How
do laboratory studies of the effects of power-frequency fields on the pineal
gland and melatonin relate to the question of cancer risk? Is there
any evidence that power-frequency fields cause any effects on human health,
such as miscarriages, birth defects, Alzheimer's disease, multiple sclerosis,
suicide or sleep disorders?
What are
some good overview articles?
What
do the 1999 and 2002 reports from the U.S. National Institute of
Environmental Health Sciences (NIEHS) say about power-frequency fields and
cancer?
What
does the 2002 report from the State of California say about possible human
health hazards from exposure to power-frequency fields? What
about the claim that exposure to radon and other chemical carcinogens is
increased by the presence of high-strength electric fields.
Are some
people sensitive to (allergic to) the presence of power-frequency fields?
Revision notesThis FAQ sheet owes much to the many readers of USENET who have sent me comments and suggestions over the years.
Initial conversion of the FAQ into html was done by Bob Mueller and Dennis Taylor of theGeneral Clinical Research Center at the Medical College of Wisconsin, and server space for these documents is provided by the General Clinical Research Center of the Medical College of Wisconsin.
Most of the concern about power lines and cancer stems from studies of people living near power lines (Q12) and people working in "electrical" occupations (Q15). Some of these studies appear to show a weak association between exposure to power-frequency magnetic fields and the incidence of cancer.
However, epidemiological studies done in recent years show little evidence that power lines are associated with an increase in cancer (Q19A and Q19B,Q19H thru Q19K), laboratory studies have shown little evidence of a link between power-frequency fields and cancer (Q16), and a connection between power line fields and cancer remains biophysically implausible (Q18).
A 1996 review by a prominent group of scientists at the U.S. National Academy of Science concluded that:
"No conclusive and consistent evidence shows that exposures to residential electric and magnetic fields produce cancer, adverse neurobehavioral effects, or reproductive and developmental effects."(Q27E).
A 1999 review by the U.S. National Institutes of Health concluded that:
"The scientific evidence suggesting that [power-frequency electromagnetic field] exposures pose any health risk is weak."(Q27G).
A 2001 review by the U.K. National Radiation Protection Board (NRPB) concluded that:
"Laboratory experiments have provided no good evidence that extremely low frequency electromagnetic fields are capable of producing cancer, nor do human epidemiological studies suggest that they cause cancer in general." (Q27H)
A review of the epidemiological literature by the International Commission on Non-Ionizing Radiation Protection [B22] concludes that:
"In the absence of evidence from cellular or animal studies, and given the methodological uncertainties and in may cases inconsistencies of the existing epidemiologic literature, there is no chronic disease for which an etiological [causal] relation to [power-frequency fields] can be regarded as established".
The largest studies of childhood leukemia and power lines ever done reported in 1997-2000 that they could find no significant evidence for an association of power lines with childhood leukemia (Q19H through 19K). In contrast, a pair of studies published in 2000 [C54,C57] reported that if all the studies in which magnetic fields could be measured or estimated were pooled, a statistically significant association could be found for childhood leukemia in the children with the highest average fields.
On the other hand, a series of studies have shown what life-time exposure of animals to power-frequency magnetic fields does not cause cancer (Q16B).
Overall, most scientists consider the evidence that power line fields cause or contribute to cancer to be weak.
X-rays, ultraviolet (UV) light, visible light, infrared light (IR), microwaves (MW), radio-frequency radiation (RF), and magnetic fields from electric power systems are all parts of the electromagnetic (EM) spectrum. The parts of the electromagnetic 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 electromagnetic field goes through one complete oscillation (cycle) and is usually given in Hertz (Hz), where one Hz is one cycle per second.
The Electromagnetic Spectrum |
|
Power-frequency fields in the US vary 60 times per second (60 Hz), and have a wavelength of 5,000 km. Power in most of the rest of the world is at 50 Hz. Broadcast AM radio has a frequency of around 10^6 (1,000,000) Hz and a wavelength of around 300 m. Microwave ovens have a frequency of 2.54 x 10^9 Hz, and a wavelength of about 12 cm. X-rays have frequencies above 10^15 Hz, and wavelengths of less than 100 nm.
This FAQ sheet will use the term "power frequency" to refer to both the 50- and 60-Hz alternating current (AC) frequencies used in electric power systems, and the term "power frequency field" to refer to the sinusoidal electric and magnetic fields produced by 50- and 60-Hz lines and devices. The phrase "EMF" will be avoided since it is an imprecise term that could apply to many very different types of fields, and because the term has a long-standing usage in physics to refer to an entirely different quantity, electromotive force. The terms "electromagnetic radiation" and "nonionizing radiation" will be avoided since power-frequency sources produce no appreciable radiation (seeQ5).
Power-frequency fields are also properly referred to as extremely low frequency (or ELF) fields. In strict electrical engineering terms, ELF refers to frequencies between 30 and 300 Hz, but the term is often used in the biological and occupational health literature to cover the range from above 0 Hz to 3000 Hz (everything above static fields and below radio-frequency).
The interaction of biological material with an electromagnetic source depends on the frequency of the source. We usually talk about the electromagnetic spectrum as though it produced waves of energy. However, sometimes electromagnetic energy acts like particles rather than waves, particularly at high frequencies. The particle nature of electromagnetic energy is important because it is the energy per particle (or photons, as these particles are called) that determines what biological effects electromagnetic energy will have [A12].
At the very high frequencies characteristic of "vacuum" UV and X-rays (less than 100 nanometers), electromagnetic particles (photons) have sufficient energy to break chemical bonds. This breaking of bonds is termed ionization, and this part of the electromagnetic spectrum is termed ionizing. The well-known biological effects of X-rays are associated with the ionization of molecules. At lower frequencies, such as those characteristic of visible light, radio-frequency radiation, and microwaves, the energy of a photon is very much below those needed to disrupt chemical bonds. This part of the electromagnetic spectrum is termed non-ionizing. Because non-ionizing electromagnetic energy cannot break chemical bonds there is no analogy between the biological effects of ionizing and nonionizing electromagnetic energy [A12].
