Power-Frequency Fields and Cancer (J. Moulder, v8, 17-Nov-93) Notice: This FAQ sheet may b

Power-Frequency Fields and Cancer (J. Moulder, v8, 17-Nov-93) Notice: This FAQ sheet may be redistributed as long at remains correctly attributed. If it is edited or condensed prior to redistribution, please add a note to that effect. Revision notes: v8: (17-Nov-93): Discussion of newly published European studies of powerlines and cancer expanded. Meta-analysis sections updated to include the new studies and more cautionary notes. Section of bioeffects expanded, and reference to the EMF-melatonin-cancer hypothesis added. Several Gauss to Tesla conversion errors corrected. 1) Why is there a concern about power lines and cancer? Most of the concern about power lines and cancer stems from epidemiological studies of people living near distribution and transmission lines, and epidemiological studies of people working in "electrical occupations". Some of these epidemiological studies appear to show a relationship between exposure to power-frequency fields and the incidence of cancer. Laboratory studies have shown little evidence of a link between power-frequency fields and cancer. 2) What's the difference between the electromagnetic [EM] energy associated with power lines and other forms of EM energy such as microwaves or x-rays? X-rays, ultraviolet (UV) light, visible light, infrared light, microwaves (MW), radiowaves (RF), and magnetic fields from electrical power systems are all parts of the EM spectrum. The parts of the EM spectrum are characterized by their frequency or wavelength. The frequency and wavelength are related, and as the frequency rises the wavelength gets shorter. The frequency is the rate at which the EM field changes direction and is usually given in Hertz (Hz), where one Hz is one cycle per second. Power-frequency fields in the US vary 60 times per second, so they are 60 Hz fields, and have a wavelength of 3000 miles (5000 km). Power in most of the rest of the world is at 50 Hz. Broadcast AM radio has a frequency of around one million Hz and a wavelength of around 1000 ft (300 m). Microwave ovens have a frequency of about 2 billion Hz, and a wavelength of about 5 inches (12 cm). X-rays and UV light have frequencies of millions of billions of Hz, and wavelengths of less than a thousandth of an inch (10 nm or less). 3) What differences are there in the biological effects of these different portions of the EM spectrum? The interaction of biological material with an EM source depends on the frequency of the source. We usually talk about the EM spectrum as though it produced waves of energy. This is not strictly correct, because sometimes EM energy acts like particles rather than waves; this is particularly true at high frequencies. This double nature of the EM spectrum is referred to as "wave-particle duality". The particle nature of EM energy is important because it is the energy per particle (or photons, as these particles are called) that determines what biological effects EM energy will have. At the very high frequencies characteristic of UV light and X-rays, EM particles (photons) have sufficient energy to break chemical bonds. This breaking of bonds is termed ionization, and this portion of the EM spectrum is termed ionizing radiation. At lower frequencies, such as those characteristic of visible light, radiowaves, and microwaves, the photons don't carry enough energy to break chemical bonds; but they do carry enough energy to cause molecules to vibrate, causing heating. These are called thermal effects, and this portion of the EM spectrum is termed the thermal, non-ionizing portion. At frequencies below those used in commercial broadcast radio (such as the 50/60 Hz frequencies generated in the production and distribution of electricity), the photons have insufficient energy to cause heating, and this portion of the EM spectrum is termed the non-thermal, non-ionizing portion. 4) What is difference between EM radiation and EM fields? In general, EM sources produce both radiant energy (radiation) and non- radiant energy (fields). Radiated energy exists apart from its source, travels away from the source, and continues to exist even if the source is turned off. Non-radiant energy is not projected away into space, and it ceases to exist when the energy source is turned off. When a person or object is more than several wavelengths from an EM source, a condition called far-field, the radiation component of the EM source dominates. In the far- field the electrical and magnetic components are closely related. When a person or object is less than one wavelength from an EM source, a condition called near-field, the field effect dominates, and the electrical and magnetic components are unrelated. For ionizing frequencies where the wavelengths are less than a thousandth of an inch (less than 10 nm), human exposure is entirely in the far-field, and only the radiation from the EM source is relevant to possible health effects. For MW and RF, where the wavelengths are in inches to a few thousand feet (a few cm to a km), human exposure can be in both the near- and the far-field, so that both field and radiation effects are relevant. For power-frequency fields, where the wavelength is thousands of miles (thousands of km), human exposure is always in the near-field, and only the field component is relevant to possible health effects. 