Xref: helios.physics.utoronto.ca sci.med.physics:2459 sci.answers:1478 news.answers:27582

Xref: helios.physics.utoronto.ca sci.med.physics:2459 sci.answers:1478 news.answers:27582 Path: admin-one.radbio.mcw.edu!user From: jmoulder@its.mcw.edu (John Moulder) Newsgroups: sci.med.physics,sci.answers,news.answers Subject: Powerlines & Cancer FAQs 3/6: FAQ 2 Supersedes: Followup-To: sci.med.physics Date: Mon, 15 Aug 1994 16:31:50 -0600 Organization: Medical College of Wisconsin Lines: 551 Approved: new-answers-request@MIT.edu Distribution: world Expires: 12 Sep 1994 00:00:00 GMT Message-ID: References: Reply-To: jmoulder@its.mcw.edu (John Moulder) NNTP-Posting-Host: admin-one.radbio.mcw.edu Summary: Q&As on the connection between powerlines, electrical occupations and cancer. Discussion of the biophysics of interactions with EM sources, summaries of the laboratory and human studies, information on standards, and references. Keywords: powerlines, magnetic fields, cancer, EMF, non-ionizing radiation, FAQ Archive-name: powerlines-cancer-FAQ/part3 Last-modified: 1994/8/15 Version: 2.6a Maintainer: jmoulder@its.mcw.edu FAQs on Power-Frequency Fields and Cancer (Q&A, Part 2 of 3) 16) Do laboratory studies indicate that power-frequency fields can cause cancer? Carcinogens, agents that cause cancer, can be either genotoxic or epigenetic (in older terminology these were called initiators and promoters). Genotoxic agents (genotoxins, initiators) can directly damage the genetic material of cells. Genotoxins often affect many types of cells, and may cause more than one kind of cancer. Genotoxins generally do 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 goes away. An epigenetic agent is something that increases the probability that a genotoxin will damage the genetic material of cells or that a genotoxin will cause cancer. Promoters are a particular kind of epigenetic agent that increase the cancer risk in animals already exposed to a genotoxic carcinogen. Epigenetic agents (including promoters) usually affect only certain types of cells, and may cause only certain types of cancer. Epigenetic agents generally have thresholds for their effect; in other words, as the dose of an epigenetic agent is lowered a level is reached at which there is no risk. 16A) Are power-frequency fields genotoxic? There are many approaches to measuring genotoxicity. Whole-organism exposure studies can be used to see whether exposure causes cancer or causes mutations. Cellular studies can be done to detect DNA or chromosomal damage. Very few whole-organism exposure studies have been published (see Loscher & Mevissen [K6] for summaries of some of the unpublished work). Bellossi et al [G13] exposed leukemia-prone mice for 5 generations and found no effect on leukemia rates; however, since the study used 12 and 460 Hz pulsed fields at 60 G (6 mT), the relevance of this to power- frequency fields is unclear. Otaka et al [G19] showed that power- frequency magnetic fields did not cause mutations in fruit flies. Rannug et al [G20] found that power-frequency magnetic fields did not significantly increase the incidence of skin tumors or leukemia in mice. Benz et al [G4] conducted a multi-generation mouse exposure study in 1983-1985 as part of the NY State Powerlines Project; this study reported no increase in mutations rates, fertility, or sister chromatid exchanges, but has never been published in the peer-reviewed literature. Published laboratory studies have reported that power-frequency magnetic fields do not cause DNA strand breaks [G5,G9,G17], chromosome aberrations [G1,G7,G16], sister chromatid exchanges [G2,G7,G11,G21], micronuclei formation [G11], or mutations [G3,G16,G18]. Many of the above laboratory studies also examined power-frequency electrical fields and combination of power-frequency electrical and magnetic fields [G1,G2,G5,G9,G11,G17]. As with the studies of magnetic fields alone, the studies of electrical fields and combined fields showed no evidence of genotoxicity. There are two published reports of genotoxicity. Khalil & Qassem [G12] reported that a 10.5 G (1.05 mT) pulsed field caused chromosome aberrations. Nordenson et al [E4] reported that switchyard workers exposed to spark discharges had an increased rate of chromosomal defects, but Bauchinger et al [E2] found no such increase in chromosomal defects in a similar study. 16B) Are power-frequency magnetic fields cancer promoters? Epigenetic agents influence the development of cancer without directly damaging the genetic material. Promoters are a specific class of such epigenetic agents. In a 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 are exposed to the genotoxin, plus another agent. If the agent plus the genotoxin results in more cancers than that seen for the genotoxin alone, then that agent is a promoter. It has been suggested that power-frequency EMFs could promote cancer [L2]. Published studies have reported that power-frequency magnetic fields do not promote chemically-induced skin cancer [G10,G15,G20,G27,G29] or chemically-induced liver cancers [G22,G25]. For chemically-induced breast cancer, two published study have reported promotion [G14,G23], but two others have not found promotion [G24,G28]. Interpretation of the breast cancer promotion data is complicated by the fact that one of the positive studies [G14] has been published only in preliminary form, and also reports that a static magnetic field of less than the intensity of the earth's magnetic fields can promote chemically induced-breast cancer. 