FAQs on Power-Frequency Fields and Cancer (part 1 of 2) Notice: This FAQ sheet may be redistributed as long at remains correctly attributed. If it is edited prior to redistribution, please add a note to that effect. Revision notes: v1.9 (19-Dec-93): Sections added on whether powerlines radiate, how close one has to live to a powerline to be considered exposed, reducing fields, and on the impact of powerlines on property values. The sections on laboratory studies and on arguments pro and con were broken up into sections. The sections on confounders, and on application of the Hill criteria were expanded and broken up into sections. The sections on new European epidemiological studies and on standards were updated. v2.2 (28-Dec-94): Schreiber study and Ahlbom et al meta-analysis added to Q18. First version approved for and posted to *.answers newsgroups. Converted to two parts. 1) Why is there a concern about powerlines and cancer? Most of the concern about power lines and cancer stems from epidemiological studies of people living near powerlines, and epidemiological studies of people working in Òelectrical occupationsÓ. Some of these epidemiological studies appear to show a relationship between exposure to power-frequency magnetic fields and the incidence of cancer. Laboratory studies have shown little evidence of a link between power-frequency fields and cancer. 2) What is 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 electromagnetic 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. The power-frequency fields are often referred to as extremely low frequencies or ELF. 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 2.5 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, RR and MW, the photons do not carry enough energy to break chemical bonds; but they do carry enough energy to cause molecules to vibrate, causing heating (thermal effects). 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? When dealing with fields from an electromagnetic source it is customary to distinguish between near fields (which do not transmit energy to infinity from the source) and radiation (which does). 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. 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 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 can be 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) Do power lines produce electromagnetic radiation? The fields associated with transmission lines are purely near-field. While the lines theoretically might radiate some energy the efficiency of this is so low that this effect can for all practical purposes be ignored. To be an effective radiation source, and antenna must have a length comparable to its wavelength. Power-frequency sources are clearly too short compared to their wavelength (3000 miles, 5000 km) to be effective radiation sources. This is not to say that there is no loss of power during transmission. There are many sources of loss in transmission lines that have nothing to do with "radiation" (in the sense as it is used in electromagnetic theory). Loss of energy is a result of resistive heating, not "radiation". This is in sharp contrast to radiofrequency antennas, which "lose" energy to space by radiation. Likewise, there are many ways of transmitting energy from point A to point B that do not involve radiation. Electrical circuits do it all the time. The only ÒpracticalÓ exception to the statement that power-frequency fields do not radiate is the use of extremely-low-frequency antennas to broadcast to submerged submarines. The US Navy runs a power-frequency antenna in Northern Wisconsin and the Upper Peninsula of Michigan. To overcome the inherent inefficiency of the frequency, the antenna is several hundred kilometers in length. 6) 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. 7) 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. Well-accepted safety standards exist to prevent significant thermal damage to persons exposed to MW and RFs [45] and also for persons exposed to lasers and UV light. 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. 8) 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 Q9 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 (Q25 for safety standards). 9) 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 G equals 1 T (1 G = 100 microT; 1 microT = 10 mG). 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 (1200 volt) distribution line 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 Theriault [24] for further details. Occupational exposures in excess of 100 mG (10 microT) have been reported (e.g., in arc welders and electrical cable splicers). In ÒelectricalÓ occupations mean exposures range from 5 to 40 mG (0.5 to 4 microT). See Theriault [24] for further details. 