cycle of increased loading begins again. If the wire configurations remain unchanged over the course of a particular epidemiologic study, contemporary magnetic-field measurements might be somewhat higher than historical measurements, and the absolute magnetic-field cut points between wire-code categories would be somewhat higher than they would have been at the time of disease etiology. This trend causes no problem for the long-term validity of wire codes if the relative discriminating ability of wire codes is consistent, a point which could be examined.
There is a tendency, seen in the 1,000-homes study (EPRI 1993a) and elsewhere, for older homes to have higher wire codes, higher power-line fields and higher total fields. Possible reasons for such a trend could be that most homes built over 50 years ago and 7.1% of homes built 30-50 years ago have knob and tube wiring (which can cause higher in-home fields); none of the homes built less than 30 years ago had knob and tube wiring (EPRI 1993a). In addition, homes seem to accumulate a larger number of wiring errors as they age; wiring practices in older homes might have permitted more wiring irregularities; or power lines might be more fully loaded in older neighborhoods.
Assuming that the trend is correct, it could be interpreted to mean that in the historical period, when homes were newer, a greater spread in the magnetic fields was represented by the wiring codes. Therefore, wire codes might have somewhat greater discriminating power when applied to a historical period rather than a current period, a point which should be examined.
A serious barrier to acceptance of a possible weak connection between human health and exposure to extremely-low-frequency electric and magnetic fields in residences is the absence of a plausible physical mechanism to account for such a connection. Because any biologic effect of exposure to electric or magnetic fields at residential field strengths is, at best, at the margins of detectability, laboratory experiments to identify such a mechanism at those field strengths appear not to be feasible. However, at higher magnetic-field strengths, there appear to be genuine biologic effects, which are accessible to experiment. Mechanistic studies at high field strengths, accompanied by attention to dose-response relationships, might illuminate the situation at lower field strengths.
Laboratory experiments must be designed with appropriate positive and negative controls, conducted under blind or double-blind conditions as appropriate, and described in sufficient detail to permit independent replication. The committee found that a large fraction of published reports failed to meet those criteria.
The areas of biologic research that seem most likely to be productive include the following: