The Use of ‘Spot’ Measurements in Epidemiological Studies of the Health Effects of Magnetic Field Exposure

Abstract

In several countries, epidemiological studies are being planned, or are in progress, to test the hypothesis that a causal relation exists between exposure to extremely low frequency magnetic fields and cancer incidence. One of the major difficulties in these studies is the development of valid and efficient protocols to assess magnetic field exposure. In studies focusing on residential magnetic fields, many researchers are turning to recently developed stationary automated magnetic field monitors to characterize exposure. We argue that a relatively small number of manually collected 'spot' measurements may be an adequate alternative which has several advantages. We compared a dichotomous exposure classification based on continuous magnetic field monitoring of 40 houses with that obtained through 'spot' measurements randomly sampled from the continuous records. We found that a single spot measurement had at least an 80% chance of classifying houses correctly and that this probability did not increase significantly as the number of readings was increased. We also calculated the sensitivity and specificity of various simulated measurement protocols and, from these, the effect of misclassification on estimates of relative risk. Since relatively large spatial variations in background magnetic field exist in many homes, we suggest that a small number of readings collected manually at several points within a residence may characterize the magnetic field better than continuous monitoring at one fixed location.

The determinants of Canadian children's personal exposures to magnetic fields

“…Personal exposure measures are expensive and, if the etiologically relevant period is years before the adverse effect, often impossible. Proxy measures based on known determinants of personal exposure are often available, but although the reliability of spot vs. continuous area magnetic ®eld measurement has been examined [Delpizzo, 1991], as has the association between wire code and 24 h bedroom measurement [Tarone et al, 1998], the relative or absolute validity of each is little known. There have been two previous studies examining the relationship between spot area measurements and/or 24 h measurements and personal exposure measurements, but both were based on relatively small numbers Friedman et al, 1996].…”

Use of short-term measurements for assessing temporal variability of residential ELF magnetic field exposure

“…Results on the validity of short-term measurements for estimating long-term time-average exposure have been published (Delpizzo et al, 1991;London et al, 1991;Delpizzo and Salzberg, 1992;Yost et al, 1992;Kaune and Zaffanella, 1994;Schuz et al, 2000, but not much is known about the usability of short-term measurements for estimating other exposure metrics. It has been shown that residential point-intime ''spot'' measurements can be used to estimate (in addition to time-average MF) time spent above an MF threshold (Juutilainen et al, 1996;Eskelinen et al, 2002).…”

Use of spot measurements for assessing residential ELF magnetic field exposure: A validity study

“…This study was designed as a``validity study'' [Armstrong et al, 1995] to evaluate the quality of exposure assessment of a study on maternal exposure to residential 50 Hz magnetic ®eld (MF), conception delay and birth weight, which used spot measurements for exposure assessment. Results on the validity of spot measurements have been published [Delpizzo et al, 1991;London et al, 1991;Delpizzo and Salzberg, 1992;Yost et al, 1992;Kaune and Zaffanella, 1994;Auvinen et al, 2000;Schuz et al, 2000], but there is limited understanding of the applicability of the results in different areas with varying electric wiring practices and climates.…”