Non-ionizing electromagnetic sources can produce biological effects. Many of the biological effects of ultraviolet (UV), visible, and infrared (IR) frequencies depend on the photon energy, but they involve electronic excitation rather than ionization, and do not occur at frequencies below that of infrared (IR) light (below 3 x 10^11 Hz). Radio-frequency and microwaves sources can cause effects by inducing electric currents in tissues, which cause heating. The efficiency with which a nonionizing electromagnetic source can induce electric currents, and thus produce heating, depends on the frequency of the source, and the size and orientation of the object being heated. At frequencies below that used for broadcast AM radio (about 10^6 Hz), electromagnetic sources couple poorly with the bodies of humans and animals, and thus are very inefficient at inducing electric currents and causing heating [A12].
Thus in terms of potential biological effects the electromagnetic spectrum can be divided into four portions (see diagram ofelectromagnetic spectrum):
In general, electromagnetic sources produce both radiant energy (radiation) and non-radiant fields. Radiation travels away from its source, and continues to exist even if the source is turned off. In contrast, some electric and magnetic fields exist near an electromagnetic source that are not projected into space, and that cease to exist when the energy source is turned off.
The fact that exposure to power-frequency fields occurs at distances that are much shorter than the wavelength of 50/60-Hz radiation has important implications, because under such conditions (called "near-field"), the electric and magnetic fields can be treated as independent entities. This is in contrast to electromagnetic radiation, in which the electric and magnetic fields are inextricably linked.
To be an effective radiation source an antenna must have a length comparable to its wavelength. Power-frequency sources are clearly too short compared to their wavelength (5,000 km) to be effective radiation sources. Calculations show that the typical maximum power radiated by a power line would be less than 0.0001 microwatts/cm^2, compared to the 0.2 microwatts/cm^2 that a full moon delivers to the Earth's surface on a clear night. The issue of whether power lines could produced ionizing radiation is covered inQ21B.
This is not to say that there is no loss of power during transmission. There are sources of loss in transmission lines that have nothing to do with "radiation" (in the sense as it is used in electromagnetic theory). Much of the loss of energy is a result of resistive heating; this is in sharp contrast to radiofrequency and microwave antennas, which "lose" energy to space by radiation. Likewise, there are many ways of transmitting energy that do not involve radiation; electric circuits do it all the time.
Ionizing electromagnetic radiation carries enough energy per photon to break 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 which may lead to cancer. If these changes occur in reproductive cells, they can also lead to inherited changes (mutation). All of the known human health hazards from exposure to the ionizing portion of the electromagnetic spectrum are the result of the breaking of chemical bonds in DNA. For frequencies below that of hard UV, 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 electromagnetic radiation [M2].
A principal mechanism by which radiofrequency radiation and microwaves cause biological effects is by heating (thermal effects). This 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. At the whole-animal level, tissue injury and other thermally-induced effects can be expected when the amount of power absorbed by the animal is similar to or exceeds the amount of heat generated by normal body processes. Some of these thermal effects (also seeQ9) are very subtle, and do not represent biological hazards [A12].
It is possible to produce thermal effects even with very low levels of absorbed power. One example is the "microwave hearing" phenomenon; these are auditory sensations that a person experiences when his head is exposed to pulsed microwaves such as those produced by radar. The "microwave hearing" effects is a thermal effect, but it can be observed at very low average power levels.
Since thermal effects are produced by induced currents, not by the electric or magnetic fields directly, they can be produced by fields at many different frequencies. Well-accepted safety standards exist to prevent significant thermal damage to persons exposed to radiofrequency radiation and microwaves (seeQ31C), and also for persons exposed to lasers, infrared (IR) and ultraviolet (UV) light [M3].
The electric fields associated with the power-frequency sources exist whenever voltage is present, and regardless of whether current is flowing. These electric fields have very little ability to penetrate buildings or even skin. The magnetic fields associated with power-frequency sources exist only when current is flowing. These magnetic fields are difficult to shield, and easily penetrate buildings and people. Because power-frequency electric fields do not penetrate the body, it is generally assumed that any biologic effect from residential exposure to power-frequency fields must be due to the magnetic component of the field, or to the electric fields and currents that these magnetic fields induce in the body [A12].
The argument that biological effects of power-frequency fields must be due to the magnetic component of the field has been the subject of recent debate [A14]. In particular, King [F27] has argued that the electrical fields from power lines do penetrate most buildings, and that the electrical currents induced in the body by power line electrical fields may be greater than those induced by power line magnetic fields. This issue is discussed further inQ16G andQ19L.
At power frequencies, the photon energy is a factor of 10^10 smaller than that needed to break even the weakest chemical bond. There are, however, well-established mechanisms by which power-frequency electric and magnetic fields could produce biological effects without breaking chemical bonds [A12,F3,F23,M6,M9,M10]. Power-frequency electric fields can exert forces on charged and uncharged molecules or cellular structures within a tissue. These forces can cause movement of charged particles, orient or deform cellular structures, orient dipolar molecules, or induce voltages across cell membranes. Power-frequency magnetic fields can exert forces on cellular structures, but since biological materials are largely nonmagnetic these forces are usually very weak.
Power-frequency magnetic fields can also cause biological effects via the electric fields that they induce in the body. These electric and magnetic forces occur in the presence of random thermal agitation (thermal noise) and electric noise from many sources; and to cause significant changes in a biological system applied fields must generally far exceed those that exist in typical environmental exposure conditions [A12,F3,F17,F23,F34,M6].
