Health effects of exposure to extremely low frequency fields
In this chapter, epidemiological studies for both occupational and public ELF field exposure environments have been examined and summarized. Special emphasis has been put on childhood leukemia, brain and breast cancer, and neurodegenera- tive and reproductive diseases. In addition, experimental and clinical studies of exposure to ELF fields have been examined with a focus on melatonin hypothesis, genotoxicity and carcinogenicity, reproduction, and perception. Major large-scale national and international programs and reviews have been presented. Detailed information and suggestions for future research are also included.
The hypothesis
The electrification of homes during last century caused peak leukemia mortality among children 2-4 years of age. This occurred as domestic, urban, and rural reticulation of electric power was extended. This new age-related peak occurred in the UK in the 1920s, US in the 1930s, and in other countries as they reticulated power [39].
Introduction
Health is a state of total physical, mental, and social well-being, and not just the absence of disease or sickness. A biological effect occurs when exposure to EM fields causes some noticeable or detectable physiological change in the living system. Such an effect may sometimes, but not always, lead to an adverse health effect, which means a physiological change that exceeds the normal range for a brief period of time. However, increased biological action does not necessarily cause noticeable health effects, since there are adaptive mechanisms operating at cellular-tissue-organism levels in response to ever- occurring changes. However, these mechanisms may not always be effective, especially when the organism is under extra stress or has increased metabolic needs [1]. It occurs when the biological effect is outside the normal range for the body to compensate, and therefore leads to some detrimental health condition. Health effects are often the result of biological effects that accumulate over time and depend on the EM exposure dose.
Determining adverse health effects from ELF exposure is complex. Such effects have been studied for several decades. Following the emergence of questions and debates in society, the potential health effects of ELF in connection with the development of worldwide electrification are attracting renewed interest. Not all investigators agree about such adverse health effects. In its ELF field assessment, the National Institute of Environmental Health Sciences (NIEHS), based on the report of its expert Working Group [2], stated that biological effects are plausible at a tissue dose of 1 mV/m. According to Dawson et al. [3], contact current levels in the order of 10 pA or less, considerably below ICNIRP exposure limits, can produce electric fields in some tissues that are well above the NIEHS's 1 mV benchmark [4]. In addition, a growing number of studies in the literature suggest that there may be health effects at such ELF field levels, possibly depending on many variables, including duration of exposure, strength of the field, person's mass and age, general health, and probably genetic predisposition or vulnerability to cancer. For an effect to be established, most of the evidence from the epidemiological, human, and laboratory studies should indicate that an effect exists. Therefore, detailed knowledge of biological outcomes is important to understand the generated health risks.
Since 1979, there has been a flurry of scientific activity to evaluate the possibility that exposure to ELF fields from power lines, substations, and other sources may cause cancer. Overall, the currently available epidemiological and toxicological data do not provide clear evidence that ELF fields is associated with an increased risk of cancer, although there is some epidemiological evidence of linkages to childhood leukemia. There is also no convincing evidence from cellular and animal studies that ELF fields can directly damage DNA or promote tumor growth.
During the past several decades, a large number of studies and major scientific reviews have been conducted to assess the biological effects of ELF fields. Considering the interaction mechanism of these fields with biological systems, the effect of magnetic fields has been the central point of research, focusing primarily on fields of the magnitude encountered in everyday life (below 100 pT).
Epidemiological studies
Epidemiology is the scientific field that examines the patterns of disease occurrence in human populations and the factors that influence those patterns. Public concern over human effects of exposure to EM fields is largely based on a series of key epidemiological assessment studies. Those studies identify the association between diseases and particular environmental characteristics. It may indicate a cause-and-effect relationship, depending upon the strength of the observed association. Epidemiological studies correlate historical biological data for a large population of people. Any biological data is purely statistical in nature; however, people usually fit a particular category based on location or occupation. The results may only show an association with a stimulus, since there are many factors involved with each person.
The major objectives of most epidemiological studies are to determine whether a specific factor is likely to cause a given disease and to quantify the strength of the relationship. Epidemiological studies are critical for determining the causes of diseases and play a primary role in a human health risk assessment. They correlate EM exposure and health effects on human populations to establish quantitative dose-response relations. At best, the epidemiological findings indicate a correlation between EM field exposure and a health effect, but not necessarily a causal relation.
Two major types of studies are used to evaluate whether an exposure is linked with a given disease: the cohort and the case-control study designs. In a cohort study, exposed and unexposed populations are ascertained, then followed up to compare risks of developing particular disease outcomes. In an ideal case-control study, cases are those who have developed a particular disease in a specified population during the study period, and control subjects are a random sample of those in the population who have not developed disease [5, 6]. In a case-control study, an odds ratio (OR) is used to estimate the association quantitatively. An OR is the ratio of the odds of being exposed among the cases to the odds of being exposed among the controls. If OR is equal to 1, the understanding is that there is no association between the exposure and disease. If OR is greater than 1, it means there is a positive association between the exposure and disease. On the other hand, a cohort study is evaluated statistically in a similar manner as a case-control study, although the risk estimate is referred to as a relative risk (RR). The RR is equal to the risk of disease in the exposed group divided by the risk of disease in the unexposed group. When RR is greater than 1, it implies that the exposed group has a higher risk of disease. A strong association is one with a RR of 5 or more. In summary, OR or RR is simply a measure of association and it does not mean that there is a known or causal relationship. In the end, all studies of the relationship between the exposure and disease must be identified and evaluated to verify the possible impact that other factors such as random error, bias, and confounding may have influenced the results.
