Biological Studies of Power-Frequency Fields and Carcinogenesis ...

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where peer-review is not rigorous. What Biological Effects are. Relevant to Carcinogenesis? Power-frequency fields intense enough to induce electric currents in ...
Biological Studies of

Power Frequency Fie1ds and carcinogenesis =

ost of the public concern about electric power and cancer stems from epidemiological studies that appear to show a relationship between exposure to power-frequency magnetic fields and the incidence of cancer. While there is disagreement among scientists concerning the strength and consistency of this epidemiology, essentially all reviewers agree that the epidemiological evidence for a link between power-frequency fields and cancer falls far short of that needed to conclude that a causal relationship exists [ 1,2]. The biophysics of the interaction of power-frequency fields with biological systems has also been extensively reviewed; and it is generally concluded that significant biological interactions are implausible at the field intensities encountered in residential a n d in m o s t occupational settings [2, 31. In a case such as this, where the epidemiologicalevidence for a link between a physical agent and a disease is not defiiitive and the interaction is biophysically implausible, laboratory studies become critical for risk evaluation. If there were strong in vitro or in vivo evidence that power-frequencies were carcinogenic, it would make the epidemiology more convincing. In fact, replicated laboratory evidence for carcinogenesis at relevant field intensities, combined with the existing epidemiology, would probably be sufficient to support a conclusion that a causal relationshipexisted. Conversely, if appropriate laboratoq studies were done and those studies consistently failed to show any evidence for carcinogenicity, then we would dismiss the epidemiology, particularly in view of the biophysical implausibility. In other words, while epidemiologythis weak cannot establish causality on its own, weak epidemiology combined with strong laboratory evidence and plausible mechanisms could satisfy the criteria for causality. The laboratory studies could have additional roles. A particular problem with the epidemiology of power-frequency fields is that there is no consensus as to the

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John E. Moulder Deportment of Radiation Oncology, Medical College of Wisconsin

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appropriate exposure metric; arguments have been made for peak fields, timeweighted fields, the presence of transients, and for particular combinations of powerfrequency fields and the geomagnetic field. If biological effects relevant to carcinogenesis were demonstrated in the lab, they could be used to define an appropriate exposure metric and to provide a doseresponse model for the epidemiological studies. The demonstration of consistent laboratory effects for power-frequency fields below 50 pT (or better yet, below 5 pT) would also stimulate development of biophysical models. After all, if bioeffects exist at these field levels, there must be a biophysical mechanism, and the nature of the bioeffects should provide clues to that mechanism. Thus, both epidemiology and biophysical modeling would be greatly enhanced by relevant laboratory data.

literature on Power-Frequency Field Bioeffects The literature on biological effects of power-frequency fields is voluminous, but is plagued by reports in which exposure conditions are poorly defined and where statistical validity is uncertain. The literature is also replete with reports of effects that have either failed attempts at replication or where replication has never been attempted. If power-frequency fields are associated in some way with carcinogenesis, it is clear that we do not understand the underlying biophysical mechanism(s) [2, 31. As a result, we do not know what characteristics of exposure might be relevant to biological effects. Given this level of ignorance, it is critical to characterize fully the exposure conditions used in laboratory studies. Unfortunately, complete information is rarely available. In general, the best that can be determined is the frequency, the field intensity, the exposure duration, and whether the wave-form is sinusoidal or pulsed; often even this minimal information is not available. As 0739-51 75/96/$5.0001996

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Table I: In vivo Assessment of the Genotoxicity of Power-Frequency Fieldsa

No effect of Dulsed 12 and 460Hz fields on leukemia incidence in mice 11 11.

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No effect of sinusoidal fields on tumor incidence in rats [17].

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Endpoint

Results

Carcinogenesis

No significant effect of sinusoidal fields on the incidence of skin tumors or leukemia in mice [12].

Sinusoidal fields may cause an increase in lymphoma [I61and breast tumors 1131 in mice. Mutagenesis

No mutagenesis by sinusoidal fields in mice [14,15]or fruit flies [75].

Chromosome aberrations

Possible excess of lymphocyte chromosome aberrations in current or former smokers who were occupationally exposed to power-frequency fields [21,23]. Possible excess of lymphocyte chromosome aberrations in non-smokers who were occupationally exposed to power-frequency fields [19,20,22], but the effect was not seen in other studies [18,21,23,24].

No increase in chromosome aberrations in plants exposed to 75Hz sinusoidal magnetic, or combined electric and magnetic fields [76]. Sister chromatid exchanges (SCEs)

No increase in SCEs in lymphocytes of men occupationally exposed to electric and magnetic fields 118,20-231. No increase in lymphocytes SCEs in rats exposed to sinusoidal fields [41].