In general, the fields or currents that are induced in the body by power-frequency electric or magnetic fields are too low to be hazardous; and well-accepted safety standards exist to protect persons from exposure to power-frequency fields that would induce hazardous currents [M4,M5,M6,M8,M9,M10]. These safety standards for fields (as opposed to those that protect against shock from contact with conductors) are set to limit induced currents in the body to levels below those that occur naturally in the body. The well-known hazards of electric power, shock and burns, generally require that the subject directly contact a charged surface (e.g., a "hot" conductor and ground) allowing current to pass directly into the body.
One distinction that is often made in discussions of the biological effects of non-ionizing electromagnetic sources is between "nonthermal" and "thermal" effects. This refers to the mechanism for the effect: non-thermal effects are a result of a direct interaction between the field and the organism (for example, photochemical events like vision and photosynthesis); and thermal effects are a result of heating (for example, heating with microwave ovens or IR light). There are many reported biological effects of non-ionizing electromagnetic sources whose mechanisms are totally unknown, and it is difficult (and not very useful) to try to draw a distinction between "thermal" and "nonthermal" mechanisms for such effects [A12].
In the US magnetic fields are often still measured in Gauss (G) or milligauss (mG), where:
1,000 mG = 1 G.
In the rest of the world and in the scientific community, magnetic fields are measured in tesla (T), were:
10,000 G = 1 T
1 G = 100 microT (µT)
1 microT = 10 mG
In the FAQ magnetic fields will generally be specified in microT.
Electric fields are measured in volts/meter (V/m).
Measurement techniques are discussed inQ29 andQ30.
Within the path of a power line (known in the U.S. as a right-of-way or ROW) of a high-voltage (115-765 kV, 115,000-765,000 volt) transmission line, fields can approach 10 microT and 10,000 V/m. At the edge of a high-voltage transmission ROW, the fields will be 0.1-1.0 microT and 100-1,000 V/m. Ten meters from a 12 kV (12,000 volt) distribution line fields will be 0.2-1.0 microT and 2-20 V/m. Actual magnetic fields depend on distance, voltage, design and current; actual electric fields are affected only by distance, voltage and design (not by current flow) [F7].
Fields within residences vary from over 150 microT and 200 V/m a few cm from certain appliances to less than 0.02 microT and 2 V/m in the center of many rooms. Appliances that have the highest magnetic fields are those with high currents or high-speed electric motors (e.g., vacuum cleaners, microwave ovens, electric washing machines, dishwashers, blenders, can openers, electric shavers) [F22]. Electric clocks, and clock radios, which have been mentioned as major sources of night-time exposure of children, do not have particularly high magnetic fields (0.04-0.06 microT at 50 cm [F22]). Appliance fields decrease rapidly with distance [F7,F22]. Of the appliances assessed in British homes, only microwave ovens, electric washing machines, dishwashers and can openers produced fields greater than 0.20 microT at 1 meter [F22].
A 2002 analysis of power-frequency field levels in Spanish primary schools found a median level in classrooms of 0.012 microT (0.12 mG) with a maximum of 0.88 microT (8.8 mG). In playgrounds, the median level was 0.0095 microT (0.095 mG) and the maximum was 0.46 microT (4.6 mG).
Because electric fields from powerlines have little ability to penetrate buildings, there is little correlation between electric and magnetic fields within homes [C11,C12]. In particular, while magnetic fields are elevated inside buildings near powerlines, electric fields do not appear to be similarly elevated [C11,C12].
Occupational exposures in excess of 100 microT and 5,000 V/m have been reported (e.g., in arc welders and electrical cable splicers). In "electrical" occupations typical mean exposures range from 0.5 to 4 microT and 100-2,000 V/m [F7,F11,F16,D19]. Exposure to power-frequency electric and magnetic fields are poorly correlated in occupational settings [F16].
Electric trains can also be a major source of exposure, as power-frequency fields at seat height in passenger cars can be as high as 60 microT [F28]
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 [F2,F29]. Once the fields are produced, however, shielding is very difficult. Small areas can be shielded by the use of Mu metal (a nickel-iron-copper alloy) but Mu metal shields are very expensive. Larger area can be shielded with less expensive metals; but such shielding is still expensive, and generally successful use requires considerable technical knowledge.
Increasing the height of towers, and thus the height of the conductors above the ground, will reduce the field intensity at the edge of a power line corridor [F2F29]. The size, spacing and configuration of conductors can be modified to reduce magnetic fields, but this approach is limited by electrical safety considerations. Placing multiple circuits on the same set of towers can also lower the field intensity at the edge of the ROW, although it generally requires higher towers. Replacing lower voltage lines with higher voltage ones can also lower the magnetic fields.
Burying transmission lines can substantially reduce their magnetic fields. The reduction in the magnetic field occurs because the underground lines use rubber, plastic or oil for insulation rather than air; this allows the conductors to be placed much closer together and allows greater phase cancellation. The reduction in magnetic fields for underground lines is not due to shielding. Placing high voltage lines underground is very expensive, adding costs that may exceed one million US dollars per mile.
The reduction in magnetic fields from burying a line is greatest at a distance from the line. At the center of a transmission line corridor, fields from a buried line can actually be higher than those from an overhead line [F29]. For example, in a comparison of overhead and underground 400 kV lines [F29], the fields at the center of the corridor were 25 microT for the overhead line and 100 microT for the buried line; but at 20 m, the fields were 10 microT for the overhead line and 1-2 microT for the buried line.
