Experiments that appear to show an effect but have no biologic or physical explanation might be given less weight. In weighing the evidence, some assessors might weigh evidence differently than others in reaching their conclusions.

At the end of the risk-assessment process, the body of evidence is weighed together to reach an overall assessment of a possible hazard. If the results from several areas of research (e.g., epidemiologic studies, tests in cell systems, or whole-animal studies) are consistent and have been replicated and if a biologically plausible mechanism of action for the effect is evident, the evidence for the effect is given great weight. In contrast, a body of evidence that includes inconsistent and conflicting results, no replication of results, and effects that are often at the threshold of detection might be given little weight in reaching a conclusion.

In this report, the committee attempts to explain carefully why individual studies are given more or less weight and how the body of evidence is weighed in reaching its conclusions.

SOURCES OF EXPOSURE

Electric Power Lines

Electric transmission lines are generally built on "rights-of-way" and vary in voltage, height, and configuration of suspension from the towers. Electric power lines commonly observed in typical neighborhoods are not transmission lines as described above, but are lower-voltage distribution lines. The magnetic fields immediately under distribution lines are generally of the order of 0.5 µT (5 mG), although in some densely populated areas, fields as high as 5.0 µT (50 mG) have been measured. The fields decrease rapidly as the distances from the power lines increase.

Electric substations are installations where the voltages used with transmission lines are stepped down to lower voltages used with distribution lines. Electric and magnetic fields produced by substation equipment are generally not appreciable beyond the substation boundaries, but the fields can be somewhat stronger near them than in other parts of the neighborhood, because the power lines converge at the substation and might be closer to the ground as they go in and out of the substation.

Electric Appliances

Electric appliances in residences and workplaces are all potential sources of exposure to electric and magnetic fields. The strengths of magnetic fields vary widely: the magnetic fields from household appliances might be as low as a few tenths of a microtesla or as high as 150 µT (1,500 mG). The magnetic fields from all sources tend to decrease rapidly as the distance from the source increases. It has been noted that the earth's natural static magnetic field of about 50 µT (500 mG)



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