General Information;  Power Lines;  Substations;  Transformers;  Appliances;  Trains;  Field Measurement

GENERAL INFORMATION

The United States electric power system operates at 60 cycles a second or hertz (Hz). This means that the electric charges (current) flowing in the system change direction 60 times a second. This changing of direction of current is called alternating current or AC.  AC fields induce weak electrical currents in conducting objects, including humans, and have been the focus for research on how electric and magnetic fields could affect human health.

Electric and magnetic fields have different properties. Electric fields are easily shielded or weakened by conducting objects (trees, buildings, etc.) but magnetic fields are not. Both electric and magnetic fields diminish with increasing distance from the source. Magnetic fields are typically measured in gauss, a unit of magnetic field strength or magnetic flux density. 1,000 milligauss (mG) = 1 gauss. Recent interest and research have focused on the potential health effects of magnetic fields. Some epidemiological studies have suggested that a link may exist between exposure to these fields and certain types of cancer, primarily leukemia and brain cancer. Other studies have found no such link. Laboratory researchers are studying how such an association is biologically possible. At this point, there is no scientific consensus about this issue, except a general agreement that there is a cause for concern and that more information is needed. A national research effort is under way, and major study results are expected in the next few years.

It is not know at this point whether exposure to magnetic fields from power frequency sources constitutes a health hazard. Therefore, it can not be determined what levels of exposure are "safe" or "unsafe". Some studies have shown that exposure to higher levels of this radiation is not necessarily worse than exposure to lower levels. More research is required to identify dose-response relationships. There is some evidence from laboratory studies to suggest that there may be "windows" for effects. This means that biological effects are observed at some frequencies and intensities but not at others. Also, it is not known if continuous exposure to a given field intensity causes a biological effect, or if repeatedly entering and exiting of the field causes effects. In light of all this uncertainty, it is impossible to say what is a "safe" distance from any magnetic field source or what is a "safe" exposure. The only thing possible at this point in time is to make comparisons. For instance, the typical home has a background magnetic field level (away from appliances) that ranges from 0.1 to 4.0 milligauss. Although some experiments with cells have reported effects at field levels as low as 2 milligauss, there is no laboratory evidence for adverse human health effects at this level.

POWERLINES  (To the Top)

Transmission and distribution lines can be collectively referred to as power lines. Magnetic field levels from power lines will be determined by the amount of current flowing through the line, the arrangement and proximity of the lines themselves with respect to each other, the height of the line above the ground, and the proximity of the lines to other power lines.

Transmission lines carry electricity over long distances and usually operate at voltages of 100 kilovolts and above. For any transmission line in New Jersey, at a perpendicular distance of 400 feet from the center of the line configuration, the magnetic field level on the ground from the line will be approximately 1 milligauss or less. At distances closer than 400 feet, it is difficult to predict what the magnetic field level will be as each situation becomes unique to that particular line. Some transmission lines carry very little current and expose people to lower magnetic field levels than what they would encounter from a distribution line. Measurements made by Department staff under transmission lines in New Jersey have ranged between 8 - 130 milligauss. In general, fields from both transmission lines and distribution lines will vary, depending on the time of day, the day of the week, the time of year and the ambient temperature. However, for transmission lines, magnetic fields will rarely vary by more than a factor of two.

Distribution lines operate at lower voltages and bring power from substations to businesses and homes. Distribution lines may expose people to magnetic field levels as high or higher than transmission lines. This is because they are physically closer to the ground than transmission lines. For this same reason, distribution lines that are buried underground can sometimes expose one to a higher magnetic field if one is standing directly over top of them than what one would receive from the same line mounted overhead on a pole. The Department has received information from electrical utilities in New Jersey that some underground distribution lines operating at a voltage of 69 kilovolts may produce magnetic field levels as high as 55 milligauss directly above the line. 50 feet from the center of the line, this level drops to 1 milligauss. In general, most magnetic fields from distribution lines will be a lot lower and may even be as low as 1 milligauss, directly above or below the line.

SUBSTATIONS  (To the Top)

Electrical substations serve many functions in controlling and transferring power on an electrical system. Substations may utilize transmission lines, distribution lines or a combination of both. In general, the strongest magnetic fields around the outside of the substation comes from the power lines entering and leaving the station. While transformers inside the substation can produce high magnetic fields, the fields remain localized around the transformers. Beyond the substation fence, the magnetic fields produced by the equipment within the station are typically indistinguishable from background levels.