16C) Do power-frequency magnetic fields enhance the effects of other genotoxic agents? There are some types of studies that are relevant to the carcinogenic potential of agents, but that are neither classic genotoxicity nor promotion tests. The most common of these are cellular studies that test whether an agent enhances the genotoxic activity of a known genotoxin; these studies could be regarded as the cellular equivalent of a promotion study. Published studies have reported that power-frequency magnetic fields do not enhance the mutagenic effects of known genotoxins [G3], and do not inhibit the repair of DNA damage induced by ionizing [G8,G9] or UV [G16] radiation. One study [G7] has reported that power-frequency fields may increase the frequency of sister chromatid exchanges induced by known genotoxins. 17) Do laboratory studies indicate that power-frequency fields have any biological effects that might be relevant to cancer? There are biological effects other than genotoxicity and promotion that might be related to cancer. In particular, agents that have dramatic effects of cell growth, on the function of the immune system, or on hormone balances might contribute to cancer without meeting the classic definitions of genotoxicity or promotion. 17A) How do laboratory studies of the effects of power-frequency fields on cell growth relate to the question of cancer risk? There are substances (called mitogens) that cause non-growing normal cells to start growing. Some mitogens appear to be carcinogens. There have been numerous studies of the effects of power-frequency fields on cell growth (proliferation) and tumor growth (progression). Most recent studies of the effects of power-frequency magnetic fields on cancer growth have shown no effect [G6,G10,H3], but one has reported that a 20 G (2 mT) enhanced tumor growth [G15]. Most recent studies of effects of power-frequency magnetic fields on cell growth have also shown no effect [G1,G11,G17,G21,H2,H8,H9], but some have shown increased [G7] or decreased [G12] cell growth after exposure to intense (greater than 10 G, 1 mT) fields. With one controversial exception [H1] there have been no reported effects on proliferation or progression for fields below 2000 mG (200 microT). 17B) How do laboratory studies of the effects of power-frequency fields on immune function relate to the question of cancer risk? In the early 1970¹s there was speculation that damage to the immune system had a major role in preventing the development of cancer; this theory was known as the ³immune surveillance hypothesis² [E3,E7]. Subsequent studies have shown that this hypothesis is not generally valid [E3,E6,E7]. Suppression of the immune system in animals and humans is associated with increased rates of only certain types of cancer, particularly lymphomas [E6,E7]. Immune suppression has not been associated with an excess incidence of leukemia, brain cancer or breast cancer in either animals or humans [E3,E6,E7]. Some studies have shown that power-frequency fields can have effects on cells of the immune system [K2], but no studies have shown the type or magnitude of immune suppression that is associated with an increased incidence of lymphomas. 17C) How do laboratory studies of the effects of power-frequency fields on the pineal gland and melatonin relate to the question of cancer risk? It has also been suggested that power-frequency EM fields might suppress the production of the hormone melatonin, and that melatonin has "cancer- preventive" activity [H7,H8,L3]. This is still highly speculative. There have been a number of reports that electrical fields and static magnetic fields can affect melatonin production [H7], but studies using power-frequency magnetic fields have not shown reproducible effects. Kato et al [H10] reported that exposure to circularly-polarized power- frequency fields of 10-2500 mG (1-250 microT) caused a small decrease melatonin production in rats, but that lower-intensity fields [H10] and vertically- or horizontally-polarized power-frequency fields [H13] had no effect. Loscher et al [G28] reported that 3-10 mG (0.3-1.0 microT) power-frequency fields caused a small decrease in melatonin production in mice, but that this decrease did not lead to promotion of chemically- induced mammary tumors. Lee et al [H11] reported that exposure to a 500 kV transmission line field (40 mG, 4 microT, 6 kV/m) had no effect on melatonin levels in sheep. The second component of the powerline-melatonin-cancer hypothesis, that a decrease in melatonin levels will lead to an increase in cancer, is also unproven. While there is some evidence that melatonin has activity against transplanted and chemically-induced breast tumors in rats, there is little evidence that melatonin affects other types of cancer in animals, or that it has any effect on breast or other types of cancer in humans. 