5) How do ionizing EM sources cause biological effects? Ionizing EM radiation carries sufficient energy per photon to break chemical bonds. In particular, ionizing radiation is capable of breaking bonds in the genetic material of the cell, the DNA. Severe damage to DNA can kill cells, resulting in tissue damage or death. Lesser damage to DNA can result in permanent changes in the cells which may lead to cancer. If these changes occur in reproductive cells, they can lead to inherited changes, a phenomena called mutation. All of the known hazards from exposure to the ionizing portion of the EM spectrum are the result of the breaking of chemical bonds in DNA. For frequencies below that of UV light, DNA damage does not occur because the photons do not have enough energy to break chemical bonds. Well- accepted safety standards exist to prevent significant damage to the genetic material of persons exposed to ionizing EM radiation. 6) How do the thermal non-ionizing EM sources cause biological effects? Visible light, MW, and RF can cause molecules to vibrate, causing heating. This molecular heating can kill cells. If enough cells are killed, burns and other forms of long-term, and possibly permanent tissue damage can occur. Cells which are not killed by heating gradually return to normal after the heating ceases; permanent non-lethal cellular damage is not known to occur. All of the known hazards from exposure to the thermal non-ionizing portion of the EM spectrum are the result of heating. For frequencies below about the middle of the AM broadcast spectrum, this heating does not occur because the photons do not have enough energy to cause molecular vibrations. The molecular vibration caused by MW is how and why a MW oven works - exposure of the food to the microwaves causes water molecules to vibrate and get hot. MW and RF penetrate and heat best when the size of the object is close to the wavelength. For the 2450 MHz (2.45 billion Hz) used in microwave ovens the wavelength is 5 inches (12 cm), a good match for most of what we cook. 7) How do the power-frequency EM fields cause biological effects? The electrical and magnetic fields associated with power-frequency fields cannot break bonds or cause molecular heating because the energy per photon is too low. Thus the known mechanisms through which ionizing radiation, MWs and RFs effect biological material have no relevance for power-frequency fields. The electrical fields associated with the power-frequency fields exist whenever voltage is present, and regardless of whether current is flowing. These electrical fields have very little ability to penetrate buildings or even skin. The magnetic fields associated with power-frequency fields exist only when current is flowing. These magnetic fields are difficult to shield, and easily penetrate buildings and people. Because power-frequency electrical fields do not penetrate, any biological effects from routine exposure to power-frequency fields must be due to the magnetic component of the field. Exposure of people to power-frequency magnetic fields results in the induction of electrical currents in the body. These currents are similar to naturally-occurring currents. It requires a power-frequency magnetic field in excess of 5 Gauss (500 microT, see Q8 for typical exposures) to induce electrical currents of a magnitude similar to those that occur naturally in the body. Electrical currents that are above those that occur naturally in the body can cause noticeable effects, including direct nerve stimulation. Well-accepted safety standards exist to protect persons from exposure to power-frequency fields that would induce such currents (see Q16 for safety standards). 8) What sort of power-frequency magnetic fields are common in residences and workplaces? In the US magnetic fields are commonly measured in Gauss (G) or milliGauss (mG), where 1,000 mG = 1G. In the rest of the world, they are measured in Tesla (T), were 10,000 Gauss equals 1 Tesla (1 Gauss = 100 microT; 1 microT = 10 milliGauss). Power-frequency fields are measured with a calibrated gauss meter. Measurements must be done in multiple locations over a substantial period of time because there are large variations in fields over space and time. Within the right-of-way (ROW) of a high voltage transmission line, fields can approach 100 mG (0.1 G, 10 microT). At the edge of a high-voltage transmission ROW, the field will be 1-10 mG (0.1-1.0 microT). Ten meters from a 12 kV distribution line will be fields will be 2-10 mG (0.2-1.0 microT). Actual fields depend on voltage, design and current. Fields within residences vary from over 1000 mG (100 microT) a few inches (cm) from certain appliances to less than 0.2 mG (0.02 microT) in the center of some rooms. Appliances that have the highest fields are those with high currents (e.g., toasters, electric blankets) or high-speed electric motors (e.g., vacuum cleaners, electric clocks, blenders, power tools). Appliance fields decrease very rapidly with distance. See ref. 24 for further details. Occupational exposures in excess of 100 mGauss (10 microT) have been reported (e.g., in arc welders and electrical cable splicers). In "electrical" occupations mean exposures range from 5 to 40 mG (0.5 to 4 microT). See ref. 24 for further details. 