10) Can power-frequency fields in homes and workplaces be reduced? There are engineering techniques that can be used to decrease the magnetic fields produced by power lines, substations, transformers and even household wiring and appliances. Once the fields are produced, however, shielding is very difficult. Small area can be shielded by the use of Mu metal, a nickel-iron-copper alloy with Òhigh magnetic permeability and low hysteresis lossesÓ. Mu metal shields are very expensive, and limited to small volumes. 11) 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 [1-3, 38, 45]. The correlations are 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 [4-6, 37]. 12) How big is the Òcancer riskÓ 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 ÒexposedÓ person getting cancer divided by the risk of an ÒunexposedÓ person getting cancer. Since no one is unexposed to power-frequency fields, the comparison is actually Òhigh exposureÓ versus Òlow exposureÓ. 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 (see Olsen et al, [38], Fig. 2 for an example of a multiple comparison adjustment). An overview of the epidemiology requires that studies be combined using a technique known as Òmeta-analysisÓ. Meta-analysis is not easy to do, since the epidemiological studies of residential exposure use a wide variety of methods for assessing ÒexposureÓ. 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 Hutchison [7] and Doll et al [39] by inclusion of the new European studies (Q18). 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 (0.8 - 3.0) 8 studies childhood brain cancer: 1.9 (0.9 - 3.0) 6 studies childhood lymphoma: 2.5 (0.3 - 40) 2 studies all childhood cancer: 1.5 (0.9 - 2.5) 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 [26]. 13) How close do you have to be to a power line to be considered exposed to power-frequency magnetic fields? The epidemiological studies that show a relationship between cancer and powerlines do not provide any consistent guidance as to what distance or exposure level is associated with increased risk. The studies have used a wide variety of techniques to measure exposure, and they differ in the type of lines that are studied. The US studies have been based predominantly on neighborhood distribution lines [1-3], whereas the European studies have been based strictly on high-voltage transmission lines [4-6, 37, 38, 44, 46]. Field measurements: Several studies have measured power-frequency fields in the residences [2, 3, 45]. Both one-time (spot), peak, and 24-hour average measurement have been made; none of the studies using measured fields have shown a relationship between exposure and cancer risk. Proximity to lines: Several studies have used the distance from the power line corridor to the residence as a measure of power-frequency fields [4-6, 44, 46]. When something we can measure (distance to the line), is used as an index of what we really want to measure (the magnetic field), it is called a surrogate (or proxy) measure. With one exception, studies that have used distance from power lines as a surrogate measure of exposure have shown no significant relationship between proximity to lines and risk of cancer. The exception is a childhood leukemia study [46] that showed a RR of 2.9 (1.0-7.3) for residence within 50 m of high-voltage transmission lines. This same study showed no elevation of child leukemia risks at 51-100 m, and no increase in childhood brain cancer, overall childhood cancer, or any types of adult cancer at any distance. Wire Codes: The original US powerline studies used a combination of the type of wiring (distribution vs transmission, number and thickness of wires) and the distance from the wiring to the residence as a surrogate measure of exposure [1-3]. This technique is known as ÒwirecodingÓ. Three studies using wirecodes [1-3] have shown a relationship between childhood cancer and Òhigh-current configurationÓ wirecodes. Two of these studies [2, 3] failed to show a significant relationship between exposure and cancer when actual measurements were made. Wirecodes correlate with measured fields, although the correlation is not very good [47]. The wirecode scheme was developed for the U.S., and does not appear to be readily applicable elsewhere. Calculated Historic Fields: The recent European studies have used utility records and maps to calculate what fields would have been produced by powerlines in the past [37, 38, 44, 46]. Typically, the calculated field at the time of diagnosis or the average field for a number of years prior to diagnosis are used as a measure of exposure (Q17). These calculated exposures explicitly exclude contributions from other sources such as distribution lines, household wiring, or appliances. When the field calculations are done for contemporary measured fields they correlate reasonably well [46]. Of course, there is no way to check the accuracy of the calculated historic fields. 