by wire codes but might be associated with lower socioeconomic status and higher exposure.

Residential Mobility High wire codes in Columbus, Ohio (Jones et al. 1993) have also been associated with homes in which residents are more mobile. In Columbus, the inner city, the oldest region, had the highest proportion of high wire codes, and the suburbs, the newest region, had the lowest proportion of high wire codes. Low electric-energy consumption was also associated with high wire codes (Jones et al. 1993).

Age of Home Older homes tend to have higher wire codes, higher power-line fields, and higher total fields than new homes (EPRI 1993a; Bracken et al. 1992). This information is counter to the argument that average residential exposures have increased over time because of increased use of electricity (Jackson 1992; ORAU 1992). Also, within each wire-code category, the homes with the lowest magnetic fields tend to be the newest homes (less than 30 years old), and except for the very-high-current-configuration (VHCC) category, homes with the highest magnetic fields tend to be the oldest homes (more than 30 years old). Homes with VHCC, regardless of age, are likely to have fields above 0.1 µT (1 mG) (EPRI 1993a). Jones et al. (1993) reported that the inner city of Columbus, Ohio, has the highest proportion of high wire codes, and the suburbs have the highest proportion of low wire codes.

A possible reason for the trend of increasing exposure with increasing age of home could be a greater prevalence of knob and tube wiring in older homes, a practice that can cause higher magnetic fields in homes. Homes newer than 30 years old had no such wiring, but 28.8% of homes over 50 years old and 7.1% of those 30-50 year old did (EPRI 1993a). Homes might also accumulate a larger number of wiring errors as they age, and wiring practices in older homes might have permitted more wiring irregularities. Power lines might be more fully loaded in older neighborhoods. The lines are designed to accommodate the maximum capacity needed, and that might be reached more often in older neighborhoods than newer neighborhoods. Finally, the population movement out of central cities, with small yards and greater housing density, to suburban areas with greater average distance from power lines to residences, might result in reduced wire codes and reduced in-home exposures.

Because older homes tend to fall into the high-wire-code category more often than new homes, confounding might be introduced if a real or imposed disparity exists between the ages of homes of cases and controls. Wertheimer and Leeper (1980) noted that the Fulton et al. (1980) study design, which included dates of birth and diagnosis, generated more recent average occupancy dates for cases than for controls.4 More recently constructed and occupied homes will


 Homes at the time of diagnosis were provided for cases, but only homes at the time of birth were available for controls.

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