9) What is known about the relationship between powerline corridors and cancer rates? Some studies have shown that children (but not adults) living near certain types of powerlines (high current distribution lines and transmission lines) have higher than average rates of leukemia, lymphomas and brain cancers (Refs 1-3, 38). The correlation is not strong, and none of the studies have shown dose-response relationships. When power-frequency fields are actually measured, the correlation vanishes. Several other studies have shown no correlations between residence near power lines and cancer risk (Refs 4-6, 37). 10) How big is the "cancer risk" associated with living next to a powerline? The excess cancer found in epidemiological studies is usually quantified in a number called the relative risk (RR). This is the risk of an "exposed" person getting cancer divided by the risk of an "unexposed" person getting cancer. Since no one is unexposed to power-frequency fields, the comparison is actually "high exposure" versus "low exposure". A RR of 1.0 means no effect, a RR of less the 1.0 means a decreased risk in exposed groups, and a RR of greater than one means an increased risk in exposed groups. Relative risks are generally given with 95% confidence intervals. These 95% confidence intervals are almost never adjusted for multiple comparisons even when multiple types of cancer and multiple indices of exposure are studied. An overview of the epidemiology requires that studies be combined using a technique known as "meta-analysis". Meta-analysis is not easy to do, since the epidemiological studies of residential exposure use a wide variety of methods for assessing "exposure". Meta-analysis also gets out-of-date rapidly in this field. The following RRs (called summary RRs in meta- analysis) for the residential exposure studies are adapted from refs. 7 and 39 by inclusion of the new European studies (see Q13). The confidence intervals should be viewed as measures of the diversity of the data, rather than as strict tests of the statistical significance of the data. childhood leukemia: 1.5 (0.7 - 3.0) 8 studies childhood brain cancer: 1.9 (0.9 - 2.9) 6 studies childhood lymphoma: 2.5 (0.3 - 40) 2 studies all childhood cancer: 1.5 (0.9 - 3.0) 5 studies adult leukemia: 1.1 (0.8-1.6) 3 studies adult brain cancer: 0.7 (0.4 - 1.3) 1 study all adult cancer: 1.1 (0.9-1.3) 3 studies As a base-line for comparison, the age-adjusted cancer incidence rate for adults in the United States is 3 per 1,000 per year for all cancer (that is, 0.3% of the population gets cancer in a given year),and 1 per 10,000 per year for leukemia (ref. 26). 11) What is known about the relationship between "electrical occupations" and cancer rates? Several studies have shown that people who work in electrical occupations have higher than average leukemia, lymphoma, and brain cancer rates (refs 8- 10). Most of the cautions listed for the residential studies apply here also: many negative studies, weak correlations, no dose-response relationships. Additionally, these studies are mostly based on job titles, not on measured exposures. Meta-analysis of the occupational studies is even more difficult than the residential studies. First, a variety of epidemiological techniques are used, and studies using different techniques should not really be combined. Second, a wide range of definitions of "electrical occupations" are used, and very few studies actually measured exposure. The following RRs (see Q10) for the occupational exposure studies are adapted from refs 7 and 40. Again, the confidence intervals should be viewed as measures diversity rather than as tests of the statistical significance. leukemia: 1.15 (1.0-1.3) 28 studies brain: 1.15 (1.0-1.4) 19 studies lymphoma: 1.2 (0.9-1.5) 6 studies all cancer: 1.0 (0.9-1.1) 8 studies The above relative risks do not take into account the recent European studies (Q13). Adding these new studies raises the summary RR for leukemia to about 1.2, and lowers the summary RRs for brain cancer and lymphomas to essentially one. Another new study of cancer in the electrical power industry (ref 30) shows no significant elevation of leukemia, brain cancer or lymphoma risks. 12) What do laboratory studies tell us about power-frequency fields and cancer? Carcinogens, agents that cause cancer, are generally of two types: genotoxins and promoters. Genotoxic agents (often called initiators) directly damage the genetic material of cells. Genotoxins usually effect all types of cells, and may cause many different types 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 never goes away. A promoter (often called an epigenetic agent) is something that increases the cancer risk in animals already exposed to a genotoxic carcinogen. Promoters usually effect only certain types of cells, and may cause only certain types of cancer. Promoters generally have thresholds for their effect; in other words, as the dose of the promoter is lowered a level is reached in which there is no risk. Power-frequency fields show none of the classic signs of being genotoxins - they do not cause DNA damage or chromosome breaks, and they are not mutagenic (refs 11-15 and see ref 31 for a comprehensive review). No studies have shown that animals exposed to power-frequency fields have increased cancer rates. There are agents (for example, promoters) that influence the development of cancer without directly damaging the genetic material. It has been suggested that power-frequency EMFs could promote cancer (refs 17 & 18). Most promotion studies of power-frequency fields have been negative (refs 14, 19- 21); but recently there was a positive report of promotion of breast cancer in rats (ref 31). There are other biological effects that might be related to cancer. 