is about 100 times stronger than residential magnetic fields normally associated with the alternating current (ac) of power lines and electric appliances; exposure to the earth's field is constant, and exposure to constructed alternating fields is intermittent. However, that comparison might be of little relevance, because it is not known which aspect of the magnetic fields could be of significance to health.

Transportation Systems

Some transportation systems, including subways and intercity trains, operate on ac current and generate ac electric and magnetic fields. Measurements in the Baltimore-Washington commuter trains indicate exposures to magnetic fields at 25 Hz with peak strengths as large as 50 µT (500 mG) in the passenger areas at seat height. The fields vary greatly with the position in the car as well as with the particular type and model of the car; detailed measurements indicate an average field of approximately 12.5 µT (125 mG).

WHY KNOWLEDGE ABOUT ELECTRIC AND MAGNETIC FIELDS IS IMPORTANT

Because electricity is used so extensively and sources of electric and magnetic fields are everywhere, every person in modern society is unavoidably exposed to them. Thus, understanding any biologic effects that might be associated with exposure to electric and magnetic fields is fundamentally important.

It is easy to see why so much attention has been given to the possibility that power-frequency electric and magnetic fields are associated with adverse effects. People who study how individuals respond to risk have learned that certain types of risks elicit stronger responses than others (Slovic 1987). One of the health effects that has been associated with exposure to electric and magnetic fields is an especially dreaded one, namely, cancer. Children, a group of particular concern, are a reported target for leukemia and possible reproductive and behavioral effects. The sources of the reported electric-and magnetic-field risks are largely imposed on people and not under their control. Furthermore, the fields that are the source of the reported risks are invisible and mysterious to many. All these factors cause many people to respond with concern and anxiety to potential risks associated with exposure to electric and magnetic fields (MacGregor et al. 1994).

When such an indispensable resource as electricity is reported to be associated with adverse health effects, it is not difficult to understand why concerns have arisen. It is also apparent that the potential health effects are only one part of the concern. If extreme steps are taken to reduce exposure to power-frequency electric and magnetic fields, large sums of money will need to be expended (e.g., to bury transmission and distribution lines, to redesign residential wiring and electric appliances, or to retrofit existing ones). Every citizen would contribute to the implementation of these measures through higher utility bills and greater



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