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Electric Power Basics Powerline Terminology This article uses six basic electrical terms- conductor, current, voltage, load, power, and circuit. The conductor is the wire you see between power poles or towers; it carries the electricity. Current is the movement of electrons in the conductor. Voltage is the electric force that causes current in a conductor. Load is the electric power needed by homes and businesses. When a conductor energized with voltage is connected to a load, a circuit is completed, and current will flow. Electric Power Facilities There are two basic types of power lines: transmission lines and distribution lines. Transmission lines are high-voltage power lines. The high voltage allows electric power to be carried efficiently over long distances from electrical generation facilities to substations near urban areas. In the United States, most transmission lines use alternating current (AC) and operate at voltages between 50 and 765 kV (lkV or kilovolt = 1000 V). Utilities use lower-voltage distribution lines to bring power from sub-: ;0iV stations to businesses and homes. Distribution lines operate at voltages below 50 kV. For residential customers, these levels are further reduced to 120/240 V once the power reaches its destination. Electrical substations serve many functions in controlling and transferring power on an electrical system. Several different types of equipment may be present, depending on the functions of the particular substation. For example, transformers change the high voltages used by transmission lines to the lower voltages used by distribution lines. Circuit breakers are used to turn lines on and off. Alternating Current and Direct Current DC Battery Appliances that operate either with batteries or by plugging into the household wiring usually come equipped with an AC /DC switch. If switched to AC, the appliance uses electric power that flows back and forth or "alternates" at a (U.S.) rate of 60 cycles per second (60 hertz, or Hz). If DC ("direct current") is chosen, current flows one way from the batteries to the appliance. AC fields induce weak electric currents in conducting objects, including humans; DC fields do not, unless the DC field changes in space or time relative to the person in the field. In most practical situations, a battery-operated appliance is unlikely to induce electric current in the person using the appliance. Induced currents from AC fields have been a focus for research on how EMFs could affect human health. Scientific Principles Q. What are EMFs? DC Magnetic field image A. Power lines, electrical wiring, and appliances all produce electric and magnetic fields. EMFs are invisible lines of force that surround any electrical device. Electric and magnetic fields have different properties and possibly different ways of causing biological effects. Note that while electric fields are easily shielded or weakened by conducting objects (e.g., trees, buildings, and human skin), magnetic fields are not. However, both electric and magnetic fields weaken with increasing distance from the source. Even though electric and magnetic fields are present around appliances and power lines, more recent interest and research have focused on potential health effects of magnetic fields. This is because epidemiological studies have found associations between increased cancer risk and power-line configurations, which are thought to be surrogates for magnetic fields. No such associations have been found with measured electric fields. Q. What is power-frequency EMF and how does it compare to other types of fields? A. The electromagnetic spectrum (right) covers an enormous range of frequencies. These frequencies are expressed in cycles per second (i.e., Hz). Electric power (60 Hz in North America, 50 Hz in most other places) is in the extremely-low-frequency range, which includes frequencies below 3000 Hz. The higher the frequency, the shorter the distance between one wave and the next, and the greater the amount of energy in the field. Microwave frequency fields, with wavelengths of several inches, have enough energy to cause heating in conducting material. Still higher frequencies like X-rays cause ionization-the breaking of molecular bonds, which damages genetic material. In comparison, power frequency fields have wavelengths of more than 3100 miles (5000 km) and consequently have very low energy levels that do not cause heating or ionization. However, AC fields do create weak electric currents in conducting objects, including people and animals. Q. Doesn't the earth produce EMFs? A. Yes, the earth produces EMFs, mainly in the form of DC (also called static fields). Electric fields are produced by thunderstorm activity in the atmosphere. Near the ground, the DC electric field averages less than 200 volts per meter (V/m). Much stronger fields, typically about 50,000 V/m, occur directly beneath electrical storms. Earth's magnetic currents Magnetic fields are thought to be produced by electric currents flowing deep within the earth's molten core. The DC magnetic field averages around 500 milligauss (mG). This number is larger