February 9, 2012
TECHNOS QUARTERLY Winter 1994 Vol. 3 No. 4
Q & A on EMF
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Q: What are electric and magnetic fields, or EMFs?
A: Power lines, electrical wiring, and appliances all produce AC electric and magnetic fields. EMFs are invisible lines of force that exist in the area surrounding any electrical device. They are measured in gauss or tesla and are not easily shielded or weakened by most material. EMFs are reduced in strength with increasing distance from the source.
Q: How do electric power EMFs compare to other types of fields?
A: The electromagnetic spectrum covers an enormous range of frequencies, which are expressed in cycles per second, called Hertz, or Hz. Electric power (60 Hz in North America, 50 Hz elsewhere) is in the extremely low-frequency, or ELF, range, which includes frequencies below 3,000 Hz. In general, the higher the frequency, the greater the amount of energy in the field.
Q: How do EMFs from transmission lines, appliances, and video display screens compare?
A: Electric fields close to transmission lines are much stronger than the fields found near electrical appliances, but they are reduced in strength by objects such as buildings, trees, and vehicles. Magnetic fields very close to electrical appliances are often stronger than the fields directly beneath power lines, but they decrease in strength with distance more quickly than do power-line fields. Some appliances also produce both 60-Hz and higher-frequency fields. For example, video displays such as TV sets and video display terminals, or VDTs, used with computers produce radio-frequency fields as well as ELF fields.
Q: Are there any standards for 60-Hz EMFs?
A: In the United States, there are no federal health standards specifically for 60-Hz EMFs. A professional society, the American Conference of Governmental Industrial Hygienists has developed “Threshold Limit Values” for EMFs that include 60 Hz. These are guidelines for use by industrial hygienists in occupational settings and are not considered a line between safe and unsafe EMF levels. At least six states have set standards for transmission-line electric fields; two of these also have standards for magnetic fields. Interim guidelines on limits of exposure to 50/60-Hz EMFs have been developed by the International Commission on Non-Ionizing Radiation Protection. These limits are based on established effects of EMFs, such as nerve stimulation, and they are much higher than EMF levels found typically in occupational and residential environments.
Q: What happens when a person is exposed to EMFs?
A: Strong electric fields, such as those found beneath large transmission lines, can cause the hair on a person's head to vibrate at 60 Hz. This is felt by some people as a tingling sensation. EMFs from transmission lines can also cause nuisance shocks from voltages created by EMFs on objects like metal fences. 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. Currents from electric and magnetic fields are distributed differently within the body. The amount of this current, even if a person is beneath a large transmission line, is extremely small—millionths of an ampere. The current, which is too weak to penetrate cell membranes, is present mostly between the cells. 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. For this reason, some scientists argue that it is impossible for EMFs to have any important effects. But others point to results of studies that show that biological effects can be caused by exposure to EMFs. In most cases, however, it is not clear how EMFs actually produce these demonstrated effects.
Q: What can be done to limit EMF exposure?
A: Individuals and organizations can reduce exposure to EMFs in a number of ways, some of which are not expensive. The simplest way is to increase the distance from the source of EMFs, such as a student's computer monitor. Another way is to design a source to shield or cancel the field, such as a screen over the monitor. Time spent in the field can be reduced.
—from Electric Power Lines: Questions and Answers on Research into Health Effects, published by Bonneville Power Administration, May 1994
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