14) What is known about the relationship between Òelectrical occupationsÓ and cancer rates? Several studies have shown that people who work in electrical occupations have higher than average leukemia, lymphoma, and brain cancer rates [8-10, 36]. 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 Òelectrical occupationsÓ are used, and very few studies actually measured exposure. The following RRs (Q12) for the occupational exposure studies are adapted from Hutchison [7] and Davis et al [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 (Q18). 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 [30] shows no significant elevation of leukemia, brain cancer or lymphoma risks. 15) 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 [11-15, 31]. 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 [17, 18]. Most promotion studies of power-frequency fields have been negative [14, 19-21]; but recently there was a positive report of promotion of breast cancer in rats [32]. 16) How do laboratory studies of the effects of power-frequency fields on cell growth, immune function, and melatonin relate to the question of cancer risk? 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 [11, 12, 15, 20-22, 33]. With one possible exception [33] there have been no reported effects on proliferation or progression for fields below 2000 mG (200 microT). Suppression of the immune system in animals and humans is associated with increased rates of certain types of cancer, particularly lymphomas [34, 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 Òcancer-preventiveÓ 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 Òcancer-preventiveÓ 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. 17) Do power-frequency fields show any effects at all 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 ÒbioeffectsÓ, particularly at high field strength [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 Q8) have shown reproducible effects, including effects on humans [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 ÒbioeffectsÓ with cancer causation [16, 18]. 18) What about the new ÒSwedishÓ study showing a link between power lines and cancer? There are new residential and occupational studies from Sweden [46], Denmark [36, 38], Finland [37] and the Netherlands [44]. Some of the Swedish studies are still 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. The authors of the Scandnavian childhood cancer studies [37, 38, 46] have produced a collaborative meta-analysis of their data [51]. The RRs from this meta-analysis are shown below in comparison to meta-analysis of the prior studies [7, 39]. Childhood leukemia, Scandanavian: 2.1 (1.1-4.1) Childhood leukemia, prior studies: 1.3 (0.8-2.1) Childhood lymphoma, Scandanavian: 1.0 (0.3-3.7) Childhood lymphoma, prior studies: none Childhood CNS cancer, Scandanavian: 1.5 (0.7-3.2) Childhood CNS cancer, prior studies: 2.4 (1.7-3.5) All childhood cancer, Scandanavian: 1.3 (0.9-2.1) All childhood cancer, prior studies: 1.6 (1.3-1.9) - 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 1960 and 1985. For children all types of tumors were analyzed, for adults only leukemia and brain tumors were studied. The data on childhood cancer has been published [46]. ÒExposureÓ 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 over 2 mG (0.2 microT) at the time of diagnosis, and for residence within 50 m of the powerline. The increased relative risk of leukemia is found only in one-family homes; there is no excess risk in apartments. The retrospective fields calculations do not take into account sources other the transmission lines. No significantly elevated cancer risks were found for measured fields. - Verkasalo et al [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 ÒotherÓ tumors in either sex. - Olsen and Nielson [38]. Case-control study based on all childhood leukemia, brain tumors and lymphomas diagnosed in Denmark between 1968 and 1986. ÒExposureÓ 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 (corrected for multiple comparisons) was 5.6 (1.2 - 30). 