IEEE EMBS Committee on Man and Radiation

In theory, exposure to strong power frequency magnetic fields, without contact with a conductor, can also result in shock and burn. However, these hazards require field strengths that are far higher than those normally encountered in the workplace or at home; magnetic fields above 50 mT can stimulate excitable tissues (muscles, nerves), and above 500 mT may cause ventricular fibrillation ⁽¹⁾. Environmental levels common in residences and most workplaces are magnetic fields below 1 uT (more usually below 0.5 uT, or 0.00001 times the nerve stimulus level) and electric fields below 100V/m. Since European countries use a higher voltage, lower current power system, electric fields are stronger and magnetic fields weaker than in the US, where the bulk of surveys have been done.  Summarizing the literature on residential magnetic fields, Swanson and Kaune found the mean magnetic flux density in US homes is about 0.07 uT, and the comparable level in Europe is lower⁽²⁾. The epidemiological studies that have aroused concern, particularly about childhood leukemia, have focused largely on magnetic fields. Electric fields have received less attention, partly because they are readily shielded by building materials, so that fields due to overhead power lines are very much weaker inside a building than outside ^{(3) (4)}. Environmental exposure to power-frequency electric fields greater than 100 V/m is rare in the US except near high-voltage power lines, although the body can experience localized fields of tens of thousands of volts per meter close to domestic appliances and wiring - electric blankets, for example. Those few studies looking at cancer incidence associated with measured electric fields have found no correlation ^{(5, 18, 24, 29)}.

Public concern and scientific controversy has arisen about reports of possible links between exposure to electric or magnetic fields in the home or workplace and disease (various forms of cancer, in particular). Two conditions exist that amplify the controversy: 1) The subject encompasses the extremely varied fields of physics, engineering, biology, biochemistry and epidemiology, and few if any scientists have the training to fully grasp all of these subjects. 2) The effects that have been reported are mostly very weak ones, so that both alternative hypotheses (fields either are or are not associated with disease) can be logically supported. This Technical Information Statement is concerned with putative health hazards of power-frequency electric and/or magnetic fields at levels people commonly experience at work and at home⁽²⁾. In general this means electric fields below 100 V/m and magnetic field levels below 10 uT, though some occupations involve exposure to electric fields above 10 kV/m, or magnetic fields greater than 1000 uT. The document focuses on scientific developments over the past three years related to this issue, and is particularly concerned with cancer, though other effects are mentioned. It is addressed to a technical but nonspecialist audience.
 

These different kinds of studies have different advantages and disadvantages:

Epidemiology is the study of the risk of disease and its determinants in the human population. Thus it provides direct evidence about health in human populations. However, epidemiologic studies of exposures to environmental agents or conditions are often difficult to interpret, because of the complexity of human populations. In addition, epidemiology is unsuitable for detecting small risks, or for detecting risks that are confined to small, but unidentified, subpopulations. In studies of low levels of exposure that report inconsistent results, or weak associations, it will be unclear whether an association is causal or a result of confounding factors. An important means to resolve such uncertainties is to examine the results of laboratory experiments, in which complexity can be limited by the experimental design.

The mechanisms of carcinogenesis are sufficiently well established that studies of cells and animals in the laboratory can be used to assess whether an agent has the potential to cause or contribute to cancer. They cannot, however, convincingly establish whether an agent will cause cancer in humans. Animal studies can be closely controlled, both in the uniformity of the test animals and in their exposure. One of the major forms of tests used to identify carcinogens consists of standardized laboratory animal studies, in which multiple groups of animals are exposed to an agent at various levels of intensity. The standard protocol is designed to observe a trend in dose-response: how an agent applied in increasing amounts appears to cause increasing numbers of diseased animals, or more rapid disease progression in individual animals. Studies of this type typically involve very high exposures to the animals, and the relevance of their results to the human situation (which may involve much lower levels of exposure) is often uncertain. Epidemiologic evidence is used to confirm results from animal studies, and vice versa.

