric field of 20 V/m. The field was sinusoidal and was pseudo-randomly oscillated between 72 and 80 Hz (approximately every 1/8 s). This exposure regimen differs from the other studies reviewed since magnetic and electric fields were applied concurrently. The animals were monitored for growth rate and blood chemistry and (in the third year) for spermatogenesis. The only difference noted was increased growth rate in males during the first year. Sham and exposed animals were matched for weight rather than for age so that it is possible that differences noted were due to a difference in the initial growth rate of exposed and sham animals rather than the exposure itself. Using the same exposure

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regimen, Grissett et al. [58] reported a similar experiment using younger animals. In this study, no differences in growth and development were noted in animals exposed from 4 to 67 weeks of age. McRobbie and Foster [59] exposed Swiss-Webster (CD-1) mice to pulsed magnetic fields ranging from 3.5-12 kT with pulse periods in the range of 0.33-0.56 ms. The exact date of the beginning of gestation was not known and therefore the mice were exposed for varying and unspecified periods during gestation. Animals were allowed to give birth and the numbers of live young and growth rates recorded after birth. No treatment-related effects were noted. The protocol utilized by these workers does not follow standard test guidelines. The mating of the mice was done among siblings although this strain is outbred. The gestational period during which exposure took place is not known, which is contrary to standard acceptable teratology bioassays or in vivo screens. These data are therefore of little value in the evaluation of the potential developmental effects of E/MF on mammals. In two reports of preliminary results from ongoing studies, Tribukait et al. [60,61] exposed pregnant C3H mice continuously from gestation day 0-14 to either (a) 0.5 ms wide 100-Hz rectangular-shaped field pulses of amplitude 1 t~T or 15 t~T (2 #s rise time) or (b), 20 kHz sawtooth fields 1 #T or 15/~T p-p with a rise time of 45 #s and a fall time of 5 #s. In both reports, the authors conclude that increases in malformations were noted in fetuses from dams exposed to the 15-#T sawtooth pulses. No additional data were accumulated on rectangular pulse exposure in the 1987 study [61], while additional data were reported for the control and two sawtooth pulse groups. Thus, the 1987 report comprises the data from experiments reported in 1986 [60] with those gathered subsequently. In both of the Tribukait reports, the incidence of external malformations in fetuses was significantly increased in fetuses exposed to 15-#T sawtooth fields. However, if the latter study is analyzed separately (Table II in this review) it is apparent that no significant increase in external malformations was noted. Thus, the adverse effects noted in the first study were not seen in the second. Although the combined results of both studies may still be statistically significant [61], the inability to replicate effects raises doubts concern!ng biological significance. An additional difficulty in the interpretation of these studies is the authors' use of the fetus rather than the litter as the statistical unit, which is contrary to general practice with teratology bioassays. TABLE I1 S U M M A R Y OF DATA G E N E R A T E D BY TRIBUKAIT ET AL. IN THE C3H MOUSE BETWEEN THE 1986 A N D 1987 REPORTS OF AN O N G O I N G STUDY Litters

Controls 1 #T (sawtooth) 15 /~T (sawtooth)

48 22 37

Fetuses

220

86 164

Malformations Total

External

5 (2%) 1 (1%) 4 (2%)

1 (0.5%) 0 2 (1%)