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). Studies of effects on proliferation and progression have had very mixed results: 75% show no effect on growth, while the rest are about equally mixed between studies showing increased growth and studies showing decreased growth (refs 11, 12, 15, 20- 22, 33). With one possible exception (ref 33) there have been no reported effects on proliferation or progression for fields below 2000 mGauss (200 microTesla). Suppression of the immune system in animals and humans is associated with increased rates of certain types of cancer, particularly lymphomas (refs 34 and 35). Immune suppression has not been associated with excess leukemia and brain cancer. Some studies have shown that power frequency fields can have effects on cells of the immune system (41), but no studies have shown the type or magnitude of immunesuppression that is associated with increased cancer risks. 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 (42, 43). This is highly speculative. There have been some reports that EM fields effect melatonin production, but studies using power-frequency magnetic fields have not shown reproducible effects. In addition, while there is evidence that melatonin has "cancer-preventive" activity against breast cancer in rats, there is no evidence that melatonin effects other types of cancer, or that it has any effect on breast cancer in humans. 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 (refs 16, 17, 41). Power-frequency fields intense enough to induce electrical currents in excess of those that occur naturally (above 5 G, 500 microT see Q7) have shown reproducible effects, including effects on humans (ref 16). Below about 2 G (200 microT) there are few published (and replicated) reports of bioeffects, although there are unreplicated reports of effects for fields as low as about 200 mG (20 microT). Even among the scientists who believe that there may be a connection between power-frequency fields and cancer, there is no consensus as to mechanisms which would connect these "bioeffects" with cancer causation (refs 16, 18). 13) What about the new "Swedish" study showing a link between power lines and cancer? There are new residential and occupational studies from Sweden, Denmark (36, 38), Finland (37) and the Netherlands (44). The Swedish studies are available only as translations of the unpublished preliminary reports. The published studies are considerably more cautious in there interpretations of the data than were the unpublished preliminary reports and the earlier press reports. - Fleychting & Ahlbom [Magnetic fields and cancer in people residing near Swedish high voltage powerlines]. A case-control study of everyone who lived within 300 meters 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 powerlines. No increased overall cancer risk was found for either children or adults. An increased risk for leukemia (but not other cancers) was found in children for *calculated* fields at the time of diagnosis. It is not clear if or how the retrospective fields calculations take into account sources other the transmission lines. No significantly elevated cancer risks were found for measured fields or proximity to transmission lines. - Verkasalo et al (ref 37). Study design similar to Fleychting & Ahlbom (above). Cohort study of cancer in children in Finland living within 500 m 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 such as distribution lines, household wiring or appliances. Both average fields and cumulative fields (microT - years) were used as exposure metrics. For all cancers the RR was 1.5 (0.7 - 2.7) for average exposure above 0.20 microT (2 mG), and 1.4 (0.8 - 2.3) for cumulative exposure above 0.50 microT-years. A significant excess risk of brain cancer way found in boys, the excess was due entirely to one exposed boy who developed three independent brain tumors. No significantly increased risks were found for brain tumors in girls or for leukemia, lymphomas or "other" tumors in either sex. - Olsen and Nielson (ref 38). 