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 [36]. Case-control study based on all cancer in actively employed Danes between 1970 and 1987 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 ÒexposureÓ category; women in similar exposure categories showed no excess risk. For men in the highest ÒexposureÓ 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 1980. ÒExposureÓ 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. -Schreiber et al [44]. Retrospective cohort study of people in an urban area in the Netherlands. People were considered exposed in they lived within 100 m of transmission equipment (150 kV lines plus a substation). Fields in the ÒexposedÓ group were 1-11 mG (0.1-1.1 microT), fields in the ÒunexposedÓ group were 0.2-1.5 mG (0.02-0.15 microT). For all cancers the RR (ÒexposedÓ group vs the general Dutch population) was 0.85 (0.63-1.14). No cases of leukemia or brain cancer were seen in the ÒexposedÓ group. End: powerlines-cancer-FAQ/part1 ****************************************************************************** FAQs on Power-Frequency Fields and Cancer (part 2 of 2) 19) 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 such groups of epidemiological and laboratory studies. These are often called the ÒHill criteriaÓ [23]. Under the Hill criteria one examines the strength (Q19A) and consistency (Q19B) of the association between exposure and risk, the evidence for a dose-response relationship (Q19C), the laboratory evidence (Q19D), and the biological plausibility (Q19E). These criteria are viewed as a whole; no individual criterion is either necessary of sufficient for the conclusion that there is a causal relationship between an exposure and a disease. Overall, application of the Hill criteria shows that the current evidence for a connection between power frequency fields and cancer is quite weak, because of the weakness (Q19A) and inconsistencies (Q19B) in the epidemiological studies, combined with the lack of a dose-response relationship in the human studies (Q19C), and the negative laboratory studies (Q19D&E). 19A) 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 (Q12) 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. 19B) 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 new Swedish study [46] shows an increased risk for childhood leukemia for one measure of exposure, it contradicts prior studies that showed a risk for brain cancers [7, 39], and a parallel Danish study [36] shows a risk for childhood lymphomas, but not for leukemia. Many of the studies are internally inconsistent. For example, where the Swedish study [46] 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. 19C) 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 risk is a major reason why many 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, cancer induced in rodents by ionizing radiation shows curves in which the risk rises with dose, but only up to a certain point; beyond that point the risk plateaus or even drops. Without an understanding of the mechanisms connecting dose and risk it is impossible to predict the shape, let alone the magnitude of the dose-response relationship. 19D) 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 effects on cells, tissues or animals that point towards their being a cause of cancer, or to their contributing to cancer. 19E) 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. 20) If exposure to power-frequency magnetic fields does not explain the positive residential and occupations studies, what other factors could? There are basically four factors that can result in false associations in epidemiological studies: inadequate dose assessment (Q20A), confounders (Q20B), inappropriate controls (Q20C), and publication bias (Q20D). 20A) Could problems with dose assessment affect the validity of the epidemiological studies of power lines and cancer? 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 do not know who is really exposed, and who is not, we will usually (but now always) underestimate the true risk. 20B) Are there other cancer risk factors that could be causing a false association between exposure to power-frequency fields and cancer? Associations between things are not always evidence for causality. Power lines (or electrical occupations) might be associated with a cancer risk other than magnetic fields. If such an associated cancer risk were identified it would be called a ÒconfounderÓ of the epidemiological studies of power lines and cancer. An essential part of epidemiological studies is to identify and eliminate possible confounders. Many possible confounders of the powerline studies have been suggested, including PCBs, herbicides, traffic density, and socioeconomic class. - PCBs: Many transformers contain polychlorinated biphenyls (PCBs) and it has been suggested that PCB contamination of the power-line corridors might be the cause of the excess cancer. This is unlikely. First, PCB leakage is rare. Second, PCB exposure has been linked to lymphomas, not leukemia or