Different methods of household wiring can greatly affect magnetic fields inside houses. For example, the tube-and-knob method of wiring older houses produces higher fields than modern methods that use conduit or other methods that put the wires very close together; the fields are lower because the conductors are closer together and there is greater phase cancellation. Other strategies for reducing fields from household wiring include avoidance of ground loops, and care in how circuits with multiple switches are wired. In general conformance with modern electrical wiring codes will result in decreased magnetic fields.
Some studies have reported that children living near certain types of power lines (high-current distribution lines and high-voltage transmission lines) have higher than average rates of leukemia [C1,C6,C12,C19,C46], brain cancers [C1,C6] and/or overall cancer [C5,C17]. The correlations are not strong, and the studies have generally not shown dose-response relationships. When power-frequency fields are actually measured, the association generally vanishes [C6,C12,C19,C35,C44]. Many other studies have shown no correlations between residence near power lines and risks of childhood leukemia [C3,C5,C9,C10,C16,C17,C33,C35,C44,C45,C48,C51,C53], childhood brain cancer [C5,C9,C16,C17,C19,C28,C29,C33], or overall childhood cancer [C16,C19,C33].
All but one of the recent studies of powerlines and either childhood leukemia or brain cancer [C28,C29,C33,C35,C43,C44] have failed to show significant associations. The exception is a Canadian study [C45, C46] which showed an association between the incidence of childhood leukemia and some measures of exposure (see full discussion inQ19J).
With two exceptions [C2,C32] all studies of correlations between adult cancer and residence near power lines have been negative [C4,C7,C9,C13,C18,C21,C31,C32,C38,C40,C47,C61]. The exception are Wertheimer et al [C41] who reported an excess of total cancer and brain cancer, but no excess of leukemia; and Li et al [C33] who reported excess leukemia, but no excess breast cancer or brain cancer.
The excess cancer found in epidemiologic 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 (seeQ21E) even when multiple types of cancer and multiple indices of exposure are studied (see Olsen et al, [C17], Fig. 2 for an example of a multiple-comparison adjustment).
No simple overview of the epidemiology is possible because the epidemiologic techniques and the exposure assessment in the various studies are so different. Meta-analysis, a method for combining studies [L15], has been attempted [A7,B3,B5,B9,B12,B18,C54,C57], but the results are problematical because of a lack of consensus as to the correct way to measure exposure. Meta-analyses also tend to get out-dated rather quickly. A 1999 meta-analysis of childhood cancer [B18], for example, was already missing the 4 big 1999 studies at the time it was published.
The following table summarizes the relative risks (RR) for the studies of residential exposure.
| Type of Cancer | Number of Studies |
Median RR |
Range of RR's |
| childhood leukemia | 20 | 1.25 | 0.8-2.0 |
| childhood brain cancer | 9 | 1.2 | 0.8-1.7 |
| childhood lymphoma | 8 | 1.8 | 0.8-4.0 |
| all childhood cancer | 7 | 1.3 | 0.9-1.6 |
| adult leukemia | 6 | 1.15 | 0.85-1.65 |
| adult brain cancer | 5 | 0.95 | 0.70-1.30 |
| all adult cancer | 8 | 1.10 | 0.80-1.35 |
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.
Most public and scientific attention has focused on childhood leukemia, with lesser attention given to adult leukemia, childhood and adult brain cancer, lymphoma and overall childhood cancer (see table inQ13A). The original studies which suggested an association between power lines and childhood cancer used a combination of the type of wiring and the distance to the residence as a surrogate measure of exposure, a system called "wire codes" [C1,C3,C6]. Other studies have used distance from transmission lines or substations as measures of exposure, and some studies have used contemporary measured fields or calculated historic fields. In general, the different methods of exposure assessment do not correlate well with each other, or with contemporary measured fields; none of these measures of exposure is obviously superior, and none is common to all the major studies (see figure below).
Historically, one of the more puzzling features of the childhood leukemia studies was that the correlation of "exposure" with cancer incidence appeared to be higher when wire codes or proximity to power lines were used as an exposure metric, rather than when fields were directly measured in the homes (see figure below). This has led to the suggestion that the association of childhood cancer with residence near power lines might be due to a factor other than the power-frequency field. For example, it has been suggested that socioeconomic class might be a confounder, since socioeconomic class is associated with cancer risk, and "exposed" and "unexposed" groups in some studies may be from different socioeconomic classes. This is of particular concern in the U.S. residential exposure studies that are based on wire codes, since the types of wire codes that are correlated with childhood cancer are found predominantly in older, poorer neighborhoods, and/or in neighborhoods with a high proportion of rental housing [A7,C20,C25]. However, in 1997 and 1999 the largest studies to date of power lines and childhood leukemia [C35,C44] found no association of leukemia with either wire codes or measured fields, and the most recent studies of brain cancer [C28,C29] have found no correlation with wire codes. These latest studies indicate that the "wire code paradox" does not actually exist.
The figure below shows the variety of endpoints that have been used in the childhood leukemia studies. Because of the lack of consensus as to the correct exposure metric, and the lack of an exposure metric that is common to most of the studies, no simple overview of the epidemiology can be provided. Attempts to provide an overview of these diverse data have been frustrated by the fact that no "unique" analysis can be produced. Rather one gets a family of analyses based on different definitions of exposure, most of which exclude some of the studies, and no one of which can be assumed to be the best. For example, in 1997 the U. S. National Research Council [A7] conducted a complex meta-analysis and concluded that: "wire codes are associated with an approximately 1.5-fold excess of childhood leukemia, which is statistically significant". This conclusion is based on just one of the eight separate meta-analyses of the childhood leukemia data performed by the NRC Committee, an analysis that excludes seven of the 11 studies and uses an arbitrary cut-point for defining who was exposed. A second analysis of the same four studies used a higher cut-point, and found a smaller excess that was "non-significant". The other six analyses done by the NRC committee yielded summary RRs that ranged from 0.8 to 1.7.
