Personal power-frequency magnetic field exposure in women recruited at an infertility clinic: association with physical activity and temporal variability

Abstract

Current epidemiologic approaches for studying exposure to power-frequency magnetic fields and the risk of miscarriage are potentially biased due to lack of attention to the relationship of exposure with physical activity and within-individual variability in exposures over time. This analysis examines these two issues using data from a longitudinal pilot study of 40 women recruited from an infertility clinic that contributed data for up to three 24-h periods separated by a median of 3.6 weeks. Physical activity was positively associated with peak exposure metrics. Higher physical activity within environments did not necessarily lead to higher peak exposures, suggesting that movement between and not within environments increases one's probability of encountering a high field source. Peak compared with central tendency metrics were more variable over time. Future epidemiology studies associated with peak exposure metrics should adjust for physical activity and collect more than 1 d of exposure measurement to reduce bias.

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Figures

Figure 1.

Variation in daily personal magnetic field exposure metrics separated by several weeks for a randomly selected subset of the same 10 participants.

Figure 2.

Personal magnetic field exposure over a 24-h sampling period for two representative data sets from two participants with low and high average counts, respectively, relative to the entire data set.

Figure 3.

Change in log-transformed daily personal magnetic field exposure metrics associated with tertiles of daily average counts. Derived from linear mixed models with one random effect as the random intercept for each subject and fixed effects for tertiles of daily average counts. Low = 26 sampling days from 19 women, medium = 25 sampling days from 20 women and high = 26 sampling days from 19 women.

Figure 4.

Change in log-transformed daily personal magnetic field exposure metrics associated with the daily total number of changes in environments experienced. Derived from linear mixed models with one random effect as the random intercept for each subject and fixed effects for categories of the daily total number of changes in environments experienced. Less than 6 changes = 21 sampling days from 14 women, 6–9 changes = 34 sampling days from 23 women and ≥10 changes = 34 sampling days from 21 women.