than typical AC electric power magnetic fields, but DC fields do not create currents in objects in the way that AC fields do. Q. What happens when I am exposed to EMFs? Electric Field Lines A. AC fields create weak electric currents in the bodies of people and animals. This is one reason why there is a potential for EMFs to cause biological effects. As shown on the right, currents from electric and magnetic fields are distributed differently within the body. The amount of this current, even if you are directly beneath a large transmission line, is extremely small (millionths of an ampere). The current is too weak to penetrate cell membranes; it is present mostly between the cells. Magnetic Field Lines Currents from 60-Hz EMFs are weaker than natural currents in the body, such as those from the electrical activity of the brain and heart. Some scientists argue that it is therefore impossible for EMFs to have any important effects. Other scientists argue that, just as a trained ear can pick up a familiar voice or cry in a crowd, so a cell may respond to induced current as a signal, lower in intensity yet detectable even through the background "noise" of the body's natural currents. Numerous laboratory studies have shown that biological effects can be caused by exposure to EMFs (see Biological Studies). In most cases, however, it is not clear how EMFs actually produce these demonstrated effects. Strong electric fields, such as those found beneath large transmission lines, can cause hair on your exposed head or arms to vibrate slightly at 60 Hz. This is felt by some people as a tingling sensation. EMFs from transmission lines can also in some circumstances cause nuisance shocks from voltages created by EMFs on objects like ungrounded metal fences. HUMAN HEALTH STUDIES Q. How do scientists study possible effects of EMFs on people? A. They use a type of research called epidemiology-the study of patterns and possible causes of diseases in human populations. Epidemiologists study short-term epidemics such as outbreaks of food poisoning and long-term diseases such as cancer and heart disease. Results of these studies are reported in terms of statistical associations between various factors and disease. The challenge is to discover whether the statistical results indicate a true causal association. This includes assessing possible effects of other factors "confounders" that could affect study results. A "statistically significant" finding is one in which researchers are 95% confident that an association exists. However, a statistically significant finding does not necessarily prove a cause-effect association. Usually, supplemental data are needed from studies of laboratory animals before scientists can conclude that a given factor is a cause of disease. The language of epidemiology can appear, to the uninitiated, more precise than it actually is. An odds ratio (see example below) is an estimate. Epidemiologists must calculate, along with the odds ratio, the range over which they are confident that this estimate is reliable. Sample size is a key factor in this calculation. The smaller the sample, the less reliable the information. How Epidemiologists Conduct Case-Control Studies The Process 1. A list of people with a particular disease is assembled. These are the cases. 2. A list is assembled of people who similar to the cases, but who do not have the disease. These are the controls. 3. The numbers of cases and controls who were previously exposed to factor X are estimated. This is often one of the most difficult parts of the study because exposures have often occurred many years in the past. 4. The exposure ratio of the cases is compared to that of the controls. If the ratios are the same, there is no association between factor X and the disease. If the cases have a higher ratio, there is a positive association, and factor X may be the cause of the disease. If the cases have a lower exposure ratio than the controls, there is a negative association. This would suggest that factor X may help protect people from the disease. Examples Here are 2 examples of possible outcomes of a study of a potential risk factor X, based on 300 cancer cases and 300 controls: If 71 cases were exposed to factor X and 229 were not exposed, the case exposure ratio = 71/229 = 0.31. If 71 controls were also exposed, the control exposure ratio is also 0.31. Dividing the case exposure ratio by the control ratio gives the odds ratio (OR), sometimes called the relative risk (.031/.031 = 1.00). An "OR" of 1.00 means that the odds that the cases were exposed to factor X was the same as for the controls. Therefore, in this example, there is no association between factor X and cancer. Now suppose that 110 of the total 300 cases were exposed (ratio = 110/190 = 0.58), and 71 controls were exposed (ratio = 0.31). The "OR" is 0.58/.031 = 1.87. If the "OR" is above 1.00, there is a positive association between factor X and the disease. In this example, people exposed to factor X had an 87% increased risk of having cancer. Q. What have the studies of cancer in people living near power lines found? A. To date, 14 studies have analyzed a possible association between