Micronuclei formation

An increase in micronucleus incidence was found in lymphocytes of men who were occupationally exposed to electric and magnetic fields [20], but the effect increase was not seen in a replicate 1231.

a Studies are of 50160 Hz magnetic fields unless otherwise sDecified. -

a result, when studies disagree, it is usually impossible to determine whether they were actually done under the same conditions. Interpretation of many studies is also complicated by multiple comparison issues. While the problem is not as severe in laboratory studies as it is in the epidemiology, many laboratory studies include multiple exposure conditions and/or multiple endpoints, allowing the investigator to make many different comparisons. Each comparison (by commonly accepted statistical criteria) has a 5 % probability of yielding a “statistically significant” result, even if there were no real differences. If only one assay in a study that contains multiple assays is “statistically significant,” it is often impossible to determine whether this is a real effect or a piece of statistical noise. Clearly, the closer the p-value is to 0.05 and the more comparisons there are, the higher the probability is that a reported association is a “false positive.” There are also replication problems in much of the power-frequency literature. Some “positive” reports of biological effects have never been replicated, even by the same laboratory. In many other cases, there are only partial replications; that is studies done under similar, but not identi32

cal exposure conditions, or studies done in different model systems. There are also a disturbingly large number of “positive” reports where replication attempts have failed to find the earlier reported effect [4-91. These replication problems are exacerbated by the absence of underlying biophysical models, and the resulting lack of standard laboratory models or exposure conditions. Compounding the above problems are the number of possibly relevant studies that have been reported at meetings but not published, or published in sources where peer-review is not rigorous.

What Biological Effects are Relevant to Carcinogenesis? Power-frequency fields intense enough to induce electric currents in excess of those that occur naturally (above 500 pT) have shown reproducible effects, including effects on humans [lo]. However, these effects have no obvious connection to carcinogenesis, and have not been observed at the field intensities encountered in occupational and residential settings. More important, these effects are apparently caused by induced electric currents; and on biophysical grounds, induced currents are not expected to play IEEE ENGINEERING IN MEDICINE AND BIOLOGY

any role at field intensities below about 50

pT R 3 , 101. The magnetic fields associated with power-frequency easily penetrate buildings or tissue and are difficult to shield. By contrast, power-frequency electric fields are easily shielded by conductive objects and have little ability to penetrate buildings or tissue. Because power-frequency electric fields do not penetrate the body, it is generally assumed that any biologic effect from routine exposure to power-frequency fields must be due to the magnetic component of the field, or to the electric fields and currents that these magnetic fields induce in the body [2, 3, lo]. For these reasons, this review will concentrate on studies of the biological effects of magnetic or combined electric-magnetic field exposure. If power-frequency fields were carcinogenic, they could be either genotoxic or epigenetic (in older terminology, either initiators or promoters). Genotoxic agents directly damage the genetic material of cells (the DNA). Genotoxins may not have thresholds for their effect; in other words, as the dose of the genotoxin is lowered the risk gets smaller, but it may never disappear. There are a number of standard tests for genotoxic activity, and it is relatively easy to establish whether an July/August 1996

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Table 11: In vitro Assessment of the Genotoxicity of Power-Frequency Fieldsa Endpoint

Results

Mutagenesis

N o increased mutagenesis in yeast [28], mammalian cells [26], or bacteria __ [26]. N o increased mutagenesis in bacteria exposed to a 100Hz field [27].

Chromosome aberrations

N o increased aberrations in human lymphocytes exposed to magnetic [25,29,34], or combined electric and maanetic fields 1341. No increased aberrations in human amniotic cells for continuous exposure to sinusoidal fields [37,38], but an increase for intermittent exposure [37].

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Increased aberrations in human lymphocytes for pulsed fields in one study [46], but not in a replicate 1361 or in CHO cells 1351. Sister chromatid exchanges (SCEs)

No increase in SCEs in human lymphocytes for magnetic [25,29,40-421 or combined electric and magnetic [39] fields.

DNA strand breaks

N o increased DNA damage in human cells [31], CHO cells [30] or plasmids [32] exposed to and magnetic fields human cells exposed to pulsed fields [33]

Cell transformation

No increased transformation in mammalian cells [45]

Micronuclei formation

No increased micronuclei formation in mammalian cells exposed to sinusoidal [25,40,43] or pulsed 136,441 fields.

1 Increased micronucleiformation in human lymphocytes at 32Hz field when the geomagnetic field 1 1 was present, but not when it was nulled [47].

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1 a Studies are of 50/60 Hz sinusoidal rnaanetic fields unless otherwise wecified.

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agent is, or is not, a genotoxin. Partly as a result of this standardized genotoxicity testing, most of the carcinogens whose mechanisms we understand are found to be genotoxins. An epigenetic agent is something that increases the probability that a genotoxin will damage DNA, or that a genotoxic exposure will result in cancer. Promoters are a particular type of epigenetic agent that are defined by their ability to increase cancer incidence in animals already exposed to a genotoxin. Epigenetic agents generally have thresholds for their effect, so that as the dose of an epigenetic agent is lowered, a level is reached at which there is no (rather than very little) effect. Except for tests for promotion of certain types of tumors, there are no standard assays for epigenetic activity, so there is no easy way to establish that an agent has no epigenetic activity.