18) Do power-frequency fields show any reproducible biological effects in laboratory studies? While the laboratory evidence does not suggest a link between power- frequency magnetic fields and cancer, numerous studies have reported that these fields do have "bioeffects", particularly at high field strength [H4,H6,K1,K2]. Power-frequency fields intense enough to induce electrical currents in excess of those that occur naturally (above 5 G, 500 microT, see Question 8) have shown reproducible effects, including effects on humans [K1]. 18A) Do power-frequency fields of the intensity encountered in occupational and residential settings show reproducible biological effects? 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 data appearing elsewhere; then there may be no reproducible effects below about 2 G (200 microT). While there are reports of effects for fields as low as about 5 mG (0.5 microT), few, if any, of these reports have been validated. The lack of validation of the laboratory studies is due to many factors. First, many reports on the biological effects of power-frequency fields have never been published in the peer-reviewed literature, and cannot be scientifically evaluated. Second, no attempts have ever been made to replicate many of the published reports of biological effects; and one positive report, standing in isolation, is hard to evaluate. Third, when attempts have been made to replicate some of the published studies, these replications have often failed to show the effect [H5,H2,H12]. Lastly, the investigators in this field use a wide variety of biological systems, endpoints, and exposure conditions, which makes studies extremely hard to compare and evaluate. 18B) Are there known mechanisms by which power-frequency fields of the intensity encountered in occupational and residential settings could cause biological effects? The known biological mechanisms through which intense (greater than 5 G, 500 microT) power-frequency magnetic fields cause biological effects are not relevant to fields below about 500 mG (50 microT). The currents induced in the body by fields of less than 500 mG (50 microT) are similar to, but much weaker than, the currents that occur naturally [F2]. The currents induced by a 50 mG (5 microT) are less than those induced in the body by walking through the Earth¹s static magnetic field [F2]. Thus, as emphatically pointed out by Adair [F2,F8], if weak power-frequency magnetic fields do have biological effects, they are not mediated by induced currents. It has been suggested that power-frequency magnetic fields could cause biological effects by acting directly on magnetic biological material [F3], but analysis of the biophysics indicates that this would require power-frequency fields of at least 50 mG (5 microT) [F3,F8]. Some of the biophysical constraints on possible mechanisms for biological effects of weak power-frequency magnetic fields could be overcome is there were resonance mechanisms that could make cells (or organisms) uniquely sensitive to power-frequency fields [F1,F8]. Several such resonance mechanisms have been proposed, but none have survived scientific scrutiny [F1,F8,H5,H2,H12]. There are also severe incompatibilities between known biophysical characteristics of cells and the conditions required for such resonances [F8]. Thus if power-frequency fields below 50 mG (5 microT) do actually have biological effects, the mechanisms must be found, in Adair¹s [F2] words: ³outside the scope of conventional physics². 19) What about the "new epidemiological studies" showing a link between power frequency fields and cancer? New studies, particularly epidemiological studies, appear frequently. When these studies show "positive" effects they generate considerable media coverage. When they fail to show "positive" effects they are generally ignored. This section will cover the more recent (1993 and 1994) studies in some detail. 19A) What about the new "Swedish" study showing a link between power lines and cancer? There are new residential exposure studies from Sweden [C13,C18], Denmark [C16], Finland [C15] and the Netherlands [C17]. The published studies are considerably more cautious in their interpretations of the data than were the unpublished preliminary reports and the earlier press reports. The authors of the Scandinavian childhood cancer studies [C15,C16,C18] have produced a collaborative meta-analysis of their data [B5]. The RRs (Question 13) from this meta-analysis are shown below in comparison to meta-analysis of the prior studies [B3,B4]. Childhood leukemia, Scand: 2.1 (1.1-4.1); prior: 1.3 (0.8-2.1) Childhood lymphoma, Scand: 1.0 (0.3-3.7); prior: none Childhood CNS cancer, Scand: 1.5 (0.7-3.2); prior: 2.4 (1.7-3.5) All childhood cancer, Scand: 1.3 (0.9-2.1); prior: 1.6 (1.3-1.9) - Fleychting & Ahlbom [C13,C18]. A