brain cancer. - Herbicides: It has been suggested that herbicides sprayed on the powerline corridors might be a cause of cancer. This is an unlikely explanation, since herbicide spraying would not effect distribution systems in urban areas (where 3 of 5 positive childhood cancer studies have been done). - Traffic density: Transmission lines frequently run along major roads, and the Òhigh current configurationsÓ associated with excess childhood leukemia in the US studies [1-3, Q13] are associated with major roads. It has been suggested that power lines might be a surrogate for exposure to cancer-causing substances in traffic exhaust. This may be a real confounder, since traffic density has been shown to correlate with childhood leukemia risk [28]. Note that this would explain only the residential connection, not the occupational connection. - Socioeconomic class: Socioeconomic class may be an issue in both the residential and occupational studies, as socioeconomic class is clearly associated with cancer risk, and "exposed" and "unexposed" groups in many studies are of different socioeconomic classes [29]. This is of particular concern in the US residential exposure studies that are based on "wirecoding", since the type of wirecodes that are correlated with childhood cancer are found predominantly in older (poorer?) neighborhoods, and/or neighborhoods with a high proportion of rental housing. 20C) Could the epidemiological studies of power lines and cancer be biased by the methods used to select control groups? An inherent problem with many epidemiological studies is the difficulty of obtaining a ÒcontrolÓ group that is identical to the ÒexposedÓ group for all characteristics related to the disease except the exposure. This is very difficult to do for diseases such as leukemia and brain cancer where the risk factors are poorly known. An additional complication is that often people must consent to be included in the control arm of a study, and participation in studies is known to depend on factors (such socioeconomic class, race and occupation) that are linked to differences in cancer rates. See Jones et al [48] for an example of how selection bias could effect a powerline study. 20D) Could analysis of the epidemiological studies of power lines and cancer be skewed by publication bias? It is a known that positive studies in many fields are more likely to be published than negative studies (see Dickersin et al [49] for examples from cancer clinical trials). This can severely bias meta-analysis studies such as those discussed in Q12 and Q14. 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!). Several specific examples of publication bias are known in the studies of electrical occupations and cancer (see Doll et al [39], page 94). In their review Coleman and Beral [8] report the results of a Canadian study that found a RR of 2.4 for leukemia in electrical workers. The British NRPB review [39] found that further followup of the Canadian workers showed a deficiency of leukemia (a RR of 0.6), but that this followup study has never been published. This is an anecdotal report, but publication bias, by its very nature is usually anecdotal. 21) What is the strongest evidence for 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 [1-3, 45]. b) The epidemiological studies that show a significant correlation between work in electrical occupations and cancer, particularly leukemia and brain cancer [8-10, 36]. c) The lab studies that show 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 G (100 - 500 microT) [17, 18]. d) The one laboratory study that provides evidence that power-frequency magnetic fields can promote chemically-induced breast cancer [32]. 22) What is the strongest evidence against a connection between power-frequency fields and cancer? The best evidence that there is not a connection between cancer and power-frequency fields is probably: a) Application of the Hill criteria (Q19) to the entire body of epidemiological and laboratory studies [24, 27]. b) The fact that all studies of genotoxicity, and all but one study of promotion have been negative (Q15). c) Adair's [25] biophysical analysis that indicates that Òany biological effects of weak [less than 40 mG, 4 microT] ELF fields on the cellular level must be found outside of the scope of conventional physics" d) JacksonÕs [26] and OlsenÕs [38] epidemiological analysis that shows that childhood and adult leukemia rates have been stable over a period of time when per capita power consumption risen dramatically 23) 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 would not 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). 