Power-Frequency Fields and Genotoxicity There are many approaches to measuring genotoxicity. Whole-organism exposure studies can be used to assess whether exposure causes cancer, mutations, or chromosomal injury. Cellular studies can be done to detect DNA or chromosomal damage. In reviewing the genotoxicity litJuly/August 1996

erature, non-mammalian as well as mammalian systems have been included. The coverage of exposure conditions has also been broad, including pulsed as well as sinusoidal fields, and covering frequencies from 10 to 3000 Hz. Such broad coverage is warranted, since any evidence for genotoxicity from any system exposed to any related type of field could be relevant to the question of carcinogenicity.

Whole Animal Genotoxicity Studies The biggest gap in the range of endpoints assessed is that very few long-term whole organism exposure studies have been published (Table I). Bellossi, et al., [l 11 exposed leukemia-prone mice for S generations and found no effect on leukemia rates; however, the study used nonpower-frequency pulsed fields at 6000 pT, so the relevance of this to environmental power-frequency exposure is unclear. Rannug, et a1.,[12]found that SO and SO0 pT fields at 50Hz did not significantly increase the incidence of skin tumors or leukemia in mice, but the number of animals in the study was small. Beniashvili, et al., [13]reported that exposure of mice to a 50Hz field at 20 pT resulted in an increased incidence of mammary tumors; however, the study has been reported only in preliminary form with incomplete inIEEE ENGINEERING I N MEDICINE AND BIOLOGY

formation about exposure conditions and experimental design. More recently, Kowalczuk, et al., [14] reported that 8 weeks of exposure to a 10,000 pT SOHz field did not increase the rate of dominant lethal mutations in mice. Three other rodent exposure studies have been reported, but none have appeared in the peer reviewed literature. In the largest of these studies, Benz, et al., [ l S ] exposed mice for multiple generations to 60Hz fields at 300 pT (plus 15 kV/m) or at 1000 pT (plus 50 kV/m). The study reported no increase in mutation rates, fertility, or sister chromatid exchanges, but so far the study has appeared only as an appendix to a report by the New York State Powerline Project. Mickail and Fam [16]have reported that lifetime exposure of mice to a 25,000 pT 60Hz field led to an excess incidence of lymphoma. This mouse lymphoma study has been presented only at meetings, where considerable concern about noise, heating, and vibration artifacts have been raised. Lastly, Yasui, etal., [ 171 have reported the absence of increased cancer in rats after 2 years of exposure to SOHz fields at SO0 and 5000 pT, but the study has only been presented as a preliminary meeting report. 33

Endpoint

Results

Skin tumors induced by DMBAb

Skin tumor promotion at 2000 pT [58].

1 No skin tumor promotion at 2000 UT 148,491.

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No skin tumor co-promotion with TPAb at 2000 pT [48,49]. No skin tumor promotion at 50 or 500 pT delivered continuously 1121 or intermittentlv 1501. Mammary tumors induced by DMBAb

No mammary tumor promotion at 0.3-1.O pT [55,56], 100 pT [57],or 30,000 pT [55]. Mammary tumor promotion at 100 pT [54].

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1 Mammary tumors induced by NMUb 1 Mammary tumor promotion at 20 pT 1131.

1 Liver tumors induced by DENAb I

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No liver tumor promotion at 0.5, 5, 50 or 500 pT [51]. No liver tumor co-Dromotion with Dhenobarbital at 0.5 or 500 UT 1521.

Lymphoma induced by ENUb a All studies are

No lymphoma promotion for continuous exposure at 2, 200, or 1000 pT or at 1000 pT [53].

of sinusoidal 50/60 Hz magnetic fields.