The childhood leukemia studies as a whole show no consistent association between residence near power lines and the incidence of leukemia.
However, a pair of studies published in 2000 [C54,C57] found that if certain reports were pooled and certain exposure metrics were chosen, there appeared to be an increased risk of leukemia in the highest exposure group.
Relative Risk of Childhood Leukemia |
 |
| Relative risk (RR) of childhood leukemia and exposure to power-line fields. RRs are shown with 95% confidence intervals and the expected number of exposed cases (a measure of the statistical power of the study) is shown in parentheses. Where more than one exposure cut-point was used by the authors, the highest cut-point with more than 5 expected exposedcases is shown. The summary weights each study on the basis of the numbers of expected exposed cases, and treats all exposure measures equally. Pooled 1980-1994 data is from Moulder [A12]. |
The studies that show a relationship between cancer and power lines do not provide any consistent guidance as to what distance or exposure level might be associated with increased cancer incidence. 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, whereas the European studies have been based strictly on high-voltage transmission lines and/or transformers.
Since no human health hazards from residential exposure to power-frequency fields have been proven to exist, it is impossible to rationally define a safe distance or safe exposure level. To develop a rational (science-based) human safety standard, it is necessary to have a specific confirmed or strongly suspected hazard to protect people from. It is also necessary to have some concept of the mechanistic basis for the hazard, so that there is a rational basis for deciding what to measure.
Field measurements: A number of studies have measured power-frequency fields in residences [C6,C7,C12,C19,C21,C29,C34,C35,C44,C45, C46,C59]. Both one-time (spot), peak, 24-hour and 48-hour average measurements have been made. Two of the studies [C46,C59] using measured fields have shown a statistically-significant relationship between exposure and childhood leukemia. No other types of cancer in either adults of children have been show to be associated with measured fields.
A report published in 2000 [C54] calculated that if all the studies that included long-term measurements of magnetic fields were pooled, a statistically significant association could be found for children with 24-48 hr average exposures of 0.4 microT or greater. A second study published in 2000 [C57] reported that if all the studies for that included estimated or measured magnetic fields were pooled, a statistically significant association could be found for children with exposures of 0.3 microT or greater. For children with lower average exposures, no significant elevation of childhood leukemia was found in either analysis of the pooled studies.
Proximity to lines: Many studies have used the distance from the power line corridor to the residence as a measure of power-frequency fields [C4,C5,C9,C10,C13,C19,C20a,C21,C32,C33,C53,C58]. 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. Three [C5,C19,C32] of the 12 studies that have used distance from power lines as a surrogate measure of exposure have shown a relationship between proximity and cancer. The most notable are a childhood study [C19] that showed an increase in leukemia incidence for residence within 50 m of high-voltage transmission lines, and an adult study [C32] that showed an increase in leukemia incidence for residence within 100 m of high-voltage transmission lines. The largest study of proximity to power lines and childhood cancer found no association with any kind of cancer in children living within 50 m of power lines or substations [C58].
If there is a human health hazard from residential exposure to power-frequency fields it is highly unlikely to depend on anything as simple as the distance of the residence from the nearest powerline.
Depending of the type of line and its current, magnetic fields from power lines become less than those produced by the typical residence at a distance of 20-70 meters.
Wire codes: The original US power line 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 [C1,C2,C3,C6,C7,C12,C28,C29,C35,C44,C45, C46]. This technique is known as "wire coding" [F21]. Three studies using wire codes [C1,C6,C12] have reported a relationship between childhood cancer and "high-current configuration" wire codes. Two of these studies [C6,C12] failed to show a relationship between exposure and cancer when actual measurements were made, the third study [C1] made no actual measurements. The most recent studies of wire codes and childhood cancer [C28,C29,C35,C44,C45, C46] have found no significant associations.
Wire codes are stable over time [F6], but correlate poorly with measured fields [A7,F6,F7,F10,F21]. The wire code scheme was developed for urban areas in the U.S., and is not readily applicable elsewhere. It has been suggested that wire codes might be a better measure of long-term magnetic fields than actual magnetic field measurements, but analyses have shown that this is unlikely [A7,F21]. A more serious problem with using wiring codes to estimate magnetic field exposure is that wire codes correlate strongly with things that have nothing to do with magnetic fields (such as age of houses, traffic density and socioeconomic status) [C40].
Calculated Historic Fields: Many recent studies (Q19) have used utility records and maps to calculate what fields would have been produced by high voltage power lines in the past [C16,C17,C19,C21,C26a,C31,C32,C32,C33,C44]. 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. These calculated exposures explicitly exclude contributions from other sources such as distribution lines, household wiring, or appliances. There is no way to check the accuracy of these calculated historic fields. See Jaffa et al [F36] for a discussion of some of the reasons to question the accuracy of these calculations.
| Type of Cancer | Number of Studies |
Median RR |
Range of RR's |
| leukemia: | about 45 | 1.20 | 0.80-2.10 |
| brain: | about 35 | 1.15 | 0.90-1.90 |
| lymphoma: | about 12 | 1.20 | 0.90-1.80 |
| lung: | about 15 | 1.05 | 0.65-1.45 |
| female breast: | about 10 | 1.10 | 0.85-1.50 |
| male breast: | about 10 | 1.25 | 0.65-2.80 |
| all cancer: | about 15 | 1.05 | 0.85-1.15 |
SeeQ19 for a more detailed discussion of the recent studies [alsoB11,B12,B13,B17,B20,B20].
While the causes of specific cancers in individuals are still poorly understood, the mechanisms of carcinogenesis are sufficiently well understood that cellular and animal studies can provide information relevant to determining whether an agent causes or contributes to cancer [A8,A9,A12,A13,K5,L26,L28]. Current research indicates that carcinogenesis is a multi-step process driven by a series of injuries to the genetic material of cells. Not surprisingly, this model of carcinogenesis is referred to as the multi-step carcinogenesis model.
