Measuring temporal variability in residential magnetic field exposures

Considerable interest has developed during the past ten years regarding the hypothesis that living organisms may respond to temporal variability in EL F magnetic fields to which they are exposed. Consequently, methods to measu re various aspects of temporal variability are of interest. In this paper, five measures of temporal variability were examined: Arithmetic means (D-me an) and rms values (D-rms) of the first differences (i.e., absolute value o f the difference between consecutive measurements) of magnetic field record ings; "standardized" forms of D-rms, denoted RCMS, obtained by dividing D-r ms by the standard deviations of the magnetic field data; and mean (F-mean) and rms (F-rms) values of fractional first differences. Theoretical invest igations showed that D-mean and D-rms are virtually unaffected by long-term systematic trends (changes) in exposure. These measures thus provide rathe r specific measures of shea-term temporal variability. This was also true t o a lesser extent for F-mean and F-rms. In contrast, the RCMS metric was af fected by both shortterm and long-term exposure variabilities. The metrics were also investigated using a data set consisting of twice-repeated two-ca lendar-day recordings of bedroom magnetic fields and personal exposures of 203 women residing in the western portion of Washington State. The predomin ant source of short-term temporal variability in magnetic field exposures a rose from the movement of subjects through spatially varying magnetic field s. Spearman correlations between TWA bedroom magnetic fields or TWA persona l exposures and five measures of temporal variability were relatively low. Weak to moderate levels of correlation were observed between temporal varia bility measured during two different sessions separated in time by 3 or 6 m onths. We conclude that first difference and fractional difference metrics provide specific and fairly independent measures of short-term temporal var iability. The RCMS metric does not provide an easily interpreted measure of short-term or long-term temporal variability. This last result raises unce rtainties about the interpretation of published studies that use the RCMS m etric. (C) 2001 Wiley-Liss, Inc.