proximity to power lines and various types of childhood cancer. Of these, eight have reported positive associations between proximity to power lines and some form(s) of cancer. Four of the 14 studies showed a statistically significant association with leukemia. The first study to report an association between power lines and cancer was conducted in 1979 in Denver by Dr. Nancy Wertheimer and Ed Leeper. They found that children who had died from cancer were 2 to 3 times more likely to have lived within 40 m (131 ft) of a high-current power line than were the other children studied. Exposure to magnetic fields was identified as a possible factor in this finding. Magnetic fields were not measured in the homes. Instead, the researchers devised a substitute method to estimate the magnetic fields produced by the power lines. The estimate was based on the size and number of power line wires and the distance between the power lines and the home (p. 34). A second Denver study in 1988, and a 1991 study in Los Angeles, also found significant associations between living near high-current power lines and childhood cancer incidence. The L.A. study found an association with leukemia but did not look at all cancers. The 1988 Denver study found an association with all cancer incidence. When leukemia was analyzed separately, the risk was elevated but not statistically significant. In neither of these two studies were the associations found to be statistically significant when magnetic fields were measured in the home and used in the analysis. Studies in Sweden (1992) and Mexico (1993) have found increased leukemia incidence for children living near transmission lines. A 1993 Danish study, like the 1988 Denver study, found an association for incidence of all childhood cancers but not specifically leukemia. A Finnish study found an association with central nervous system tumors in boys. Eight studies have examined risk of cancer for adults living near power lines. Of these, two found significant associations with cancer. The following chart summarizes results from studies involving cancer in people living near power lines. Q. Are there high cancer rates in some neighborhoods close to electric power facilities? A. Scientists call unusual occurrences of cancer in an area or in time a "cancer cluster". In some cases, a cancer cluster has served as an early warning of a health hazard. For most reports of cancer clusters, however, the cause is never determined, or the perceived cluster is not really an unusual occurrence. Concerns have been raised about seemingly high numbers of cancers in some neighborhoods and schools close to electric power facilities. In recent years, three state health departments have studied apparent cancer clusters near electric power facilities. A Connecticut study involved five cases of brain and central nervous system cancers in people living near an electrical substation. The local rates for these types of cancer were found to be no different from statewide rates. Examination of cancer rates at various distances from the substation also failed to show evidence of clustering. In North Carolina, several cases of brain cancer were identified in part of a county that included an electric power generating plant. An investigation showed that brain cancer rates in the county, however, were actually lower than statewide rates. Among staff at an elementary school near transmission lines in California, 13 cancers of various types were identified. Although this was twice the expected rate, the state investigators concluded that the cancers could have occurred by chance alone. Q. Do electrical workers have higher risks of cancer? A. Several studies have reported increased cancer risks for jobs involving work around electrical equipment. To date, it is not clear whether these risks are caused by EMFs or by other factors. A report published in 1982 by Dr. Samuel Milham was one of the first to suggest that electrical workers have a higher risk of leukemia than do workers in other occupations. The Milham study was based on death certificates from Washington state and included workers in 10 occupations assumed to have elevated exposure to EMFs. A subsequent study by Milham, published in 1990, reported elevated levels of leukemia and lymphoma among workers in aluminum smelters, which use very large amounts of electrical power. About 50 studies have now reported statistically significant increased risks for several types of cancer in occupational groups presumed to have elevated exposure to EMFs. Relative risk levels in these studies are mostly less than 2, and the possible influence of other factors such as chemicals has not been ruled out. At least 30 other studies did not find any significant cancer risks in electrical workers. Most of the earlier occupational studies did not include actual measurements of EMF exposure on the job. Instead, they used "electrical" job titles as indicators of assumed elevated exposure to EMFs. Recent studies, however, have included extensive EMF exposure assessments. For this study, the category "electrical workers" included electrical engineering technicians, electrical engineers, electricians, power