Most epidemiological studies are limited by the use of surrogate indicators rather than direct measurements of exposure. An epidemiological association, if found, might not be related directly to exposure; rather, it may be due to chance, confounding factors, or some unrecognized factors related to the way the data have been collected. Consideration of the extent to which epidemiological studies may be successful in assessing EM risk is essential when reviewing the literature. Supporting evidence in laboratory studies is important to grow confidence that the epidemiological studies may be indicating a risk.
Most epidemiological studies reported in the literature have been criticized as having significant limitations, including failure to consider variability in exposure intensity, transients, intensity spikes, harmonics of the fundamental frequency, historical exposures, and concomitant exposures to other agents experienced in occupational settings.
Health outcomes of particular interest in this section are childhood and adult cancer, as well as noncancer health effects, including reproductive effects, neurodegenerative diseases, suicide and depression, and cardiovascular diseases.
Occupational environments
Occupational exposure environments are studied in the context of specific industries and workplaces, particularly in the electric power-utility industry where high exposure to ELF fields is likely. Workers can be exposed to ELF fields from electrical systems in their building and the equipment they use. A variety of methods for exposure assessment are applied to studies in occupational environments. These methods range from job classification to modeling techniques based on personal exposure measurements and occupational history. Occupational history is a collection of data for a study subject, which may contain information on jobs that the subject held during their employment. Such information is obtained through many means such as interviews or through various employment records. The information contains industry title, company name, description, and duration of the job. Medical records may also be obtained from clinics or disease registries.
Electrical appliances, tools, and power supplies in buildings are the main sources of ELF exposure that most people receive at work. People who work near transformers, electrical closets, circuit boxes, or other high- current electrical equipment may have high-field exposures. In offices, magnetic field levels are often similar to those found at homes, typically 0.5-4.0 mG. However, these levels may increase dramatically near certain types of equipment. In general, the literature is rich with more occupational studies investigating exposure of workers to ELF fields at various places using different techniques of evaluation.
Occupational exposure was studied considering various health problems as well as adult cancers, including brain tumors and leukemia [7-27], breast cancer among both men and women [28-33], lymphoma [16, 23, 34], lung cancer [15,16,21,30], other cancers [16,34-36], and noncancer [20,37,38].
Sahl et al. [8] studied utility workers at Southern California Edison. The authors noticed no difference in risk for brain cancer among electrical workers compared with the reference group. However, small but significant increases in brain cancer risk were observed for electricians (RR = 1.6) and plant operators (RR = 1.6). Researchers from Canada and France [9] conducted a study of 223,292 workers at three large utilities, two in Canada (Hydro Quebec and Ontario Hydro) and a national utility in France (Electricite de France). The result shows that workers with acute myeloid leukemia (AML) were about three times more likely to be in the half of the workforce with higher cumulative exposure to magnetic fields. In the analysis of median cumulative magnetic field exposure, no significant elevated risks were found for most types of cancer studied. Floderus et al. [14] reported an association between estimated field exposure and increased risk for chronic lymphocytic leukemia (CLL). In addition, an increased risk of brain tumors was reported for men under the age of 40 years whose work involved an average magnetic field exposure of more than 2 mG. Johansen and Olsen [18,19] reported that the workers had slightly more cancer than expected from general population statistics, but there was no excess of leukemia, brain cancer, or breast cancer. In a large population-based case-control study, Willett et al. [27] found little evidence to support an association between occupational exposure to EM fields and acute leukemia.
Most of the above studies concentrated on magnetic field exposures, assuming that they are the more biologically active components of the ELF fields and thus more likely to cause cellular damage. However, there are studies that indicate that electric field exposures may enhance cancer risk. Miller et al. [16] reported an increase in the risk of all types of leukemia, as well as some of the highest leukemia risks ever reported in a study of ELF fields and cancer. An elevated risk of leukemia was also seen among senior workers who spent the most time in electric fields above certain thresholds, in the range of 10-40 V/m [23]. In a Canadian population-based control study, Villeneuve et al. [24] reported a non-significant increased risk of brain cancer among men who had ever held a job with an average magnetic field exposure >0.6 gT relative to those with exposures <0.3 gT.
There are rather notable differences in adult cancer studies, with two kinds of results: null association [7,18,19], and mixed, but in general positive, results from a few studies of power-frequency magnetic fields [9,12, 17,21,25,33] and of electric fields [16, 23,24,34]. Most studies of adult cancers, particularly brain cancer, have been based on occupational groups, especially electrical workers with possibly high exposure.