TRANSFORMERS  (To the Top)

Transformers are electrical devices used to adjust the voltage-current relationship of an electrical power circuit for best efficiency during transmission and distribution use. There are electric and magnetic fields near a transformer and around the lines that connect to them. But the fields tend to drop off rapidly as one moves away from the transformer. Utilities use a variety of transformers throughout their systems. Step-up transformers are used at the power generating station to raise the voltage so the power can be economically delivered over transmission lines. The magnetic fields from these types of transformers are high but localized and to do not travel beyond the bounds of the substation. Step-down transformers are used to reduce line voltages.

Overhead (pole-mounted) transformers are used where distribution lines are overhead and surface (pad-mounted) transformers are used where distribution lines are underground. Frequently in urban situations, transformers can be located within buildings. If the transformer is what is referred to as a network transformer, which can supply power to an entire block, magnetic fields on the floor directly above the transformer can be as high as 700 milligauss. Since magnetic fields remain localized around the transformer itself, a pole mounted transformer will have very little impact on ground level magnetic fields, which will be dominated by the overhead distribution lines coming in and going out of the transformer.

Pad mounted transformers have magnetic fields similar in intensity to kitchen appliances. The magnetic fields near this type of transformer are elevated close to the surface of the transformer. A few feet away, the levels drop off to background.

 

 

APPLIANCES  (To the Top)

Appliances that operate either on batteries or by plugging into the household wiring usually come equipped with an AC/DC switch. If DC is chosen, current flows one way from the batteries to the appliance. DC fields, unlike AC fields, do not induce electrical currents in humans unless the DC field changes in space or time relative to the person in the field. In most situations, a battery operated appliance is unlikely to induce electrical current in the person using the appliance.

In general, appliances using AC have potentially high, localized magnetic fields that decrease rapidly with distance. Magnetic fields from appliances are often stronger than the fields directly beneath power lines. The intensity of the magnetic field from an appliance appears to be related to product function and design. Here are some examples of average magnetic field levels 6 inches away from certain appliances:

  • Hair dryer - 300 milligauss
  • Electric shaver - 100 milligauss
  • Blender - 70 milligauss
  • Can opener - 600 milligauss
  • Coffee maker - 7 milligauss
  • Microwave oven - 200 milligauss
  • Color TV (1 foot away) - 7 milligauss

An example of how rapidly magnetic fields from appliances drop over distance:

Can opener:

  1. At 6 inches - 600 milligauss
  2. At 1 foot - 150 milligauss
  3. At 2 feet - 20 milligauss
  4. At 3 feet - 2 milligauss

TRAINS  (To the Top)

Some trains run on AC while others use DC. Some trains that use AC operate at 25 or 16.75 hertz. Very little is known about the biological effects from 25 or 16.75 hertz AC or DC. Areas of strong AC magnetic fields have been measured close to the floor on some DC trains. Magnetic fields measured in trains powered by 60 hertz AC have been reported to be as high as 500 milligauss in the passenger areas at seat height. Department staff have not made any measurements on train lines.

MEASURING MAGNETIC AND ELECTRIC FIELDS  (To the Top)

The measurement of electric and magnetic fields from nonionizing radiation sources is a complex task. The Department does not have a certification program for testing or measurement firms and therefore, cannot endorse any such companies. The best way to obtain accurate readings of 60 hertz electric and magnetic fields is to contact the owner (electric utility) of the power lines in question and request that measurements be made.  Please be aware that the utility supplying power to the house may not be the same utility that owns the high voltage transmission lines running by the property of interest. Utility personnel have been trained in this area of expertise and will probably provide the most accurate readings. They are usually reluctant to interpret any readings although they may try to put them into perspective. Anyone having any questions regarding measurements should contact the Department. As a quick rule of thumb, typical magnetic field levels found in homes range from 0.1 - 4.0 milligauss. Any readings above that are not necessarily hazardous, but higher. It is not necessary to obtain readings if a power line is more than 400 feet away from the home or area of interest.

For more information on this topic, please visit the following Internet areas: Please read this Disclaimer prior to connecting to these websites.

Power Lines and Cancer: FAQ's from John E. Moulder, PH.D. of the Medical College of Wisconsin

FCC OET Bulletin No. 56 (Q&A about Biological Effects and Potential Hazards of Radiofrequency Radiation)

Back to Nonionizing Radiaiton Sections Main Page


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Department of Environmental Protection
P. O. Box 402
Trenton, NJ 08625-0402