In-vitro (cellular) studies may yield information about mechanisms of action of a chemical or physical agent, but their relevance to disease in intact organisms may be uncertain. It is useful to isolate specific cells or tissues to examine their individual response to an agent, but the same cells may not respond in the same way when they are within an intact animal. In-vitro studies attempt to control many different variables (temperature, light, vibration, growth medium, cell density and others in addition to the magnetic field parameters) and thus when one laboratory finds a significant result there may be several years' delay before it is replicated, or fails replication in independent laboratories. Many reports of effects therefore must be still considered preliminary as insufficient time has elapsed to complete replication studies. Replication does not necessarily imply exact duplication of an experiment, from procedures to results, but it does imply the completion of similar experiments with results that reinforce the conclusions of the original study.

It is not possible to prove that an agent does not cause cancer, from the results of any or all of these types of studies.  Conversely, with the exception of a very few cases (e.g. tobacco smoke) none of these three types of studies is likely to be definitive, and judgments about the carcinogenicity (or noncarcinogenicity) of an agent have a certain level of uncertainty. All three study types are each subject to many different sources of error, which is the main reason to insist upon independent replication in different laboratories (or human populations) before results can be considered established. Thus, agencies such as the Environmental Protection Agency (EPA) or International Agency for Research on Cancer (IARC) classify potential human carcinogens according to the strength of evidence available, weighing all relevant evidence according to the assumptions used by each agency, a method called by them a "weight of evidence" approach.

For these reasons, when examining evidence related to the issue of power frequency fields and cancer it is necessary to consider all relevant evidence, including studies reporting no effect of the fields, with due attention to the relevance of the study to carcinogen identification, and the quality of the study.
 

Two large prestigious reviews have appeared in the last two years. That of the National Research Council - National Academy of Sciences (NRC/NAS) was published in 1997.  Though noting the human epidemiology suggested that "wire codes", a measure of residential proximity to power lines, could be associated with childhood leukemia, this study concluded, "Based on a comprehensive evaluation of published studies relating to the effects of power frequency electric and magnetic fields on cells, tissues, and organisms (including humans), the conclusion of the committee is that the current body of evidence does not show that exposure to these fields presents a human-health hazard." ⁽⁹⁾  The NIEHS Working Group on EMF concluded in July 1998, "there is limited evidence that residential exposure to ELF magnetic fields is carcinogenic to children on the basis of the results of studies of childhood leukemia" and "there is limited evidence that occupational exposure to ELF magnetic fields is carcinogenic to humans on the basis of results of studies of chronic lymphocytic leukemia (CLL)."⁽¹⁰⁾  These conclusions, which result in an IARC classification of  "possible human carcinogen (Group 2B, Appendix B)", are based on the human epidemiologic evidence.  The report also concludes that evidence from experiments on laboratory animals does not support a conclusion of carcinogenicity, and that evidence is weak that fields below 100 uT have any biological effects. It notes that studies of cells in vitro and of mechanisms provide some evidence that  ELF magnetic fields stronger than 100 uT may affect processes or end-points commonly associated with carcinogenesis. The NIEHS Working Group was restricted in its charter to using the IARC classification scheme for potential human carcinogens, and this scheme imparts specific technical meanings to the phrases above.  In particular, "limited evidence" is defined as, "A positive association has been observed between exposure to the agent, mixture or exposure circumstance and cancer for which a causal interpretation is considered by the Working Group to be credible, but chance, bias or confounding could not be ruled out with reasonable confidence." Also, a Group 2B classification (possible carcinogen) is used "for...exposure circumstances for which there is limited evidence of carcinogenicity in humans and less than sufficient evidence of carcinogenicity in experimental animals," among other possibilities.  As the most recent extended and detailed reviews by large groups of scientists, these two documents are a good place to start in researching the science of ELF fields prior to 1998.  COMAR recommends that the reader of the NIEHS Working Group Report also read its Appendix A where the procedures and terms used in the report are clearly defined, and Appendix B, the statement on animal carcinogenicity by a minority of the Working Group.  The WHO report⁽¹¹⁾ is directed towards pointing out places where more research is needed, and is complementary to the two reports cited above.
 