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Frolen et al. [62] attempted to extend the work of Tribukait et al. [61] utilizing a different strain of mice, the CBA/S. They used 20 kHz sawtooth pulses with 45 ~ts rise time and 5 /xs fall time, with p - p field strength of 15 ~tT. This exposure was similar to that reported as effective in the Tribukait studies. Here, however, pregnant animals were exposed to the fields throughout the entire gestational period. The authors concluded that although they did not observe any increase in the malformations as seen in the Tribukait studies, they did note an increase in resorption rate and the number of dead fetuses in exposed groups. Examination of the data reveals a concomitant increase in the average number of implantations and live fetuses at term in the exposed group. Frolen and Svedenstal [63] repeated and extended their previous studies. Using the same exposure paradigm as detailed above, experiments were conducted in which groups of mice were exposed for different periods during gestation. The studies consisted of exposure periods of gestation days 1-19, 2-19 and 5-19. The same pattern of a statistically significant increase in resorption rate was seen in all three experiments. It is of interest that a pattern of increased implantation rates in exposed groups was also seen. In all of the above experiments, the difference in number of implantations between controls and exposed was greater than the difference in the number of live fetuses at term. It is difficult to interpret the significance of the data presented since the number of live fetuses at term is higher in the exposed in two of the four experiments. This lack of effect at term may indicate that resorption rates are a direct reflection of random differences in the initial implantation number rather than an embryocidal effect of the exposure. In a study by Mikolajczyk et al. [64], pregnant rats were exposed to either 1.35-1.73 V/m (from black and white TV) or 0.63-0.81 V/m (from color TV). We assume that this exposure included both 50-Hz electrical and magnetic components and an approximately 20 kHz sawtooth magnetic component of unknown flux density. The exact gestational periods of exposure were not specified but appear to include a substantial portion of organogenesis. No significant effects were noted in the number of live or dead fetuses per litter when control and experimental groups were compared. This study, which did not follow standard teratology bioassay guidelines, is irreparably flawed since there was no examination of fetuses for the presence of defects, and the in utero period of exposure was unknown. In addition, no further characterization of the E/MF exposure was presented beyond stating the intensity ranges. Stuchly et al. [65] studied the teratogenic potential of magnetic fields with an 18 kHz sawtooth waveform (44 t~s rise time, 12 ~s fall time) in Sprague-Dawley rats. Three intensities of magnetic fields (5.7, 23, or 66 txT p - p ) were used and rats were exposed 2 weeks prior to mating and throughout pregnancy for 7 h/day. Shortly before term, the pregnant animals were killed and fetuses were gathered and examined for malformations. No treatment-related differences were noted in maternal parameters, fetal weight, or terata. A significant decrease in the incidence of bipartite or semi-partite thoracic centra was noted in the two highest exposure groups. Contrary to this trend, a significantly increased incidence of minor skeletal anomalies was noted in the fetuses (but not litters) of the group exposed to the highest magnetic intensity. The biological significance of the minor skeletal anomalies reported is unknown and there is no general agreement among developmental toxicologists as

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to the categorization of various xenobiotic-induced deviations from normal embryo/fetal in utero development. Rommereim and Anderson [66] exposed pregnant rats throughout gestation to 60-Hz sinusoidal, horizontal, magnetic fields ranging from 0.001 to 10 G. The abstract reporting this study stated that 'no robust developmental toxicity was found'. Wiley et al. [67] reported on a study to test the teratogenic potential of VDT-like sawtooth magnetic fields on Swiss-Webster (CD-I) mice. Animals were exposed throughout gestation to 20 kHz fields (45 #s rise time, 5/~s fall time) ranging from 3.6 to 200 #T p-p. No treatment-related adverse effects were found on maternal weight gain, fetal weight, number of implants, or the number of live and/or dead embryos/fetuses. Postnatal effects on sexual behavior and sex hormones were tested in Sprague-Dawley rats after prenatal exposure to a 15 Hz pulsed magnetic field of 0.3 ms duration, 330 #s rise time and peak intensity of 800/zT by McGivern et al. [68]. Pregnant animals were exposed for two 15omin periods on days 15-20 of gestation. At birth, no treatment-related effects on offspring of exposed dams were noted for number live, average weight, or anogenital distance. At day 120 postpartum, reproductive morphology and function of males was analyzed by measurement of circulating testosterone, LH and FSH, testes and accessory sex organ weights and scent marking behavior. No treatment-related differences were noted in hormone levels, but significantly increased accessory-sex organ weights and significantly reduced scent marking behavior were noted. These effects are consistent with hormone-mediated effects occurring in utero. Further studies attempting to replicate and expand upon these observations would be of interest. Zusman et al. [69] studied the effects of pulsed E/MF on both in vitro and in vivo development of Sprague-Dawley rat embryos and on the in vitro development of Hebrew University mouse embryos. Pregnant rats were exposed to the E/MF at frequencies of 20, 50, and 100 Hz (0.6 V/m). Preimplantation rat and mouse blastocysts were exposed in vitro at frequencies of 20, 50 and 70 Hz (0.6 V/m) and 1, 20 and 50 Hz (0.6 V/m), respectively. Continuous exposure of pregnant rats throughout the entire gestation period did not result in significant effects on postnatal growth or viability. Exposure of day 10.5 rat embryos in vitro to 50 and 70 Hz resulted in a significant incidence of developmentally retarded embryos after 48 h of exposure. The biological process(es) underlying the apparent differences in in vitro and in vivo response in the rat is unknown. In the mouse a significant increase in the percentage of embryos with arrested development was seen in the 20 and 50 Hz groups after 72 h of exposure. Among those embryos which continued to develop irrespective of treatment, no differences were noted in the rate of development. Epidemiology of E/MF and pregnancy outcome