line and cable workers, power station operators, telephone line workers, TV and radio repairmen, and welders and flame cutters. Q. Is there any evidence that EMF exposure increases the risk of breast cancer? A. There is some epidemiological evidence for an association between EMF exposure and breast cancer, but studies have also reported evidence to the contrary. A 1994 study (Loomis et al.) examined death records of female workers and found that women employed in electrical occupations were slightly more likely to have died of breast cancer than were other working women. However, because the study could not control for factors such diet, fertility, and family history (which are known to affect breast cancer risk), the results are considered to be preliminary, not conclusive. A 1994 Norwegian study reported an excess risk of breast cancer among female radio and telegraph operators aboard ships. A 1993 Danish study found no association between occupational EMF exposure and female breast cancer. Several studies have reported an increased risk of breast cancer among men employed in EMF-related occupations. However, the 1994 study of electrical workers in Canada and France reported no such association. Several large-scale studies are now under way in the United States and in other countries to see if women living in homes with higher EMF exposures have an increased risk of developing breast cancer. The reason for the recent interest in EMFs and breast cancer has less to do with epidemiology than with biology-laboratory evidence concerning the role of EMFs and melatonin in the development and suppression of breast cancer Q. If EMFs really do cause or promote cancer, shouldn't cancer rates have increased along with the increased use of electricity? A. Not necessarily. Although use of electricity has increased greatly over the years (right), EMF exposures have probably not increased in the same way. Changes in the way that buildings are wired and in the way electrical appliances are made have in some cases resulted in lower magnetic field levels. Rates for various types of cancer have shown both increases and decreases through the years. For example, mortality rates (deaths) for the two most common cancers in children have decreased because of better treatment. Incidence rates (numbers of new cases), however, have tended to increase for unknown reasons. Reliable data on incidence rates only became available beginning in the early 1970s.) Incidence rates can reflect changes in exposures to various environmental agents, and they are also affected by changes in how cancers are diagnosed and reported. The effect of a major cancer risk factor, like smoking, is evident in the historic lung cancer rates. The possible effect of EMFs would be mixed with those of many other factors having small or moderate risks to certain segments of the population. The individual contribution of these factors would be difficult to separate in the overall cancer rates. Q. Besides cancer, what other kinds of effects have been reported in epidemiologic studies involving EMFs? A. Several epidemiologic studies have looked for EMF effects on pregnancy outcomes and general health. Various EMF sources have been studied for possible association with miscarriage risk: power lines and substations, electric blankets and heated water beds, electric cable ceiling heat, and computer monitors or video display terminals (VDTs). Some studies have correlated EMF exposure with higher than expected miscarriage rates; others have found no such correlation. Epidemiologic studies have revealed no evidence of an association between EMF exposure and birth defects in humans. Several studies looked at the overall health of high-voltage electrical workers, and a few looked at the incidence of suicide or depression in people living near transmission lines. Results of these studies have been mixed. Some studies have also investigated the possibility that certain sensitive individuals may experience allergic-type reactions to EMFs, known as "electrosensitivity." One preliminary report released in 1994 has suggested a possible link between occupational EMF exposure and increased incidence of Alzheimer's disease. This study also found a higher incidence of Alzheimer's disease among tailors and dressmakers. At the time this booklet was produced, the research related to Alzheimer's had not been peer-reviewed or published. BIOLOGICAL STUDIES Q. What effects of EMFs have been reported in laboratory studies? A. Several kinds of biological effects have been reported in studies of electric and /or magnetic fields (see below). A biological effect is a measurable change in some biological factor. It may or may not have any bearing on health. Overall, effects attributed to EMFs have been small and difficult to reproduce. Very specific laboratory conditions are usually needed for effects of EMFs to be detected. It is not known how EMFs actually cause these effects. Laboratory studies to date have not answered questions about possible human health effects. These studies are, however, providing clues about how EMFs interact with basic biological processes. The cell membrane may be an