case-control study of everyone who lived within 300 m (1000 ft) of high-voltage powerlines between '60 and '85. For children all types of tumors were analyzed; for adults only leukemia and brain tumors were studied. Exposure was assessed by spot measurements, calculated retrospective assessments, and distance from power lines. No increased overall cancer incidence was found in either children or adults, for any definition of exposure. An increased incidence of leukemia (but not other cancers) was found in children for calculated fields over 2 mG (0.2 microT) at the time of diagnosis, and for residence within 50 m (150 ft) of the power line. The increased incidence of leukemia is found only in one-family homes. The retrospective fields calculations do not take into account sources other the transmission lines. No significant elevation in cancer incidence was found for measured fields. - Verkasalo et al [C15]. A cohort study of cancer in children in Finland living within 500 m (1500 ft) of high-voltage lines. Only calculated retrospective fields were used to define exposure. The calculated fields are based only on lines of 110 kV and above, and do not take into account fields from other sources. Both average fields and cumulative fields (microT - years) were used as exposure metrics. The total incidence of childhood cancer was not significantly elevated for average exposure above 0.20 microT (2 mG), or for cumulative exposure above 0.50 microT-years (5 mG-years). A significant excess incidence of brain cancer was found in boys; the excess was due entirely to one exposed boy who developed three independent brain tumors. No significant increase in incidence was found for brain tumors in girls or for leukemia, lymphomas or other cancers in either sex. - Olsen and Nielson [C16]. A case-control study based on all childhood leukemia, brain tumors and lymphomas diagnosed in Denmark between '68 and '86. Exposure was assessed on the basis of calculated fields over the period from conception to diagnosis. No overall increase in cancer was found when 0.25 microT (2.5 mG) was used as the cut-off point to define exposure (as specified in the study design). After the data were analyzed, it was found that the overall incidence of childhood cancer was significantly elevated if 0.40 microT (4 mG) was used as the cut-off point. No significant increase was found for leukemia or brain cancer incidence for any cut-off point. A significant increase in lymphoma was found for the 0.10 microT cut-off point but not for higher cut-off points. -Schreiber et al [C17]. A retrospective cohort study of people in an urban area in the Netherlands. People were considered exposed if they lived within 100 meters of transmission equipment (150 kV lines plus a substation). Fields in the "exposed" group were 1-11 mG (0.1-1.1 microT), fields in the "unexposed" group were 0.2-1.5 mG (0.02-0.15 microT). The total cancer incidence in the ³exposed² group was insignificantly less than that in the general Dutch population. No cases of leukemia or brain cancer were seen in the "exposed" group. 19B) What about the new "Swedish" study showing a link between occupational exposure to power-frequency fields and cancer? There are new occupational studies from Sweden [D12], Denmark [D14], Norway [D16], Canada [D15], and the United States [D11,D13]. The published studies are considerably more cautious in their interpretations of the data than were the unpublished preliminary reports and the earlier press reports. -Floderus et al [D14]. A case-control study of leukemia and brain tumors in occupationally-exposed men who were 20-64 years of age in '80. Exposure calculations were based on the job held longest during the 10- year period prior to diagnosis. Many measurements were taken using a person whose job was most similar to that of the person in the study. About two-thirds of the subjects in the study could be assessed in this manner. A significant elevation in incidence was found for leukemia, but not for brain cancer. - Guenel et al [D14]. A case-control study based on all cancer in employed Danes between '70 and '87. Each occupation-industry combination was coded on the basis of supposed 50-Hz magnetic field exposure. No significant increases were seen for breast cancer, malignant lymphomas or brain tumors. Leukemia incidence was significantly elevated among men in the highest exposure category; women in similar exposure categories showed no increase in leukemia. - Tynes et al [D16]. Case-control study of workers on electrical (16.67 Hz) and non-electrical railroads in Norway. Analysis showed no significant excess of leukemia or brain cancer, and no significant trend for either magnetic or electrical fields. On the electrified railroads fields averaged 19.7 mT (197 mG), and 0.8 kV/m. Cumulative exposures were as high as 3000 mT-years (30 G-yrs) and 25 kV/m-yrs. - Theriault et al [D15. Case-control study of electric utility workers in France and Canada. Exposure to magnetic fields were estimated from measurements of current exposure of workers performing similar