Genotoxicity Evidence from point of a spark discharge can reach sevOccupation Exposure Studies eral amps [20],and electrical currents of A number of studies of lymphocyte chro- this magnitude have been associated with mosome aberrations in occupationally-ex- chromosomal injuries in other studies posed workers have been published [ 18-24]. [25].) Finally, the reported increases are At first glance (Table I), these studies appear largely limited to chromosomal aberravery contradictory, with some studies re- tions, with no effects on sister chromatid porting “significant” effects and others not. exchanges (SCEs); this is surprising, as Further examination shows statistical prob- the SCE assay is generally considered to lems and some possibly interestingpatterns. be more sensitive to genotoxic agents than The major statisticalissue is that most of the the chromosome aberration assay. studies examine multiple endpoints and subgroups, raising a massive multiple comparCellular Genotoxicity Studies son problem. Skyberg, e t al., [21] for Cellular genotoxicity studies have example, reports “significant” chromoso- been massive in scope (Table 11). Publishmal damage in exposed workers; but this ed studies have spanned many different increase is found in only one subgroup, only models, from plasmids to human cells. All for one of several different assays, and has a major genotoxicity endpoints, with the exp-value of only 0.04. With adjustment for ception of cell transformation assays, multiple comparison, the statistical signifi- have been assessed in multiple models and cance of the genotoxicity effect vanishes. multiple labs. A wide range of exposure The multiple comparison problem also ap- conditions have also been assessed, inplies, although to a lesser extent, to the cluding combined electric and magnetic “positive” findings reported by Valjus, et fields, pulsed as well as sinusoidal fields, al., [23] and Ciccone, et al., [24]. non-power-frequency fields, and field inEven with the multiple comparisonprob- tensities ranging from under 1 pT [26,27] lems, several possibly interesting patterns to over 1000 pT [28,29]. emerge. The effects reported are predomiThe cellular genotoxicity studies have nantly in smokers and former smokers, been overwhelmingly negative. Published groups in which excess chromosomal ab- laboratory studies have reported that normalities are expected. The effects are power-frequency magnetic fields do not also seen predominantlyin workers exposed cause DNA strand breaks [30-331, chroto spark discharges. (Spark discharges are a mosome aberrations [25,29,34-381, sister phenomena unique to the electricalenviron- chromatid exchanges (SCEs) [25,29, 39ment of high-voltagesources, where electric 421,micronucleifomation [25,36,40,43, fields can reach intensitiesof up to 20 kVJm. 441, mutations [26-281,or cell transformaThe peak value of the body current at the tion [45]. 34

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There are three published reports of genotoxicity from in vitro systems, but none were carried out under conditions similar to environmental and occupational exposures, and none have been replicated. Khalil & Qassem [46] reported that a 1050 pT pulsed field caused chromosome aberrations; but Scarfi, et al., [36] were unable to replicate this observation, and Takahashi, etal., [35] found no effect in similar studies with CHO cells. Nordenson, et al., [37] reported that continuous exposure of human amniotic cells to a 300 pT field did not increase chromosomal aberrations, but that intermittent exposures did; the study has not been replicated. Finally, Tofani, et al., [47], reported that exposure of human lymphocytes to a 32Hz field at 75 or 150 pT caused an increase in micronucleus formation (an indicator of genotoxicity), but only if a parallel 42 pT static field was also present. Again, the study has not been replicated. Summary of Genotoxicity Despite over 30 published studies, there is no replicated evidence for genotoxicity. Only the lack of cell transformation and long-term animal exposure studies, plus the presence of a few as yet unreplicated positive reports of genotoxicity, keeps the data from being totally convincing.

Power-Frequency Fields Even before the evidence accumulated July/August 1996

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Table IV: In vitro Assessment of the Epigenetic Potential of Power-Frequency Fieldsa

1 Endpoint

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Inhibition of DNA repair

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Results

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No inhibition of repair of UV-induced DNA damage in yeast at 1000 UT 1281. No inhibition of repair of y-ray induced DNA damage in human lymphocytes exposed to magnetic (1000 pT), or combined electric and magnetic fields (50 pT plus 0.2 Vim, 600 pT plus 6 Vim, 1000 pT plus 20 V/m) 1591.

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No inhibition of repair of y-ray induced DNA damage in human lymphocytes exposed to a 2500 pT Dulsed field 1601. Enhancement of transformation

Enhancement of TPAb-induced transformation in mammalian cells exposed to a 100 pT field [45], but the effect could not be replicated 1621.

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No enhancement of chemical' mutagenesis in bacteria at 0.12 pT [27].

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No enhancement of viral-induced mutations in human cells at 1 or 10 UT 1261. No enhancement of mitomycin-C induced micronucleus formation in human lymphocytes exposed to a 2500 UT Dulsed field 1441. No enhancement of drug-induced micronucleus formation in human lymphocytes exposed to 150 pT at 32 or 60Hz with the geomagnetic field nulled, but enhancement by a 32Hz field when the field was present [47]. Possible enhancement of chemically-induced SCEs in human lymphocytes at 5,000-10,000 UT 1291.

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Enhancement of y-ray induced chromosomal injury in human lymphocytes at 600-1500 UT [61].

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1 a All studies are of 50i60 Hz sinusoidal magnetic fields, unless otherwise stated. 1 TPA =12-O-tetradecanoylphorbol-l3-acetate 1

sodium azide or 4-nitro-o-phenylenediamine

that power-frequency fields were not genotoxic, there were suggestions that these fields were promoters. As a result, extensive investigations have been conducted of the possible epigenetic activity of power-frequency fields.