• Gurney et al. ⁽¹²⁾ found no association of brain cancer in children with residence near power lines, and no association of brain cancer with childhood or fetal exposure to fields from electrical appliances.
• Preston-Martin et al. ^{(13) (14)} found no association between brain cancer in children and residential magnetic fields or use of electric blankets or water bed heaters.
• Linet et al. ⁽¹⁵⁾ found no association of childhood leukemia with types of wiring outside the home. The authors interpret their data on measured magnetic fields as a non-significant excess of childhood leukemia with exposure to fields above 0.2 uT, though the data are borderline and the subject of some controversy. The same workers ⁽¹⁶⁾ found no consistent associations between childhood leukemia and use of various electric appliances during pregnancy or childhood.  Significant associations were found with electric blankets, hair dryers/curling irons, and television/video game use, but dose-response trends are missing.
• Michaelis et al. ⁽¹⁷⁾ found an excess of childhood leukemia associated with residential exposures to power frequency magnetic fields, which was statistically significant for children 4 years of age or less and for median night-time magnetic field, but not for all children and median 24-hour magnetic field.
• McBride et al. ⁽¹⁸⁾ did not find any statistically significant elevated odds of childhood leukemia in relation to wire code, measured magnetic fields or measured electric fields.  Field meters were carried by subjects for 48 hours.
• Tynes et al. ⁽¹⁹⁾ reported no increased risk for cancer for children living near high voltage power lines in Norway.
• Versakalo et al. ⁽²⁰⁾ found no statistically significant increase in cancer in adults living within 500 meters of a high voltage power line.
• Li et al. ⁽²¹⁾ did report an increase of leukemia (but not brain or breast cancer) in adults living within 50 meters of a high voltage power line.
• Feychting et al.⁽²²⁾ did not find a statistically significant association between female breast cancer and calculated residential magnetic fields, though the data are weakly suggestive of an association for pre-menopausal women only.
• Gammon et al. ⁽²³⁾ found no association between electric blanket use and breast cancer incidence among younger women.
• Guenel et al.⁽²⁴⁾ reported no significant associations between occupational exposure to power-frequency electric fields and adult cancers.
• Miller et al.⁽²⁵⁾ found no consistent associations between estimated occupational exposure to magnetic fields and certain types of leukemia or between cumulative electric field exposure and the occurrence of all leukemia.
• Feychting et al. ⁽²⁶⁾ found no association between occupational or residential exposure to magnetic fields and adult brain cancer, but did find elevated risks of certain leukemias with combined home and work exposure to magnetic fields.
• Harrington et al.⁽²⁷⁾ and Rodvall et al.⁽²⁸⁾ found no significant association between brain cancer and occupational exposure to ELF magnetic fields.
• Kheifets et al.⁽²⁹⁾ found that occupational exposure to electric fields does not correlate well with exposure to magnetic fields (the 5 occupations with highest magnetic field include 3 of the 5 highest electric-field-exposed), and that adult leukemia is not associated with occupational electric field exposure.
• Savitz et al.⁽³⁰⁾ found a weakly elevated risk of lung cancer associated with occupational exposure to pulsed magnetic fields, but no consistent trends with increasing exposure, for either pulsed or steady ELF magnetic fields.

Another way of dealing with the problem of low numbers of cases in the highly exposed categories is to pool data from several studies--this is called meta-analysis.  No current meta-analyses (including all relevant published epidemiologic data) are available; recent ones include Wartenberg⁽³¹⁾ (childhood leukemia) and Kheifets et al.⁽³²⁾, ⁽³³⁾ (occupational: leukemia and brain cancer).  Some workers in the field question whether meta-analysis is appropriate with EMF studies because of large differences in experimental procedures between studies.  The larger studies with comprehensive exposure assessment ⁽¹³⁾, ⁽¹⁴⁾, ⁽¹⁵⁾ do not provide compelling evidence to identify residential fields as a cause of cancer, but are sufficient to categorize power-frequency magnetic fields as a possible carcinogen according to the specific meaning of the term in IARC guidelines.  Opposing this, the work of McBride et al.⁽¹⁸⁾ supports the contention that such fields are not related to childhood leukemia, at least.

Animal studies relevant to cancer are similarly summarized below.  Most of these studies report results of many comparisons, and statistically it is likely that at least some statistically significant results are in fact due to chance.  For this reason researchers look for internal consistency within studies (trends with increasing field strength, similar reactions in related species, for example) and when such consistency is lacking the study is weaker even if it reports positive effects.