tasks. No association with magnetic fields was observed for overall cancer or for any of the other 29 cancer types studied, including melanoma, overall leukemia, brain cancer or male breast cancer. Workers with cumulative exposure above 3.1 microT-years (31 mG-years) had a significantly higher incidence of acute non-lymphocytic and acute myeloid leukemia, there were no clear dose-response trends. - Matanoski et al [D11]. Case-control study of telephone company workers in New York, with exposure defined by job titles plus some retrospective measurements. The incidence of leukemia was increased, but not significantly so, in workers with higher exposures to magnetic fields. The authors interpret their data as showing higher risk with increasing exposure, but the trend does not appear to be statistically significant. - Sahl et al [D13]. Cohort plus nested case-control of electrical utility workers in California. Dosimetry was done on selected workers. Electricians had the highest exposures, with a time-weighted mean of 30 mG (3 microT). Neither cohort nor case-control analysis showed a significant excess of total cancer, leukemia, brain cancer or lymphoma. No significant dose-response trend was found for any cancers. 19C) What about the new studies showing a link between electrical occupation and breast cancer? There are some laboratory studies [G14,G23] that suggest that power- frequency fields might promote chemically-induced breast cancer (see Question 16B), and a biological mechanism has been proposed that could explain such a connection (see Question 17C). However, there is relatively little epidemiological support for such a connection. McDowall et al [C4] found no excess female breast cancer (and no male breast cancer at all) in adults living near transmission lines or substations. Vena et al [C12] found no excess breast cancer in women who used electric blankets. Tynes & Anderson [D4] and Demers [D5] both reported a significantly elevated incidence of male breast cancer in electrical workers. Matanoski et al [D6] and Loomis et al [D7] also reported an excess incidence of male breast cancer in electrical workers, but in neither case was the increase statistically significant. More recently, studies by Theriault et al [D15], Rosenbaum et al [D17] and Guenel et al [D14] have found no excess breast cancer in electrical workers. Recently, Loomis et al [D18,D19] reported a significantly elevated incidence of female breast cancer in occupations with presumed exposure to power-frequency fields. In occupations with "potential exposure" to power-frequency fields there was no increased incidence of breast cancer. The occupations that showed an excess incidence of breast cancer were ³male-dominated² and the ones that did not were largely ³female-dominated². The authors note that breast cancer mortality is known to be elevated among women in professional and technical jobs in general. This elevation is related to the fact that women working in male-dominated jobs tend to have reproductive histories (for example, no pregnancies, delayed child-bearing, not breast-feeding) that increase their risk for breast cancer. 20) What criteria do scientists use to evaluate all the confusing and contradictory laboratory and epidemiological studies of power-frequency magnetic fields and cancer? There are certain widely accepted criteria that are weighed when assessing epidemiological and laboratory studies of agents that may pose human health risks. These are often called the "Hill criteria" [E1]. Under the Hill criteria one examines the strength (Question 20A) and consistency (Question 20B) of the association between exposure and risk, the evidence for a dose-response relationship (Question 20C), the laboratory evidence (Question 20D), and the biological plausibility (Question 20E). The Hill criteria must be applied with caution. First, when employing the Hill criteria it is necessary to examine the entire published literature; it is not acceptable to pick out only those reports that support the existence of a health hazard. Second, it is necessary to directly review the important source documents; it is not safe to base judgments solely on academic or regulatory reviews. Third, satisfying the individual criteria is not a yes-no matter; support for a criterion can be strong, moderate, weak, or non-existent. Lastly, the Hill criteria must be viewed as a whole; no individual criterion is either necessary or sufficient for concluding that there is a causal relationship between exposure to an agent and a disease. Overall, application of the Hill criteria shows that the current evidence for a connection between power-frequency fields and cancer is weak, because of the weakness and inconsistencies in the epidemiological studies, combined with the lack of a dose-response relationship in the human studies, and the largely unsupportive laboratory studies. A detailed evaluation of the criteria follows. 