Classical Promotion Assays Promoters are a specific class of epigenetic agents. In a classical promotion test, animals are exposed to a known genotoxin at a dose that will cause cancer in some, but not all animals. Another set of animals is exposed to the genotoxin plus the agent to be tested for promotional activity. If the agent plus the genotoxin results in significantly more cancers than are seen for the genotoxin alone, then that agent is a promoter. Copromotion is a related assay in which the agent is tested with both a known genotoxin and a known promoter. The promotion and co-promotion studies are summarized in Table 111. Studies of whether power frequency fields are promoters have included skin cancer, breast cancer, liver cancer, and lymphoma models; and have used field intensities ranging from less than 1 pT to over 1000 pT. Promotion studies of leukemia and brain c a n c e r are a b s e n t , d e s p i t e t h e epidemiologic evidence for a possible July/August 1996

connection, largely because there are no models for promotion of these cancers, and no known promoters to use as positive controls. Four studies have reported that powerfrequency fields do not promote chemically-induced skin cancer [ 12, 48-50]; these assays have used both continuous and intermittent fields and have used field intensities from 50 to 2000 pT. However, a recent study [58] does report promotion of chemically-induced skin cancer after exposure to a 2000 pT field. Published studies of chemically-induced liver cancer promotion by 0.5 to 500 pT fields have been negative [51, 521. A negative lymphoma promotion study has been reported at meetings [53]. The literature on promotion of chemically induced breast cancer is confusing (Table 111). One study [54] has reported that a 100 pT field can promote DMBAinduced mammary cancer in rats, but three similar studies using higher and lower [55-571 field intensities have shown no such effect. The unreplicated report by Beniashvili, et ul., [13] of promotion of NMU-induced breast cancer at 20 pT is difficult to evaluate, as the study has been published only in preliminary form, and critical experimental details are missing. IEEE ENGINEERING IN MEDICINE AND BIOLOGY

It has also been suggested that powerfrequency fields might be co-promoters; that is, they could enhance the activity of other promoters even though they have no genotoxic or promotional activity on their own. However, the published studies of co-promotion have both shown no evidence for such activity [48,49,52]. Interpretation of the tumor promotion studies is complicated by the observation in several studies [49,57] that exposu e to power-frequency fields appears to speed the growth of chemically-induced tumors, rather than increase the actual number of tumors. Such an effect on growth would be of interest if it occurred at the field intensities to which people were actually exposed, but it would not be evidence for promotion.

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Cellular Assays of Epigenetic Activity Some types of studies are relevant to the carcinogenic potential of agents, but are neither classic genotoxicity nor promotion tests (Table IV). The most common of these are cellular studies that test whether an agent enhances the activity of a known genotoxin; these studies could be regarded as the cellular equivalent of a promotion study. 35

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Table V: Assessment of Biological Effects that Might be Related to Carcinogenesisa Endpoint

Results

Effects on tumor growth

Enhanced growth of chemically-induced skin tumors at 2000 VT [49], but not at 50 or 500 pT 112,501. Enhanced growth of chemically-induced mammary tumors at 100 pT [57], but not at 0.3-1 .O or at 30,000 pT [55,56].

No effect on mammary tumor growth at 100-2000 pT [77]; on liver tumor growth at 0.5-500 pT [51,52], or on leukemia proaression at 1.4-500 UT 1781. Effects on cell growth

Increased growth of human lymphocytes at 5000 pT [29,42]. Increased growth of human cells at 80-130 pT, but no effects at higher or lower field intensities, or in a second cell line [63]. No effect on growth of human lymphocytes at 30-1000 UT [25,34,39,401.

No effect on arowth of mammalian cells at 220-30.000 UT 14.40.411 No effect on growth of human tumor cells at 0.1 9-2500 T I [31,79,80]. Growth-related effects

Possible increase in thymidine uptake in human fibroblasts at 16+ pT [64]. Possible increase in colony-forming ability of tumor cells at 100 pT [81], but an attempt at replication failed [82]. Induction of ODC, a "promotion related enzyme," in rats at 50 UT 1831

Immune suppression

Scattered reports of minor effects on components of the immune system at 1000 pT and above, but no evidence of immune-suppression 1661.

Altered hormone balance

Decreased night-time melatonin in rats at 0.02 pT [84], 0.3-1 .O pT [56], 1 pT [84,85], 5 pT [85] and 50 PT 1851.

No effect on niaht-time melatonin in rats at 0.02, 0.1 or 1 UT 185,861 Decreased night-time melatonin in Siberian hamsters at 100 pT, in one experiment, but not in two replicates 1871. No effect on melatonin levels in sheep at 4 UT 1881

No effect on melatonin levels in humans at 1 or 20 pT [89]. No evidence for significant anti-cancer activity of melatonin in humans. a All studies are of 50/60 Hz sinusoidal magnetic fields.