• Yasui et al. ⁽³⁴⁾ and Mandeville et al.⁽³⁵⁾ found no increased incidence of any cancers among rats exposed for their lifetimes to power-frequency magnetic fields at levels of 0.002, 0.5, 1 and 2 mT.
• Boorman et al.⁽³⁶⁾ and McCormick et al.⁽³⁷⁾ found no consistent effect on any cancer rates for any group of mice or rats exposed for a lifetime to power-frequency 2 or 200 uT or 1 mT magnetic fields. One of two groups of mice exposed to a 1 mT field experienced an increase in mortality, but this was not evident in rats or in any of the other mouse groups.
• Several studies have been done of skin cancer in mice treated with a carcinogenic chemical and exposed to magnetic fields of 0.5 - 2 mT ⁽³⁸⁾, ⁽³⁹⁾, ⁽⁴⁰⁾, ⁽⁴¹⁾, ⁽⁴²⁾ ; all found no effect of steady AC fields, but one⁽⁴²⁾ found mice subjected to pulsed fields (with transients) developed significantly more tumors than mice subjected to steady fields.
• Kumlin et al.⁽⁴³⁾ found that UV production of skin tumors in mice was weakly, but significantly, enhanced by exposure to magnetic fields (1.3 - 130 uT). Mice genetically altered to be more prone to skin cancer were not significantly more sensitive to magnetic fields.
• Sasser et al.⁽⁴⁴⁾ and Morris et al.⁽⁴⁵⁾ found no effect on progression of leukemia transplanted into rats exposed to 1 mT magnetic fields.
• Shen et al.⁽⁴⁶⁾ did not find lymphoma promotion in mice treated with a carcinogen and exposed to strong (1 mT) magnetic fields.
• Harris et al.⁽⁴⁷⁾ and McCormick et al.⁽⁴⁸⁾ found no increased incidence of lymphoma in lymphoma-prone mice exposed to 1 uT - 1 mT power-frequency magnetic fields.
• Mevissen et al.⁽⁴⁹⁾ found that 50 uT magnetic field exposure of rats treated with carcinogens increased their incidence of breast cancer, but did not affect tumor growth in individual animals.  A previous study⁽⁵⁰⁾ found that 100 uT exposure had effects opposite these.
• Ekstrom et al.⁽⁵¹⁾ and Boorman et al.⁽⁵²⁾ in two separate studies found no effect of power-frequency 100, 250 or 500 uT magnetic fields on promotion of chemically-induced breast cancer among rats.
• Lai and Singh ⁽⁵³⁾ found an increase in DNA strand breaks in brain cells of rats exposed to 0.1, 0.25 and 0.5 mT magnetic fields.

1. Magnetic and electric fields and melatonin in animals. Some studies suggest that exposure to power-frequency electric or magnetic fields reduces the normally occurring increase in nighttime secretion by the pineal gland of the hormone melatonin. The pineal is the remnant of a third eye, and its secretion is normally responsive to day length as sensed through the eyes (light intensity). Its sensitivity to ELF fields was first noted by Wilson et al.⁽⁵⁵⁾, ⁽⁵⁶⁾ who exposed rats to an environmental electric field of 1.7 to 1.9 kV/m, although this result has failed subsequent replication. Several studies have reported melatonin sensitivity to magnetic fields in rodents ⁽⁵⁷⁾, ⁽⁵⁸⁾, ⁽⁵⁹⁾, ⁽⁶⁰⁾ but initial studies of humans are not supportive of a similar effect ⁽⁶¹⁾, ⁽⁶²⁾, ⁽⁶³⁾ and the animal studies are inconsistent ⁽⁶⁴⁾, ⁽⁶⁵⁾, ⁽⁶⁶⁾, ⁽⁶⁷⁾, ⁽⁶⁸⁾, ⁽⁶⁹⁾. The health significance of this effect, even if it can occur in humans, is unclear. However, some experts have suggested that melatonin may limit progression of cancer by acting as scavenger of free radicals ⁽⁷⁰⁾, ⁽⁷¹⁾.

2. Cardiovascular diseases. Recent studies of humans have revealed temporary, reversible changes in heart rate variability in individuals exposed to 20uT magnetic fields ⁽⁷²⁾.  At least one epidemiology study found an association between occupational magnetic field exposure and death due to arrythmia or acute myocardial infarction, though not chronic heart disease ⁽⁷³⁾.

