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? 
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.
The Swedish study (discussed under Human Health Studies) suggested a dose/response relationship for EMF exposure: The higher the estimated magnetic field exposure, the higher the cancer risk. To deduce from the Swedish study, however, that 2 mG is some sort of safety threshold is to read far too much into the data. The Swedish government has so far concluded that current knowledge does not provide sufficient basis for setting exposure limits (see Government Actions).
*This estimate is based on the EPRI study of 992 homes. An average magnetic field measurement was calculated for each home, based on measurements conducted in each room. The average of the 992 individual “home averages” was 0.9 mG.
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.
(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.
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.
- 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.
The EMF “Infoline” number is 1-800-363-2383. (In Washington D.C., call 484-1803.) - Find out about the EMF levels produced by the particular source that concerns you. If the source is a power line, you may be far enough away that EMF levels are negligible.
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.
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.