If an agent interfered with the repair of

DNA damage caused by a genotoxin, that agent would effectively lead to an increase in DNA damage, and could increase the probability of carcinogenesis. Three studies of the effects of power-frequency fields on the repair of DNA damage induced by ionizing radiation have failed to show any effect [28,59,60]. These studies have included both magnetic and combined magnetic and electric fields, both sinusoidal and pulsed fields; and have used field intensities ranging from 50 to 2500 pT. If an agent interacted with a genotoxin in any way that increased genotoxicity, the agent would be considered to have epigenetic activity. There are no standard models for this type of assay, and there are many different genotoxins and endpoints that could be assessed. Two assays of whether power-frequency fields could enhance chemical mutagenesis have been 36

published [26,27]; both studies report no evidence for epigenetic activity. The studies of enhancement of drug-induced micronucleus formation have produced complex results. One study reports no enhancement of drug-induced micronucleus formation by a 2500 pT pulsed field [44]. The other study [47] reports that while 32 and 60Hz fields at 150 pT do not enhance drug-induced micronucleus formation when the geomagnetic field is nulled, the 32Hz field does enhance micronucleus formation when the geomagnetic field is present. Unfortunately, this study did not test 60Hz exposure with the geomagnetic field present. Three cellular studies have indicated that power-frequency fields might have some epigenetic activity. Rosenthal, et al., [29] reported that a 5000 pT power-frequency field increased the frequency of SCEs induced in lymphocytes by some alkylating agents; however, the effect was IEEE ENGINEERING I N MEDICINE AND BIOLOGY

seen only erratically, and the investigators state that the enhancement may be an artifact. Hintenlang, et al., [61] reported that 600-1500 pT fields enhanced lymphocyte chromosomal injury caused by high doses of ionizing radiation; interpretation is complicated by the fact that ionizing radiation alone was sufficient to kill most lymphocytes. Neither the Rosenthal, et al., [29] nor the Hintenlang, et aZ., [61] reports have been replicated. Finally, Cain, et al., [45] reported that a 100 pT power-frequency field could enhance cell transformation; the effect was only seen in the presence of a chemical promoter, and the authors have subsequently reported at meetings [62] that they cannot replicate the study.

Summary of Epigenetic Activity In general, power-frequency fields do not appear to have epigenetic activity, and the few studies that have shown some July/August 1996

evidence of epigenetic activity have used field intensities well above those encountered in residential and most occupational settings (Tables I11 and IV).

Other Bioeffects that Might be Related to Cancer There are biological effects other than classical genotoxicity, promotion, or epigenetic activity that might be related to cancer. In particular, agents that have dramatic effects on cell growth, on the function of the immune system, or on hormone balance, might contribute to cancer, without meeting the classic definitions (Table VI.

Cell and Tumor Growth There have been scattered reports that power-frequency fields can enhance cell proliferation or tumor growth, but the vast majority of studies have shown no effect (Table V). Many essentially harmless agents (e.g., temperature, pH, nutrients) affect the growth rates of cells and tumors, so effects of cell growth, per se, are not evidence for hazards. However, it could be of relevance to carcinogenesis if an agent caused previously non-dividing normal (as opposed to tumor or transformed) cells to begin to divide, if the growth stimulation effect persisted after the agent was removed, and/or if the effect occurred at levels to which people were actually exposed. The positive reports of effects on cell growth show none of the above features. The reports of effects on mammalian cell growth [29,42,63] have involved changes in the growth rate of cells that were already actively dividing, and have used exposures of 80 pT and above. Similarly, the positive reports of effects on tumor growth [49,57] have involved exposures of 100 pT and above, and by definition have used cells that were already dividing. Liboff, et al., 1641 are often quoted as reporting effects on cell growth for fields as low as 16 pT; but the study only reports increased uptake of a DNA precursor, an observation which may or may not indicate an effect on proliferation. In fact, despite 20+ published studies on this issue (Table V), there are no studies indicating that power-frequency fields can cause non-dividing cells to begin growing, no studies that indicate that growth effects persist in the absence of fields, and no reports of effects on growth for fields below 80 pT. July/August 1996

Immune System Suppression In the early 1970s there was speculation that the immune system had a role in preventing the development of cancer, a theory known as the “immune surveillance hypothesis” [65]. Subsequent human and animal studies have shown that this hypothesis was not generally valid [65]. Severe suppression of the immune system is associated with increased rates of certain types of cancer, particularly lymphomas [65]; but not with the tumors most commonly reported in excess in epidemiologic studies of power-frequency fields. In addition, while some investigators have reported that powerfrequency fields can have effects on cells of the immune system [66], no studies have shown actual immune suppression.

The “Melatonin Hypothesis” Some investigators have suggested that power-frequency fields might suppress the production of melatonin, and that melatonin might have “cancer-preventive” activity [67]. This speculation has been around since the early 1980s,but there is little evidence to support it. There are reports that electric fields and static magnetic fields can affect melatonin production [67], but studies using power-frequency magnetic fields have largely shown the absence of such effects (Table V). The second component of the hypothesis, that decreased melatonin levels are associated with increased cancer, is also unproven. In the late 70s there was interest in using melatonin as an anti-cancer agent, but clinical trials have shown that it is largely ineffective.