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 risk was found when 0.25 microT (2.5 mG) was used as the cut-point to define exposure (as specified in the study design). After the data were analyzed, it was found that the risk for all childhood cancer was significantly elevated if 0.40 microT (4 mG) was used as the cut-point. For the 0.40 microT cut-point the RR for all cancer was 5.6 (1.2 - 20). No significant increased risk was found for leukemia or brain cancer for any cut-point. A significant increase in lymphoma risk was found for the 0.10 microT cut-point but not for higher cut-points. - Guenel et al (ref 36). Case-control study based on all cancer in actively employed Danes between '70 and '87 who were 20-64 years old in 1970. Each occupation-industry combination was coded on the basis of supposed 50-Hz magnetic field exposure. No significant increases in risk were seen for breast cancer, malignant lymphomas or brain tumors. Leukemia incidence was elevated among men in the highest "exposure" category; women in similar exposure categories showed no excess risk. For men in the highest "exposure" category the RR for leukemia was 1.6 (1.2 - 2.2). -Floderus et al [Occupational exposure to EM fields in relation to leukemia and brain tumors]. Case-control study of leukemia and brain tumors of men, 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 significantly elevated risk was found for leukemia, but not for brain cancer. 14) How do scientists 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 such groups of studies. These are often called the "Hill criteria" (ref. 23). - First, what is the *strength of the association* between exposure and risk - - is there a clear risk associated with exposure? A strong association is one with a RR (see Q9) 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 about 1.2, while the brain cancer studies as a group have RRs of about 1.5. This is only a weak association. - Second, are there many *consistent studies* indicating the same risk -- do most studies show about the same risk for the same disease? Using the same 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 new Swedish study shows an increased risk for childhood leukemia for one measure of exposure, it contradicts prior studies that showed a risk for brain cancers, and a parallel Danish study shows a risk for childhood lymphomas, but not for leukemia. Many of the studies are internally inconsistent. For example, where the Swedish study shows an increased risk for childhood leukemia, it shows no overall increase in childhood cancer, implying that the rates of other types of cancer are decreased. In summary, few studies show the same positive result, so that the consistency is weak. - Third, is there evidence for a *dose-response relationship* -- 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 risk is a major reason why many scientists are skeptical about the significance of the epidemiology. - Fourth, is there *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's hard to convince animals to smoke). Power-frequency fields show little evidence of the type effects on cells, tissues or animals that point towards their being a cause of cancer, or to their contributing to cancer. -Fifth, are there *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. With smoking, for example, the fact that there were known cancer-causing agents in tobacco made it very easy to believe the epidemiology. 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. - Overall the 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 negative laboratory studies. 15) If power-frequency fields don't explain the positive residential and occupations studies, what could? There are basically three factors that can result in false associations in epidemiological studies. These are: a) Inadequate dose assessment - if power-frequency fields are associated with cancer, we do not know what aspect of the field is involved. At a minimum, risk could be related to the peak field, the average field, of the rate of change of the field. If we don't know who is really exposed, and who is not, we will usually (but now always) underestimate the true risk. b) Confounders - power lines (or electrical occupations) might be associated with a cancer risk other than magnetic fields. Many confounders of the powerline studies have been suggested: PCBs, herbicides, traffic density, socioeconomic class. The first two are unlikely. PCB leakage is rare, and PCB exposure has been linked to lymphomas, not leukemia or brain cancer. Herbicide spraying would not effect distribution systems in urban areas (where 3 of 4 positive childhood cancer studies have been done). Traffic density may be a real confounder (see ref. 28). Socioeconomic class may be an issue in both the residential and occupational studies, as socioeconomic class is clearly associate with cancer risk, and "exposed" and "unexposed" groups in many studies are of different socioeconomic classes (see ref. 29 for a discussion of some of these issues) c) Publication bias - it is a known that positive studies are more likely to be published than negative studies. This can severely bias meta-analysis studies such as those discussed in Q10 and Q11. Such publication bias will increase apparent risks. This is a bigger potential problem for the occupational studies than the residential ones. It is also a clear problem for laboratory studies -- it is much easier to publish studies that report effects than studies that report no effects (such is human nature!). 