The Multi-Step Carcinogenesis Model |
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This multi-step model replaced an earlier model, called the initiation-promotion model. The initiation-promotion model proposed that carcinogenesis was a two-step event, with the first step being a genotoxic injury (called initiation) and the second step being a non-genotoxic event (called promotion). It is now clear that this two-step model was too simple. In particular, it is clear that multiple genotoxic injuries are involved in many (in not all) types of cancer; and that promotion may not be involved in all types of cancer.
Our current understanding of cancer is that it is initiated by damage to the genetic information of a cell (the DNA). Agents which cause such injury are called genotoxins. It is extremely unlikely that a single genetic injury to a cell will result in cancer; rather it appears that a series of genetic injuries are required. Genotoxic carcinogens may not have thresholds for their effect; so as the dose of the genotoxin is lowered the risk of cancer induction gets smaller, but it may never reach zero. Genotoxins may affect many types of cells, and may cause more than one kind of cancer. Thus, evidence for genotoxicity of an agent at any exposure level, in any recognized test for genotoxicity, is relevant to assessing carcinogenic potential in humans [A12,A13,A8,A9,L26,L28].
There are many approaches to measuring genotoxicity. Studies of occupational-exposed humans can be done to look for genotoxic injury in white blood cell (Q16A). Animal exposure studies can be used to see whether exposure causes cancer, mutations or chromosomal injury (Q16B). Cellular studies can be done to detect DNA or chromosomal damage (Q16C) or neoplastic cell transformation (Q16D). In reviewing the genotoxicity literature, non-mammalian as well as mammalian systems have been included. The coverage of exposure conditions has also been broad, since any evidence for genotoxicity from any system exposed to any related type of field could be relevant to the question of carcinogenicity.
There are also many different types of laboratory tests that can be used to look for evidence of genotoxic activity:
| Test | Description |
| Cancer induction (in vivo) | Test for increased cancer in animals. Animals are exposed to an agent for long periods of time (often for lifetime) and examined for an increase in cancer. |
| Mutagenesis (in vivo) | Test for changes in the genetic material of eggs or sperm than can be passed on to offspring. Animals are exposed to the agent and then mated, and their offspring are examined for inherited defects. Alternatively, the off-spring are examined for changes in the sex ratio, since mutations are more likely to kill male than female offspring. |
| Mutagenesis (in vitro) | Test for changes in the genetic material of cells that can be passed on to their progeny (daughter cells). Cells are exposed to an agent, and their progeny are examined for inherited changes. |
| Sister chromatid exchanges, SCEs (in vivo or in vitro) | Test for the presence of breakage and rejoining of pieces of chromosomes. The test can be applied to white blood cells from exposed organisms (including humans) or to cells exposed in cell culture. |
| Micronucleus formation (in vivo or in vitro) | Test for the presence of pieces of chromosomes that have become detached as a result of damage to the genetic apparatus of the cell. The test can be applied to white blood cells from exposed organisms (including humans) or to cells exposed in cell culture. |
| DNA strand breaks (in vivo or in vitro) | Test for the presence of breaks in the genetic material of cells (the DNA), as opposed to breaks in the chromosomes. |
| Cell transformation (in vitro) | Tests for whether cells growing in cell culture undergo a set of changes when exposed to an agent that resemble their response to a carcinogen. These changes include loss of density-dependent inhibition of cell growth (loss of "contact inhibition") which causes cells to pile up ("focus formation"), and acquisition of the ability to grow in soft agar ("anchorage-independent cell growth"). |
It also appears that non-genotoxic (epigenetic) agents can contribute to the development of cancer, even though they may not be able to cause cancer by themselves. Epigenetic agents (non-genotoxic carcinogens) affect carcinogenesis indirectly, by increasing the probability that other genotoxic agents will cause genotoxic injury, or that genotoxic injury caused by other agents will lead to cancer. For example, an epigenetic agent might inhibit repair of potentially-genotoxic damage, affect the DNA in such a way as to make it more vulnerable to genotoxic agents, allow a cell with genotoxic injury to survive, or stimulate cell division in a previously non-dividing cell that had genotoxic injury [A8,A9,A12,L26,L28].
The actions of epigenetic agents may be tissue- and species-specific, and evidence exists that epigenetic agents have thresholds for their effects. Thus evidence that an agent has epigenetic activity must be evaluated carefully for its relevance to human carcinogenicity under real-world exposure conditions. This is significant for the issue of possible cancer risks from power-frequency fields, as the evidence, to the extent that it implicates such fields at all, suggests an epigenetic rather than genotoxic mechanism [A9,L26,L28].
Promoters are a specific class of epigenetic agents. In a classical promotion assay, 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 the agent to be tested for promotional activity. If the agent plus the genotoxin results in more cancers than are seen for the genotoxin alone, then that agent is a promoter. Promotion assays are discussed inQ16E. Some types of cellular studies are relevant to the carcinogenic potential of agents, but are neither classic genotoxicity nor promotion tests. For example, cellular systems have been used to test whether an agent enhances the activity of known genotoxins, or whether an agent inhibits repair of DNA damage. These cellular studies of epigenetic activity can be regarded as the cellular equivalent of a promotion study, and are discussed inQ16D andQ16F.
Note: The majority of agents that are known to be carcinogenic in humans are genotoxins; and no role for epigenetic carcinogens have yet been identified in leukemia or brain cancer, the types of cancer most often associated with exposure to power-frequency fields in epidemiological studies.