important site of interaction with induced currents from EMFs. Keep in mind that some of these effects are within the "normal" range of variation. A biological response to a particular stimulus does not necessarily result in a negative health effect. Q. What about effects of EMFs on the hormone melatonin? A. Melatonin is a hormone produced mainly at night by the pineal, a small gland in the brain. One reason scientists are interested in melatonin is that it could help explain results of some EMF epidemiological studies. Melatonin has been reported to slow the growth of some cancer cells, including breast cancer cells, in laboratory experiments. If power frequency EMF can affect melatonin in humans, this could be a mechanism to explain results of some EMF studies of breast cancer. In the 1980s, scientists found that in rats exposed to 60-Hz electric fields, nighttime melatonin levels were reduced. Other studies have since reported that both AC and DC magnetic fields can also affect melatonin levels in rats and hamsters. These experiments are very delicate and depend on a combination of factors such as age of the animals and length of day. Melatonin levels were not affected in sheep raised for nearly a year in the EMFs directly beneath a 500-kV transmission line. Experiments with baboons also showed no changes in melatonin. The Midwest Research Institute (MRI) has studied the effect of 60-Hz magnetic field exposure on human melatonin. In 1993 MRI reported that although subjects showed no effect on the average, those individuals with naturally lower levels of melatonin did show a small further decrease. However, in 1994 MRI reported that a second study, specifically designed to replicate the earlier results, found no such effect. YOUR ENVIRONMENT At a distance of about 300 ft. at times of average electricity demand, the magnetic field from many lines can be similar to typical background EMF levels found in most homes. The distance at which the magnetic field from the line becomes indistinguishable from typical background EMFs differs for different types of lines. Neighborhood distribution lines can also sometimes produce significant magnetic fields, depending on the amount of current they carry. Q. How strong are the EMFs from electric power substations? A. In general, the strongest EMFs around the outside of a substation come from the power lines entering and leaving the station. The strength of the EMFs from transformers decreases rapidly with increasing distance. Beyond the substation fence, the EMFs produced by the equipment within the station are typically indistinguishable from background levels. Q. What typical EMF sources do I encounter when traveling? A. Inside a car or bus, the main sources of 60-Hz magnetic field exposure are those you pass by (or under) as you drive, such as power lines. Car batteries involve DC rather than AC. Alternators can create EMFs, but not at 60 Hz. Most trains are diesel-powered. Some electrically powered trains operate on AC, such as the Baltimore-Washington commuter train. Measurements taken on this train in 1991* showed 25-Hz magnetic field strengths as high as 500 mG in the passenger areas at seat height. Other trains, such as the Washington D.C. Metro and the San Francisco Bay Area Rapid Transit, run on DC, but even these trains are not free of AC fields. Areas of strong AC magnetic fields have been measured on the Washington Metro close to the floor, presumably near equipment located underneath some subway cars. Train motors and other equipment can create alternating fields at higher than 60-Hz frequencies. In addition to sources of magnetic field exposure from the train itself, train passengers are exposed to magnetic fields from sources the train passes on its route. Q. Is there something significant about the 2-mG magnetic field level? Image of residential powerlines A. A typical American home has a background magnetic field level (away from any appliances) that ranges from 0.5 mG to 4 mG, with an average value of 0.9 mG.* Most ordinary electrical appliances produce higher localized magnetic fields. Several EMF epidemiological studies have used 2 or 3 mG as a cutoff point to define broad categories of exposure. Below this level, subjects are considered "unexposed," and above this level they are considered "exposed." In some studies, a higher cancer risk was found within the exposed group. Other studies found no such increased risk. The significance of 2 mG is as a boundary to define the exposed group in some studies, not as a safety threshold. Although some experiments with cells have reported effects at field levels as low as 2 mG, there is no laboratory evidence for adverse human health effects at this level. Q. How can I find out how strong the EMFs are where I live or work? A. For specific information about EMFs from a particular power line, contact the utility that operates the line. Most utilities will conduct EMF measurements for customers at no charge. You can make your own field measurements if you have a gaussmeter, available from several companies. Independent measurement technicians will conduct EMF measurements for a fee. In some cities, they are listed in