24) What are some good overview articles? A good review of the area was published by Oak Ridge Associated Universities [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 are in the U.K., the National Radiation Protection Board has a good review [39]. Two other good review are Theriault [24] and Bates [27]. 25) 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 (Q8). The most generally relevant are: - Board statement on restriction on human exposures to static and time varying EM fields and radiation, National Radiation Protection Board, Chilton, 1993. 50 Hz electrical field: 12 kV/m 60 Hz electrical field: 10 kV/m 50 Hz magnetic field: 1.6 mT (16 G) 60 Hz magnetic field: 1.33 mT (13.3 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 26) What effect do powerlines have on property values? There is very little hard data on this issue. There is anecdotal evidence and on-going litigation (Wall Street Journal, Dec 9, 1993). There have been Òcomparable propertyÓ studies, but I would argue that any studies done prior to about 1991 (when London et al [3] was published) would be irrelevant. So far I have found one recent ÒstudyÓ [50]. The first part of the study was a survey of homeowners in Tennessee who lived adjacent to high voltage transmission lines. Of these owners, 53% considered the lines Òan eyesoreÓ, but none considered the lines a health hazard. Of owners who thought the towers were eyesores, 28% said that the presence of the lines adversely affected then price they were willing to pay. None of the owners Òhad any knowledge of possible evidence connecting power transmission lines to certain health risks such as cancerÓ; but 87% said that if they had known of potential health risks, it would have adversely affected then price they were willing to pay. In the second part of the studies, the values of comparable houses adjacent to, and not adjacent to, the powerlines were found to have sold for the same price. It appears possible that the presence of obvious transmission lines or substations will adversely affect property values if there has been recent local publicity. It would appear less unlikely that the presence of Òhigh current configurationÓ distribution lines of the type correlated with childhood cancer in the US studies [1-3] would affect property values, since few people would recognized their existence. ----------------------- 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. Br J Cancer 60:793-798, 1989. 5) ME McDowall: Mortality of persons resident in the vicinity of electrical transmission facilities. Br J Cancer 53:271-279, 1986. 6) A Myers et al: Childhood cancer and overhead powerlines: a case-control study. Br J Cancer 62:1008-1014, 1990. 7) G.B. Hutchison: Cancer and exposure to electric power. Health Environ Digest 6:1-4, 1992. 8) M Coleman & V Beral: A review of epidemiological studies of the health effects of living near or working with electrical generation and transmission equipment. Int J Epidemiol 17:1-13, 1988. 9) JR Jauchem & JH Merritt: The epidemiology of exposure to EM fields: an overview of the recent literature. J Clin Epidemiol 44:895-906, 1991. 10) DA Savitz & EE Calle: Leukemia and occupational exposure to EM fields: Review of epidemiological studies. J Occup Med 29:47-51, 1987. 11) GK Livingston et al: Reproductive integrity of mammalian cells exposed to power frequency EM fields. Environ Molec Mutat 17:49-58, 1991. 12) M Rosenthal & G Obe: Effects of 50-Hertz EM fields on proliferation and on chromosomal aberrations in human peripheral lymphocytes untreated and pretreated with chemical mutagens. Mutat Res 210:329-335, 1989. 13) J. Nafziger et al: DNA mutations and 50 Hz EM fields. Bioelec Bioenerg 30:133-141, 1993. 14) A. Rannug et al: A study on skin tumor formation in mice with 50 Hz magnetic field exposure. Carcinogenesis 14:573-578, 1993. 15) R. Zwingelberg et al: Exposure of rats of a 50-Hz, 30-mT magnetic field influences neither the frequencies of sister-chromatid exchanges nor proliferation characteristics of cultured peripheral lymphocytes. Mutat Res 302:39-44, 1993. 16) TS Tenforde: Biological interactions and potential health effects of extremely-low-frequency magnetic fields from power lines and other common sources. Ann Rev Publ Health 13:173-196, 1992. 17) R Goodman & A Shirley-Henderson: Transcription and translation in cells exposed to extremely low frequency EM fields. Bioelec Bioenerg 25:335-355, 1991. 18) RB Goldberg & WA Creasey: A review of cancer induction by extremely low frequency EM fields. Is there a plausible mechanism? Medical Hypoth 35:265-274, 1991. 19) A Rannug et al: Rat liver foci study on coexposure with 50 Hz magnetic fields and known carcinogens. Bioelectromag 14:17-27, 1993. 