This section reviews the epidemiologic literature that has addressed the potential effect of E/MF exposure on pregnancy outcome. The literature reviewed is primarily concerned with spontaneous abortions and congenital malformations, but in some

108

cases also includes other measures, such as birth weight. This review is restricted to endpoints observed in the prenatal and perinatal period and thus does not address the literature about a potential relationship between childhood cancer and parental exposure to E/MF. In no study reviewed is exposure to E/MF directly quantified. Rather, occupational exposure is based on job title and/or personal interview and residential exposure is based on the presence or use of specific E/MF sources.

Occupational exposure." video display terminal operators The largest set of studies that has considered pregnancy outcome in women potentially exposed to E/MF focuses on female video display terminal (VDT) workers. VDTs, which operate essentially like television sets, produce ELF magnetic fields as well as ELF electric fields, both at or near 60 Hz. At the operator position, the magnetic field is several mG, for some units ranging between 5 and 7 mG; the electric field is below 10 V/m. VDTs also produce electric and magnetic fields in the very-low frequency (VLF, which includes 3-30 kHz) range, with the VLF magnetic field at the operator position typically no greater than 1-2 mG and the electric field below 10 V/m. The VLF frequency of a VDT is dependent on its design, but for most units is around 20 kHz (for review, see Ref. 13). Kurppa et al. [70] conducted a case-control study of birth defects based on 1475 cases reported to the Finnish Register of Congenital Malformations between June 1976 and December 1982. Controls were time- and area-matched by selecting the delivery preceding the case in the same maternity health district. Exposure classification was based on a review of maternal interview data by an industrial hygienist and two experts in occupational medicine. The interview, conducted shortly after delivery, did not inquire explicitly about VDT use. Fifty-one cases and 60 controls were classified as exposed during early pregnancy (first trimester). With exposure dichotomized at a 4 h/day VDT use cutpoint (and with adjustment for several confounders) an odds ratio (OR) of 0.9 (95% confidence interval (CI) 0.6-1.3) was determined. We note that 490 of 2950 women in the original study sample were classified as potentially exposed, of which exposure was evident for only 120 (111 in the first trimester who formed the basis for the main analysis). Exposures for 370 women were thus excluded from the main analysis and we cannot determine with the information provided if potentially informative data were thereby discarded. A follow-up study by Nurminen and Kurppa [71] of working mothers from the control series of Kurppa et al. [70] reported no VDT-associated complications of pregnancies. Endpoints included threatened abortion, length of gestation, birth weight, placental weight and maternal blood pressure. In 1986, Butler and Brix [72] presented a conference paper of a study of spontaneous abortions among women employed by the State of Michigan. The study covered four state departments with offices in 46 locations, mostly near Detroit and Lansing. After an initial screening phase, the study was restricted to 817 pregnancies in 728 women between 1980 and 1985. Hours of VDT use were based on personal interviews, which included a range of questions about key covariates. Butler and Brix reported a small but not statistically significant increase in spontaneous abortions and stillbirths combined among women with 21-40 VDT hr/wk (26 observed vs. 21.2 expected). For moderate VDT use (1-5 h/week), fewer adverse outcomes