In studies which blur the boundary between epidemiology and laboratory science, the white blood cells (lymphocytes) from workers with occupational exposure to an agent can be examined for chromosome aberrations, sister chromatid exchanges (SCEs) or micronuclei formation. The interpretation of these studies is complex, as they have all of the problems of exposure assessment, confounding and bias that characterize epidemiological studies. A number of such studies have been published [E2,E3,E5, E11,E12,E13,E14,E26]. At first glance these studies appear very contradictory with some studies reporting "significant" effects and others not.
A major statistical issue that must be considered is that all of the studies examine multiple endpoints and subgroups, creating a massive multiple comparison problem (seeQ21E). Skyberg et al [E12], for example, reports chromosomal damage in exposed workers; but this increase was found in only one subgroup, only for one of several assays, and has a p-value of only 0.04. With any adjustment for multiple comparison, the statistical significance of the genotoxicity effect reported by Skyberg et al vanishes. The multiple comparison problem also applies to the "positive" findings reported by Valjus et al [E11].
Even with the multiple comparison problems, several patterns emerge. The effects that are reported are predominantly seen in smokers, groups in which excess chromosomal abnormalities are expected. The effects are also seen predominantly in workers exposed to spark discharges [spark discharges are a phenomena that is unique to the electrical environment of high-voltage sources, where electric fields can reach intensities of up to 20 kV/m, and body currents can reach several amps]. Finally, the reported increases are limited to increased chromosomal aberrations, with no effects on SCEs; this is somewhat surprising, as the SCE assay is generally considered to be more sensitive to genotoxic agents than the chromosome aberration assay.
In summary, the cytogenetic studies of workers exposed to strong power-frequency electric and magnetic fields provides no consistent evidence that these fields are genotoxic. The unreplicated evidence for genotoxic effects is largely confined to current and former smokers, and to workers exposed to spark discharges.
Animal carcinogenesis studies:Until 1997, the biggest gap in the range of genotoxicity endpoints that have been assessed for power-frequency fields was that relatively few long-term whole animal exposure studies had been published.
Bellossi et al [G14] exposed leukemia-prone mice to 6000 microT fields for 5 generations (lifetimes) and found no effect on leukemia rates; however, the study used 12 and 460 Hz pulsed fields, so the relevance of this to power-frequency exposure is unclear.
Rannug et al [G23] reported that exposure of mice for 2 years to 50 and 500 microT fields did not significantly increase the incidence of skin tumors, lung tumors, or leukemia.
Beniashvili et al [G16] reported that exposure of mice for two years at 20 microT resulted in an increased incidence of mammary tumors. However, the study has been reported only in preliminary form with incomplete information about exposure conditions and experimental design.
Fam and Mikhail [G53] reported that mice exposed for three generations to a 60-Hz field at 24,000 microT had an increased incidence of lymphoma. The experiments were not conducted blind (that is, the experimenters knew which animals had been exposed and which had not), and the controls may not have been housed under conditions comparable to those of the exposed animals. When these data were presented at scientific meetings, concerns about noise, hyperthermia (overheating) and vibration were raised.
In 1997, Yasui et al [G66] reported the absence of increased cancer incidence and mortality in male and female rats after two years of exposure to 50-Hz fields at 500 and 5000 microT. In addition to finding no changes in overall cancer rates, they found no differences in the rates of individual types of cancer, including leukemia, lymphoma, brain cancer and breast cancer.
Also in 1997, Mandeville et al [G67] reported that two years of exposure of female rats to 60-Hz fields at 2, 20, 200 or 2000 microT had no effect on survival, leukemia incidence or solid tumor incidence. In addition to finding no overall changes in survival or cancer incidence, Mandeville et al found no evidence for any dose-related trends in survival or cancer incidence.
In 1998, Harris et al [G70] found that 1.5 years of exposure of lymphoma-prone mice to 50-Hz fields at 1, 100 or 1000 microT had no effect on lymphoma incidence. In addition to testing continuous exposure, Harris et al also showed that exposure of mice to intermittent (15 min on, 15 min off) fields at 1000 microT had no effect on lymphoma incidence. Similar results were reported by McCormick et al [G36]. Interestingly, these studies use the same animal model in which Repacholi et al (Rad Res, 1997) reported that exposure to 900 MHz radiofrequency (RF) radiation resulted in an increase in lymphoma incidence.
Also in 1998-1999, the U.S. National Toxicology Program (NTP) reported that two years of exposure of mice (McCormick et al [G72B]) and rats (Boorman et al [G72A]) to 60-Hz fields at 2, 200 or 1000 microT had no effect on survival or cancer incidence. In addition to testing continuous exposure, NTP showed that exposure to intermittent (1 hr on, 1 hr off) fields at 1000 microT had no effect on cancer incidence. No effects on overall cancer, leukemia, brain cancer, lymphoma or breast cancer were observed, and no exposure-response trends were found.
In a study published in late 1999 Kharazi et al [G88] reported that life-time exposure of mice to a 1420 microT field had no effect on brain tumor incidence.
In 2000 Babbitt et al [G84] reported that life-time expose of mice to a 1420 microT field had no effect on lymphoma incidence. The study also found that this field had no effect on the incidence of lymphoma induced by ionizing radiation (seeQ16E).
In 2001, Vellejo et al [G111] reported that exposure of mice for 15 or 52 weeks to a 50-Hz field at 15 microT resulted in a significant increase in leukemia.
In summary, the long-term animal exposure studies conducted to date provide no consistent evidence that long-term exposure to power-frequency fields causes cancer in animals; and no evidence at all that long-term exposure of animals to power-frequency fields is associated with brain cancer or breast cancer.
For further discussion of the animal carcinogenesis studies see McCann et al [K7] and Boorman et al [K10].
The long-term animals exposure studies with power-frequency fields are summarized in the following figures.