Miscellaneous Biological Effects of Power-Frequency Fields Even biological effects with no known relationship to cancer would be of help in establishing exposure metrics and in developing biophysical models, provided that they were robust and reproducible and occurred below about 5 pT. There are two reasons to specify a field intensity cut-off for relevance. First, human exposure to power-frequency fields seldom exceeds a time-weighted average of 1 pT, so even 5 pT is well above actual exposure levels, particularly since biological effects would be expected to scale with the square of the field strength. Second, as discussed below, there are biophysical mechanisms that can explain the existence of biological effects above 50 pT, mechanisms that are IEEE ENGINEERING IN MEDICINE AND BIOLOGY

not applicable to fields much below this intensity. Unfortunately, if a reproducible biological effect is defined as one that has been reported in the peer-reviewed literature by more than one laboratory, without contradictory reports appearing elsewhere, then there appear to be no biological effects that meet the above criteria. In fact, there may be no robust andreproducible biological effects below about 200 pT. Effects on Ca++ transport and gene expression are commonly mentioned as candidates for robust biological effects at relevant field strengths. However, neither effect appears to be widely reproducible. Effects of power-frequency fields on Ca++ transport across cell membranes have been reported for field strengths as low as 20-60 pT [68, 691, but other studies [5, 8, 91 have failed to find any evidence for such effects. Similarly, while effects of power-frequency fields on gene transcription have been reported for fields as low at 1-8 pT [70], other studies have failed to find any such effects [6,7].

Mechanisms for Biological Interactions Three general classes of biophysical mechanisms have been proposed that could account for biological effects of power-frequency magnetic fields: induced electric currents [3, 101; direct effects on magnetic biological material [7 11; or effects on rates of certain chemical reactions [72].

Induced Currents Power-frequency magnetic fields can induce electric currents, and induced electric currents definitely can cause biological effects [3,10]. However, the currents induced in the body by fields of less than 50 pT are weaker than the currents that occur naturally [3,10]. Therefore, if sinusoidal power-frequency magnetic fields of the intensity encountered in residential and most occupational settings do have biological effects, they are not mediated by induced electric currents. This argument assumes that 50 or 60Hz sinusoidal power-frequency fields are the only electromagnetic fields found in conjunction with the electric power. Howe v e r , if l a r g e t r a n s i e n t s and/or higher-order harmonics are present, it is possible that electric currents stronger than those that occur naturally in the body could be induced. 37

Magnetic Biological Material Small magnetic particles (magnetite, Fe304) have been found in bacteria that orient in the geomagnetic field. Kirschvink, et al., [71] have suggested that magnetite is present in mammalian cells, and that power-frequency magnetic fields could cause biological effects by acting directly on such particles. However, calculations show that this would require power-frequency fields of at least 2-5 pT [71, 731, so this hypothesis has little relevance for environmental or occupational exposures.

Free Radical Reactions Static magnetic fields can influence the reaction rates of chemical reactions that involve free radical pairs [72]. Since the free radicals involved in these reactions have lifetimes that are very short compared to the cycle time of power-frequency fields, a power-frequency field acts like a static field during the time scale over which these reactions occur. However, since any effects of a power-frequency field would be additive with the 30-70 pT geomagnetic field, no significant biological effects would be expected below about 50 )IT [72]. Note that if one were to hypothesize that biological effects mediated by free radical reactions were involved in carcinogenesis, the relevant studies would be those using static fields, and studies of the genotoxic and epigenetic activity of static fields have been overwhelmingly negative.

Resonance Theories Some of the constraints on mechanisms for biological effects of weak (less than 50 pT) power-frequency magnetic fields could be overcome if there were mechanisms that could make cells (or organisms) uniquely sensitive to such fields. Several such mechanisms have been proposed, most of which are based on resonances that involve an interaction of power-frequency fields with the geomagnetic field. So far, none of these proposed mechanisms have survived scientific scrutiny [3, 741, and much of the experimental evidence that prompted the speculations in the first place cannot be independently reproduced [4,5, 81. There are also severe incompatibilities between known biophysical characteristics of cells and the conditions required for such resonances [3,74]. 38

Summary of Proposed Biophysical Mechanisms None of the above mechanisms appear to explain the existence of biological interactions at field levels that are present in residential and occupational settings, although all are potential explanations for effects of fields with intensities of 50 pT and above. Thus, if power-frequency fields below 5 pT do actually have biological effects, the mechanisms must be found, in Adair’s [3] words, “outside the scope of conventional physics.”

