and use of electricity that might, by themselves (ignoring for a moment the trends in electricity usage), decrease individual exposure. The more extensive use over the past 20-30 years of three-phase transmission and distribution lines, double-circuit lines, and underground lines substantially reduces electric and magnetic fields in comparison to earlier use of single-phase and single-circuit overhead lines. In addition, the more spacious layout of suburban (in comparison to urban) house lots has moved residents farther from power lines. The change to Romex cable for wiring houses (instead of knob and tube wiring) also substantially reduces exposure of residents. The practice of making things smaller and more energy efficient (e.g., home radios and televisions) further reduces exposure. Even the recent proliferation in the use of remote control devices tends to reduce individual exposures by making it unnecessary for individuals to move near the appliances.

On the other hand, power lines, houses, and appliances are more numerous today. The residential use of electricity has certainly increased over the years, as documented above. Because of the developments in technology, it would not be surprising if there were periods (perhaps 5, 10, or 20 years long) when exposures tended to increase and other periods when average exposures actually decreased.

In summary, large increases in the residential use of electricity clearly have not been accompanied by comparable increases in the incidence of childhood leukemia. The apparent persuasiveness of the argument based on the observation that magnetic-field exposures are not related to the incidence of childhood leukemia is diminished, however, by the large and indeterminate uncertainties in the implicit assumption that population exposures to magnetic fields increase proportionately to residential energy use.

The determination of an effective exposure of a population to power-frequency electric and magnetic fields is an extremely difficult task because such fields are ubiquitous and exhibit considerable variation with time and place. At the same time, there are no clear or agreed upon indications of which metric or field characteristics might be most closely associated with biologic effects.

A number of epidemiologic studies of the possible association between magnetic-field exposures and various disease outcomes have explored exposures ascribed to the use of various electric appliances. If such associations are to be explored successfully, it is desirable to consider electric appliances that are capable of producing substantial exposures, that are not in universal use, and can be assessed for exposure on the basis of subject recall of use. Mader and Peralta (1992) evaluated the reconstruction of such exposures due to appliances and compared those exposures with residential exposures due to other sources. They concluded that the time-weighted contribution to whole-body exposure