the yellow pages of the telephone book under the heading "Engineers, environmental." Gaussmeters can be easily purchased for personal use. Q. How does the magnetic field throughout my home compare with that of other homes? A. As a source of comparison with the magnetic field throughout your home, see the figure below. Image of Magnetic Field Lines (The EPRI study of 992 homes was not designed to measure people's actual exposure to magnetic fields. Instead, it focused on identifying internal and external sources of these fields in the home. Your exposure to magnetic fields depends on how much time you spend near various sources and on the strength of the fields produced by the source) This chart summarizes data from a study by the Electric Power Research Institute (EPRI) in which spot measurements of magnetic fields were made in the center of rooms in 992 homes throughout the United States. Half of the homes studied had magnetic field measurements of 0.6 mG or less, when the average of measurements from all the rooms in the home was calculated (the all-room mean magnetic field). The all-room mean magnetic field for all homes studied was 0.9mG. Only 15% of the homes had mean magnetic fields greater than 2.1 mG. The measurements were made away from electrical appliances and primarily reflect the fields from outside power lines, household wiring, and electrical grounding sources. Q. Is it safe to live close to a transmission line? A. Living close to a transmission line can increase your overall exposure to EMFs. As discussed earlier, despite research findings to the contrary, government health or safety organizations worldwide have reportedly refused to conclude that EMFs cause cancer or other health effects. Question Graphic It is generally acknowledged that several studies have reported increased cancer risks, especially for children living close to high-current power lines. Although these studies suggest potential risks, scientists do not yet know whether EMFs, other factors, or methodological problems are responsible for their findings. It is possible that future studies will provide sufficient information to establish whether EMFs are a hazard to human health. The newer studies may also show that factors other than EMFs were responsible for effects reported in earlier studies. It is also possible that, even with more research, there will be no scientific resolution to the EMF issue in the near future. The answer to this question, therefore, involves (1) a great deal of judgment about the meaning of existing scientific evidence, (2) speculation about the possible results of future studies, and (3) individual perceptions about the relative importance of various potential health risks. During this period of uncertainty, there are some things that you can do to help answer this question. 1. Follow the EMF issue by reading various sources and talking with people who are working to resolve the issue. The EMF RAPID Program in cooperation with the U.S. Environmental Protection Agency, provides a toll-free public information telephone line to answer EMF-related questions and direct callers to further sources of information. Q. What can be done to limit EMF exposures? A. There are a number of ways to reduce exposures to EMFs. Some are as easy as standing back from an appliance when it is in use. Remember that magnetic fields from appliances drop off dramatically in strength with increased distance from the source. Other EMF reduction steps, such as correcting a household wiring problem, are worth doing anyway for safety reasons. But what about more costly actions, such as burying power lines or moving out of a home? Because scientists are still debating whether EMFs are a hazard to health, it is not clear how much should be done at this time to reduce exposures. Some EMF reduction measures may create other problems. For instance, compacting power lines to reduce EMFs can increase the danger of accidental electrocution for line workers. A concerted effort to provide scientifically valid research on which to base decisions about EMF exposures is under way, and results are expected in the next several years. Meanwhile, some authorities recommend taking simple precautionary steps, such as the following: * Increase the distance between yourself and the EMF source-sit at arm's length from your computer terminal. * Avoid unnecessary proximity to high EMF sources-don't let children play directly under power lines or on top of power transformers for underground lines. * Reduce time spent in the field-turn off your computer monitor and other electrical appliances when you aren't using them. Gaussmeter Q. Are products advertised as having reduced magnetic fields legitimate? A. This question must be answered appliance by appliance, depending on the claims of the manufacturer. According to the U.S. Food and Drug Administration's Center for Devices and Radiological Health, "low magnetic field" electric blankets do produce significantly lower magnetic fields than older versions because of wiring redesign. It is advisable, however, to be cautious about product claims that sound too good to be true, given the complexity of the EMF issue. |