What Additional Studies are Needed? No number and type of additional laboratory studies will satisfy those who demand an absolute assurance of the absence of potential human health hazards. Even if no health hazard actually exits, the ambiguities of risk research, the nature of the scientific method, and the existence of statistical noise, guarantee that there will always be loose ends and unexplained findings. Nevertheless, there appear to be certain areas where additional laboratory studies might be useful. An obvious gap in the genotoxicity studies is the relative lack of long-term animal exposure studies. However, such studies are underway in the U S . and elsewhere. A positive finding in these studies, particularly if it was for leukemia or brain cancer, would require a major reevaluation of the public health implications of exposure to power-frequency fields. However, a negative outcome would have little impact. First, almost no one expects that power-frequency fields will be found to be genotoxic. Second, proponents of the idea that power-frequency fields do cause or contribute to cancer could simply claim that the exposures were done under the wrong conditions. In addition to the long-term exposure studies, the reports of promotion of chemically-induced cancer [13, 54, 581 need replication; and if they are replicated, the dose-response relationships need to be established. Promotion studies with leukemia and brain cancer would also be valuable, but such studies are limited by the absence of established promotion models. There are also two isolated reports of enhancement of genotoxicity [29, 6 11 that need to be replicated, and if they are replicated, the dose-response relationship for the effects need to be established. IEEE ENGINEERING IN MEDICINE AND BIOLOGY

Summary Clearly, the laboratory data on powerfrequency fields does not provide any real support for an association between exposure to power-frequency fields and cancer. In fact, given the relative weakness of the epidemiology, combined with the extensive and unsupportive laboratory studies, and the biophysical implausibility of interactions at relevant field strengths, it is often difficult to see why there is still any scientific controversy over the issue of power-frequency fields and cancer. Nevertheless, the public controversy remains. This is seen in continuing litigation over cancers alleged to be caused by exposure to power-frequency fields, and by the public opposition that meets most attempts to site new powerlines and substations or to upgrade existing facilities. The public concern is sustained by periodic reports of positive findings, by the inability of scientists to guarantee that no risk exists, and by statements from scientists and government officials that more research is needed. This public concern is further encouraged by lay-oriented books that allege that there has been a conspiracy to conceal the health risks of power-frequency fields from the general public. Public concern has also been nourished by uneven reporting by the mass media on this issue. Studies regarded as “positive” receive wide press coverage that is often devoid of any discussion of statistical significance or context; whereas “negative” studies, regardless of their statistical power, get little or no coverage. As a result, the small number of “positive” reports that imply a connection between power-frequency fields and cancer gets overwhelming attention, out of a sea of studies that is largely unsupportive of any such connection. Public concern about electricity and cancer is likely to continue either until future research shows that the fields are hazardous (an outcome I personally consider unlikely), or until the public learns that science cannot provide absolute guarantees that anything is absolutely safe (an outcome, unfortunately, that I consider equally unlikely). John Moulder received his undergraduate degree from Carleton College with a dual major in Chemistry and Biology, and his Ph.D. degree in Biology from Yale University in 1972. He is Director of the Radiation Biology Program at the Medical College of Wisconsin. His priJuly/Augusf 1996

mary research interest is the biological basis for cancer radiotherapy and chemotherapy. Dr. Moulder has served on the Experimental Therapeutics and Radiation Study Sections for the NationaI Institutes of Health, and has been a member of the panels that have recently reviewed grant proposals on nonionizing radiation biology. Dr. Moulder is a member of the Radiation Research Society (in which he was formerly a Councilor), the American Society of Therapeutic Radiation Oncology (where he chairs the Radiation Biology Committee), the Environmental Mutagen Society, and the Radiation Therapy Oncology Group. He is an Associate Editor for Radiation Research and for the International Journal of Radiation Oncology Biology and Physics, and has served on the Wisconsin Radiation Protection Council. Dr. Moulder has lectured on powerfrequency fields and human health to biologists, physicists, physicians, and industry groups around the world. He has also served as a consultant and expert witness in several cases involving the alleged health effects of exposure to powerfrequency fields, and he maintains FAQ sheets on power-frequency fields and cancer on the Internet. Address for correspondence: Radiation Oncology, Medical College of Wisconsin, 8701 Watertown Plank Road, Milwaukee, WI 53226, Email: [email protected]

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melatonin concentration of albino rats. Neurosci Letters 168:205-208, 1994. 87. Yellon SM: Acute 60-Hz magnetic field exposure effects on the melatonin rhythm in the pineal gland and circulation of the adult Djungarian hamster. J Pineal Res 16:136-144, 1994. 88. Lee JM, Stormshak F, Thompson JM, Hess DL, Foster DL: Melatonin and puberty in female lambs exposed to EMF: a replicate study. Bioelectromag 16:119-123, 199.5. 89. Graham C, Cook MR, Riffle DW: Human melatonin in 60 Hz magnetic fields: Continuous versus intermittent exposure. In: Annual Review of Research on Biological Effects of Electric and Magnetic Fields, Palm Springs, p 48,1995.

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