16) What is the strongest evidence for and against a connection between power-frequency fields and cancer? The best evidence for a connection between cancer and power-frequency fields is probably: a) The four epidemiological studies that show a correlation between childhood cancer and proximity to high-current wiring (refs 1-3 plus the Fleychting & Ahlbom study described in Q13). b) The epidemiological studies that show a significant correlation between work in electrical occupations and cancer, particularly leukemia and brain cancer (refs 8-10). c) The lab studies that how that power-frequency fields do produce bioeffects. The most interesting of the lab studies are probably the ones showing increased transcription of oncogenes at fields of 1-5 Gauss (100 - 500 microT) (see ref. 17 and 18). d) The one laboratory study that provides evidence that power-frequency magnetic fields can promote chemically-induced breast cancer (ref 32). The best evidence that there is not a connection between cancer and power- frequency fields is probably: a) Application of the Hill criteria (Q14) to the entire body of epidemiological and laboratory studies (refs 24 and 27) b) The fact that all studies of genotoxicity, and all but one study of promotion have been negative (Q12). c) Adair's (ref. 25) biophysical analysis that indicates that "any biological effects of weak [less than 500 mG, 50 microT] ELF fields on the cellular level must be found outside of the scope of conventional physics" d) Jackson's (ref. 26) epidemiological analysis that shows that childhood and adult leukemia rates in the US have been stable over a period of time when per capita power consumption in the US has risen by a factor of five. 17) What studies are needed to resolve the cancer-EMF issue? In the epidemiological area, more of the same types of studies are unlikely to resolve anything. Studies showing a dose-response relationship between measured fields and cancer incidence rates would clearly affect thinking, as would studies identifying confounders in the residential and occupational studies. In the laboratory area, more genotoxicity and promotion studies may not be very useful. Exceptions might be in the area of cell transformation, and promotion of chemically-induced breast cancer. Long-term rodent exposure studies (the standard test for carcinogenicity) would have a major impact if they were positive, but if they were negative it wouldn't change very many minds. Further studies of some of the known bioeffects would be useful, but only if they identified mechanisms or if they established the conditions under which the effects occur (e.g., thresholds, dose-response relationships, frequency-dependence, optimal wave-forms). 18) What are some good overview articles? A good review of the area was published by Oak Ridge Associated Universities (ref 40). It is available from National Technical Information Service (ARAU 92/F-8) and the US Government Printing Office (029-000-00443-9). If you're in the U.K., the National Radiation Protection Board has a good review (ref 39). Two other good review are Theriault (ref. 24) and Bates (ref. 27). 19) Are there exposure standards for power-frequency fields? Yes, a number of governmental and professional organizations have developed exposure standards. These standards are based on keeping the body currents induced by power-frequency EM fields to a level below the naturally occurring fields (see Q7). The most generally relevant are: - Guidance as to restriction on exposures to time varying EM fields and the 1988 recommendations on the International Non-Ionizing Radiation Committee, National Radiation Protection Board, Chilton, 1989. 50/60 Hz E-field: ~10 kV/m (freq. dependent) 50/60 Hz H-field: 1630 A/m, 2 mT (20 G) - Sub-radiofrequency (30 KHz and below) magnetic fields, In: Documentation of the threshold limit values, American Committee of Government and Industrial Hygienists, pp. 55-64,1992. At 60 Hz: 1 mT (10 G); 0.1 mT (1 G) for pacemaker wearers - HP Jammet et al: Interim guidelines on limits of exposure to 50/60 Hz electric and magnetic fields. Health Physics 58:113-122, 1990. *H-field (rms) 24 hr general public: 0.1 mT = 1 G Short-term general public: 1 mT = 10 G Occupational continuous: 0.5 mT = 5 G Occupational short-term: 5 mT = 50 G *E-field (rms) 24 hr general public: 5 kV/m Short-term general public: 10 kV/m Occupational continuous: 10 kV/m Occupational short-term: 30 kV/m ----------------------- References: 1) N Wertheimer & E Leeper: Electrical wiring configurations and childhood cancer. Amer J Epidemiol 109:273-284, 1979. 2) DA Savitz et al: Case-control study of childhood cancer and exposure to 60- Hz magnetic fields. Amer J Epidemiol 128:21-38, 1988. 3) SJ London et al: Exposure to residential electric and magnetic fields and risk of childhood leukemia. Amer J Epidemiol 134:923-937, 1991. 4) MP Coleman et al: Leukemia and residence near electricity transmission equipment: a case-control study. 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FASEB Journal 6:2283-2287, 1992. 44) GH Schreiber et al: Cancer mortality and residence near electricity transmission equipment: A retrospective cohort study. Int J Epidem 22:9-15, 1993. John Moulder (jmoulder@its.mcw.edu) Voice: 414-266-4670 Experimental Radiotherapy Group FAX: 414-257-5033 Medical College of Wisconsin, Milwaukee