3. Immune system effects. Some studies have reported biological changes interpreted as immune system suppression ⁽⁷⁴⁾. Other laboratories have not found similar effects ⁽⁷⁵⁾; the particular combinations of field strength and duration of exposure are variable, so that study in this area is very preliminary. Effects have not been seen at lower field strengths (2, 20 uT).

4. In-vitro (cellular) effects. A large number of experiments has been carried out to study the effects of ELF magnetic and electric fields on cells in culture. Many experiments have given evidence for field-mediated effects at the cell membrane. One established effect is the increase in transport of calcium across cell membranes under exposure to strong (over 1 mT) power-frequency magnetic fields ⁽⁷⁶⁾. However, no experimental evidence exists at present to relate these changes to cancer initiation or growth. Further, most reliable results were obtained with field levels that are far larger than the fields implicated by the epidemiological findings. Several studies have reported effects on cells by magnetic fields of a few microtesla (comparable to ordinary environmental levels) and either magnetically induced or directly applied electric fields within the culture medium in the microvolt-per-meter range ⁽⁷⁷⁾, ⁽⁷⁸⁾, ⁽⁷⁹⁾, ⁽⁸⁰⁾, ⁽⁸¹⁾, or microvolt-per-centimeter range⁽⁸²⁾ but most of these experiments have not been independently replicated in more than one laboratory, and in at least one case attempts to replicate have not been successful ⁽⁸³⁾, ⁽⁸⁴⁾, ⁽⁸⁵⁾, ⁽⁸⁶⁾ . One apparently robust effect is that 1.2uT magnetic field affects the ability of melatonin to inhibit cancer growth in cultured cells.⁽⁸¹⁾  The health significance of all these studies remains unclear.

5. Mechanistic studies. Several theoretical models have been proposed to explain ways by which low-level electric and magnetic fields might affect biochemical processes. More recent studies along these lines include ⁽⁸⁷⁾, ⁽⁸⁸⁾, ⁽⁸⁹⁾, ⁽⁹⁰⁾, ⁽⁹¹⁾, ⁽⁹²⁾, ⁽⁹³⁾, ⁽⁹⁴⁾. Limited experimental support exists for some of the theoretical models, in particular the radical pair mechanism⁽⁹⁵⁾, ⁽⁹⁶⁾, ⁽⁹⁷⁾, which may well be a reasonable explanation for observed in-vitro effects.  However, experimental evidence has been insufficient for any of these models to gain widespread acceptance by the scientific community. Moreover, the physical basis of many such models has been severely criticized ⁽⁹⁸⁾, ⁽⁹⁹⁾, ⁽¹⁰⁰⁾.
 

There are numerous open scientific questions related to possible biological effects of power frequency fields. Many of the reported biological effects remain poorly understood. Most of these effects have no clear relation to human health and safety, and many involve acute high-amplitude exposure. Therefore they do not provide a basis for developing environmental exposure limits, or standards, at ambient levels. They do, however, create uncertainties that make it difficult to arrive at scientific consensus for safety standards. COMAR believes it is important for scientific funding agencies to identify the most important of these still-open questions and direct sufficient funds to resolve them.  Effects still in need of explanation and/or verification in different laboratories include: (a) in vitro results showing biological effects of low intensity magnetic fields, (b) epidemiological results suggesting health effects of relatively large ELF fields in occupational settings, and (c) the therapeutic effectiveness (e.g.bone healing) of magnetic fields of time-averaged magnitude well below shock and nerve stimulation thresholds.  It is particularly important for the sake of safety standards that experimental results be repeatable, support each other, show a logical pattern leading to a plausible biological mechanism, and have clear implications for human health.

COMAR also concludes that, since the publication of the 1997 NRC report, no convincing evidence has emerged to change the main conclusion of that review. The scientific evidence does not support the idea that cancer or other health and safety hazards exist due to power frequency fields at levels that are encountered in normal residential or most occupational environments (24-hour average magnetic fields below 1 uT, which are probably exceeded for less than 0.5 % of the U.S. population). Costs to society of major changes in the use of electrical power would be enormous, and are not justified by the present state of knowledge in this field.
 

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