Animal Carcinogenesis Studies |
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| Summary of animal carcinogenesis studies using power-frequency magnetic fields that assessed total malignant tumors or overall survival. The figure shows the ratios (exposed/sham) of the number of animals with tumors at the end of the experiment, or the number of deaths during the experiment. All data are shown with 95% confidence intervals. Typical 24-hour average residential fields are shown for comparison [F7,F22]. |
Animal Carcinogenesis Studies |
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| Summary of animal carcinogenesis studies using power-frequency magnetic fields that assessed lymphoma and/or leukemia.The figure shows the ratios (exposed/sham) of the number of animals with lymphoma or leukemia at the end of the experiment. All data are shown with 95% confidence intervals. Typical 24-hour average residential fields are shown for comparison [F7,F22]. |
Whole organism mutagenesis and genotoxicity studies:Whole organism exposure studies can be relevant to carcinogenic potential even when the end point is not cancer. The ability of an agent to cause mutations or chromosome aberrations in an organism is an indication that the agent is genotoxic, and hence potentially carcinogenic.
Benz et al [G4] reported that mice exposed for multiple generations 300 microT (plus 15 kV/m) or 1,000 microT (plus 50 kV/m) showed no increase in mutation rates, fertility, or sister chromatid exchanges (SCEs). Similarly, Kowalczuk and Saunders reported that mice exposed to 10,000 microT fields [G43] showed no increase in mutations; and Zwingelberg et al [G24] reported that a 30,000 microT field did not increase SCE rates in mice.
Kikuchi et al [G95] reported that exposure of fruit flies to 500 or 5000 microT fields for 40 generations had no effect on the mutation rate.
In 2001, Abramsson-Zetterberg and J Grawé [G106] found no evidence of chromosome injury in adult or fetal mice exposed for 18 days to a 14 microT (140 mG) power-frequency field.
The only positive reports of genotoxicity from whole organism studies are of DNA strand breaks in brain cells of rats [G60] and mice [G107] that had been exposed to 100-500 microT fields. In 2002, McNamee et al [G109] reported that they found do evidence for such genotoxic injury in the brain cells of immature mice exposed that had been exposed to a 1000 microT field. All attempts to detect DNA strand breaks after exposure of mammalian cells to power-frequency fields in culture have failed to find any significant excess [G6,G20,G37,G99,G104,G110].
In summary, the long-term animals exposure studies conducted to date provide no confirmed evidence that long-term exposure to power-frequency fields causes cancer or genotoxic injury in animals.
The traditional cellular test systems for genotoxicity have been mutagenesis assays in bacteria, yeast, and mammalian cells. A variety of other mammalian test systems for genotoxicity also exist, including chromosome aberration assays, SCE assays, DNA strand break assays, and micronuclei formation assays.
Cellular genotoxicity studies of power-frequency and ELF fields have been massive in scope. Published studies have spanned many different models, from plasmids and bacteria to human cells. All major genotoxicity endpoints have been assessed in multiple models and multiple labs. A wide range of exposure conditions have also been assessed, including combined electric and magnetic fields, pulsed as well as sinusoidal fields, non-power-frequency fields and field intensities ranging from less than 1 microT to greater than 1000 microT.
Mutagenesis assays: Studies using a wide range of exposure conditions and assay systems have shown that power-frequency fields are not generally mutagenic. Five studies have found that power-frequency electric and magnetic fields are not mutagenic in bacteria or yeast [G3,G19,G21,G51,G101]. Studies of power-frequency fields and mutagenesis in mammalian cells done at field intensities of 50,000 microT and below have also been negative[G21,G58,G83,G92,G94]; but some studies [G56,G83] have suggested that 400,000 microT fields may be mutagenic.
Chromosome aberration assays: Of eleven studies of the ability of power-frequency fields to cause chromosome aberrations, eight [G1,G8,G38,G40,G41,G75,G96,G99] have found no consistent evidence of genotoxic effects. The remaining three studies showed some unreplicated evidence that power-frequency fields could cause chromosome aberrations. In 1984, Nordenson et al [E3] reported that exposure of human lymphocytes to spark discharges caused chromosome aberrations; but in 1995, Paile et al [G40] found no evidence for this effect. In 1991, Khalil and Qassem [G17] reported that a pulsed 1050 microT field caused chromosome aberrations in humans lymphocytes, but a similar 1994 study by Scarfi et al [G38] found no such effect. Finally, in 1994 Nordenson et al [G34] reported that exposure of mammalian cells to an intermittent 30 microT field caused chromosome aberrations, but that continuous exposure did not.
Sister chromatid exchanges (SCEs): Of the nine studies of the ability of power-frequency fields to cause SCEs, eight [G2,G5,G8,G12,G40,G42,G99,G102] have found no evidence of genotoxic effects. The only "positive" study is Khalil and Qassem [G17] who reported that a pulsed 1050 microT fields caused an increase in SCE's in humans lymphocytes; the study has never been replicated.
DNA strand breaks:None of the six studies of the ability of power-frequency fields to cause DNA strand breaks in cultured mammalian [G6,G20,G37,G99,G104,G110] have found evidence of genotoxic effects. One of these studies [G110] did report that a 7000 microT field caused DNA strand breaks when a strong oxidant was present.
Micronucleus formation assays: Of the 13 studies of the ability of power-frequency fields to enhance micronucleus formation, seven [G12,G38,G40,G63,G65,G108,G112] found no evidence for such effects.
Tofani et al [G45] reported that exposure of human lymphocytes to a 32-Hz field enhanced micronucleus formation; this effect was not found at 50-Hz or if the Earth's static geomagnetic field was eliminated. Scarfi et al [G68] reported that strong (1300 microT) pulsed fields enhanced micronucleus formation in human lymphocytes.
More recently, Simkó et al [