20) MA Stuchly et al: Modification of tumor promotion in the mouse skin by exposure to an alternating magnetic field. Cancer Letters 65:1-7, 1992. 21) JRN McLean et al: Cancer promotion in a mouse-skin model by a 60-Hz magnetic field: II. Tumor development and immune response. Bioelectromag 12:273-287, 1991. 22) S Baumann et al: Lack of effects from 2000-Hz magnetic fields on mammary adenocarcinoma and reproductive hormones in rats. Bioelectromag 10:329-333, 1989. 23) AB Hill: The environment and disease: Association or causation? Proc Royal Soc Med 58:295-300, 1965. 24) G Theriault: Cancer risks due to exposure to electromagnetic fields. Rec. Results Cancer Res. 120:166-180; 1990. 25) RK Adair: Constraints on biological effects of weak extremely-low-frequency electromagnetic fields, Phys Rev A 43:1039-1048, 1991. 26) J.D. Jackson: Are the stray 60-Hz electromagnetic fields associated with the distribution and use of electric power a significant cause of cancer? Proc Nat Acad Sci USA 89:3508-3510, 1992. 27) MN Bates: Extremely low frequency electromagnetic fields and cancer: the epidemiologic evidence, Environ Health Perspec 95:147-156, 1991. 28) DA Savitz & L Feingold: Association of childhood leukemia with residential traffic density. Scan J Work Environ Health 15:360-363, 1989. 29) JM Peters et al: Exposure to residential electric and magnetic fields and risk of childhood leukemia. Rad Res 133:131-132, 1993. 30) JD Sahl et al: Cohort and nested case-control studies of hematopoietic cancers and brain cancer among electric utility workers. Epidemiology 4:104-114, 1993. 31) J McCann et al: A critical review of the genotoxic potential of electric and magnetic fields. Mut Res 297:61-95, 1993. 32) W Loscher et al: Tumor promotion in a breast cancer model by exposure to a weak alternating magnetic field. Cancer Letters 71:75-81, 1993. 33) AR Liboff et al: Time-varying magnetic fields: Effects on DNA synthesis. Science 223:818-820, 1984. 34) I Penn: Why do immunosuppressed patients develop cancer? Crit Rev Oncogen 1:27-52, 1989. 35) GR Krueger: Abnormal variation of the immune system as related to cancer. Cancer Growth Prog 4:139-161, 1989. 36) P Guenel et al: Incidence of cancer in persons with occupational exposure to electromagnetic fields in Denmark. Br J Indust Med 50:758-764, 1993. 37) PJ Verkasalo et al: Risk of cancer in Finnish children living close to power lines. BMJ 307:895-899, 1993. 38) JH Olsen et al: Residence near high voltage facilities and risk of cancer in children. BMJ 307:891-895, 1993. 39) R Doll et al, Electromagnetic Fields and the Risk of Cancer, NRPB, Chilton, 1992. 40) JG Davis et al: Health Effects of Low-Frequency Electric and Magnetic Fields. Oak Ridge Associated Universities, 1992. 41) J Walleczek: Electromagnetic field effects on cells of the immune system: the role of calcium signaling. FASEB J 6:3177-3185, 1992. 42) RG Stevens et al: Electric power, pineal function, and the risk of breast cancer. FASEB J 6:853-860, 1992. 43) RJ Reiter & BA Richardson: Magnetic field effects on pineal indoleamine metabolism and possible biological consequences. FASEB J 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. 45) RC Petersen: Radiofrequency/microwave protection guides. Health Phys 61:59-67, 1991. 46) M Feychting & A Ahlbom: Magnetic fields and cancer in children residing near Swedish high-voltage Power Lines. Amer J Epidem 7:467-481, 1993. 47) WT Kaune et al: Residential magnetic and electric fields. Bioelectromag 8:315-335, 1987. 48) TL Jones et al: Selection bias from differential residential mobility as an explanation for associations of wire codes with childhood cancer. J Clin Epidemiol 46:545-548; 1993. 49) K Dickersin et al: Publication bias and randomized controlled trials. Cont Clin Trials 8:343-353; 1987. 50) H Kung & CF Seagle: Impact of power transmission lines on property values: A case study. Appraisal J 60:413-418, 1992. 51) A Ahlbom et al: Electromagnetic fields and childhood cancer. Lancet 343:1295-1296, 1993. -------------- Acknowledgments: This FAQ sheet owes much to the many readers of sci.med.physics show have sent me comments and suggestions, including: kfoster@eniac.seas.upenn.edu (from whom I stole most of Q5) gary%ke4zv.uucp@mathcs.emory.edu (who suggested adding a quantum approach) aa2h@virginia.edu (suggestions on thermal effects and confounders) p.farrell@trl.oz.au (SI units, suggesting the pro/con arguments section) drchambe@tekig5.pen.tek.com (a start on the property value question) John Moulder (jmoulder@its.mcw.edu) Voice: 414-266-4670 Radiation Biology Group FAX: 414-257-2466 Medical College of Wisconsin, Milwaukee ******************************************************************************