20A) Criterion One: How strong is the association between exposure to power-frequency fields and the risk of cancer? The first Hill criterion is the *strength of the association* between exposure and risk. That is, is there a clear risk associated with exposure? A strong association is one with a RR (Question 13) of 5 or more. Tobacco smoking, for example, shows a RR for lung cancer 10-30 times that of non-smokers. Most of the positive power-frequency studies have RRs of less than two. The leukemia studies as a group have RRs of 1.1-1.3, while the brain cancer studies as a group have RRs of about 1.3-1.5. This is only a weak association. 20B) Criterion Two: How consistent are the studies of associations between exposure to power-frequency fields and the risk of cancer? The second Hill criterion is the *consistency* of the studies. That is, do most studies show about the same risk for the same disease? Using the same smoking example, essentially all studies of smoking and cancer showed an increased risk for lung and head-and-neck cancers. Many power-frequency studies show statistically significant risks for some types of cancers and some types of exposures, but many do not. Even the positive studies are inconsistent with each other. For example, while a recent Swedish study [C18] shows an increased incidence of childhood leukemia for one measure of exposure, it contradicts prior studies that showed an increase in brain cancer [B3,B4], and a parallel Danish study [D14] shows an increase in childhood lymphomas, but not in leukemia. Many of the studies are internally inconsistent. For example, where a recent Swedish study [C18] shows an increase for childhood leukemia, it shows no overall increase in childhood cancer. Since leukemia account for about one-third of all childhood cancer, this implies that the rates of other types of cancer were decreased; an examination of the data indicates that this is true. In summary, few studies show the same positive result, so that the consistency is weak. 20C) Criterion Three: Is there a dose-response relationship between exposure to power-frequency fields and the risk of cancer? The third Hill criterion is the evidence for a *dose-response relationship*. That is, does risk increase when the exposure increases? Again, the more a person smokes, the higher the risk of lung cancer. No published power-frequency exposure study has shown a dose-response relationship between measured fields and cancer rates, or between distances from transmission lines and cancer rates. The lack of a relationship between exposure and increased cancer incidence is a major reason why most scientists are skeptical about the significance of the epidemiology. Not all relationships between dose and risk can be described by simple linear no-threshold dose-response curves where risk is strictly proportional to risk. There are known examples of dose-response relationships that have thresholds, that are non-linear, or that have plateaus. For example, the incidence of cancer induced by ionizing radiation in rodents rises with dose, but only up to a certain point; beyond that point the incidence plateaus or even drops. Without an understanding of the mechanisms connecting dose and effect it is impossible to predict the shape, let alone the magnitude of the dose- response relationship. 20D) Criterion Four: Is there laboratory evidence for an association between exposure to power-frequency fields and the risk of cancer? The fourth Hill criterion is whether there is *laboratory evidence* suggesting that there is a risk associated with such exposure? Epidemiological associations are greatly strengthened when there is laboratory evidence for a risk. When the US Surgeon General first stated that smoking caused lung cancer, the laboratory evidence was ambiguous. It was known that cigarette smoke and tobacco contained carcinogens, but no one had been able to make lab animals get cancer by smoking (mostly because it is hard to convince animals to smoke). Currently the laboratory evidence linking cancer and smoking is much stronger. Power-frequency fields show little evidence of the type of effects on cells, tissues or animals that point towards their being a cause of cancer, or to their contributing to cancer (Question 16). 20E) Criterion Five: Are there plausible biological mechanisms that suggest an association between exposure to power-frequency fields and the risk of cancer? The fifth Hill criterion is whether there are *plausible biological mechanisms* that suggest that there should be a risk? When it is understood how something causes disease, it is much easier to interpret ambiguous epidemiology. For smoking, while the direct laboratory evidence connecting smoking and cancer was weak at the time of the Surgeon Generals report, the association was highly plausible because there were known cancer-causing agents in tobacco smoke. From what is known of power-frequency fields and their effects on biological systems there is no reason to even suspect that they pose a risk to people at the exposure levels associated with the generation and distribution of electricity (see Questions 16,17,18). Copyright (C) by John Moulder end: powerlines-cancer-FAQ/part3