109 were observed than expected, which again was not statistically significant. Given the lack of any exposure-response pattern, the heterogeneity of the job classifications considered and inconsistent patterns across those classifications and the suspicion of modest recall bias, the results of this study are not indicative of VDT-related risks to survival of the conceptus. In Sweden, Ericson and K~ill6n [73] conducted a cohort study of pregnancy outcome among women in white collar occupations. The investigators classified each woman's VDT use as high, medium, or low, based on their interpretation of the Swedish census occupation codes; thus, for example, travel agency clerks were classified 'high' and bank cashiers as 'low'. Each woman's unique personal identification number was used to link occupational classification to medical data. Birth data were obtained primarily from the Medical Birth Registry, with supplementary material provided by the Swedish Registry of Congenital Malformations. For enumerating spontaneous abortions, the Inpatient Registry for Somatic Care was used. In addition to spontaneous abortion and congenital malformation, Ericson and K~ill6n studied stillbirth, early neonatal death (< 7 days old) and low birth weight (< 1500 g and < 2500 g). Outcomes were studied for the periods 1976-77 and 1980-81, except for spontaneous abortions, which were considered for 1980-81 only. Results for all parameters, except spontaneous abortion, were expressed in standardized terms (observed/expected), with expected values based on Swedish national data and adjusted for age, parity and delivery unit. The analyses for 1976-77 included 5181 pregnancies and for 1980-81 included 4928 pregnancies. For spontaneous abortion, expected values were generated internally from the study population, standardizing for age. The spontaneous abortion analyses included 4320 pregnancies. Observed/expected (O/E) ratios were obtained for a number of endpoints. The 'medium' exposure group registered a higher incidence of low birth weight (< 1500 g) for the 1980-81 period (O/E of 2.2; 95% CI 1.1-4.4) and a higher incidence of low birth weight (30 h/week at the start of pregnancy; for stillbirth, to women employed > 30 h/week at least 2 weeks during pregnancy; for birth defects, to women employed >_ 15 h/week at the start of pregnancy. Spon-

111 taneous abortion, stillbirth and congenital defects were examined in both current and previous pregnancies, while birth weight and prematurity were analyzed for the current birth only. Depending on endpoint, from 22 000 to 25 000 current pregnancies and from 18 000 to 23 000 previous pregnancies were analyzed. Logistic regression techniques were used to obtain O/E ratios for all endpoints except birth defects, for which potential confounders could not be identified. Analyses were conducted either by (1) individual, in which exposure classification was based on self-reported hours of VDT use during the interview, or (2) group, in which all of the occupations were placed into eight categories on the basis o f the amount of VDT use that characterizes each occupation. With the exception of spontaneous abortions among current pregnancies analyzed by individual (O/E of 1.19; 90% CI 1.09-1.30), there were no relative risks associated with VDT use significantly different from 1.0. The investigators feel that the lack of confirmation of a similarly increased relative risk in the grouped analyses suggests the possibility of recall bias. Such a suspicion of bias may also be supported by the absence of an association of spontaneous abortion with VDT use in previous pregnancies (O/E of 0.97). In a study designed to test the potential effects of malathion spraying in northern California on pregnancy outcome, Goldhaber et al. [80] also obtained data on VDT use from their study subjects. A pregnancy cohort consisted of all women who reported for pregnancy testing at one of three Kaiser Permanente Medical Care Program clinics between September 1981 and June 1982. The cohort served as the base for a case-control study in which the cases consisted of women who experienced spontaneous abortions (28 weeks or less from the last menstrual period) or whose offspring was born with a defect (according to the criteria of the California Birth Defects Monitoring Program). Controls consisted of every fifth normal live birth. Questionnaires were mailed to all cases and controls and subjects who did not respond to three mailings were telephoned. The data were entered into a logistic model which adjusted for maternal age, prior history of adverse outcome, race, education, smoking, alcohol, occupation and gestational age at pregnancy diagnosis (for abortions) or hospital of delivery (for defects). The final study group consisted of 1175 women who worked during the first trimester and 408 women who did not; the majority of analyses, however, were confined to the working population. Response rates were 87.8% for controls, 82.7% for spontaneous abortions and 87.8% for birth defects. Among women who reported 5-20 h/week and > 20 h/week of VDT use, the odds ratios for birth defects were moderately but not statistically significantly elevated (OR of 1.4 for each exposure category). Because of small sample size, no further analyses were done for birth defects. Goldhaber et al. reported significantly increased miscarriages among women who reported >20 h/week of VDT use (OR 1.8, 95% CI 1.2-2.8). Among women who reported _