Alternative Magnetic Field Exposure Metrics: Occupational Measurements in Trolley Workers

Abstract

Epidemiological studies on extremely low frequency (ELF) magnetic fields have widely used personal or area sampling to evaluate exposures based on the time-weighted averaged flux density magnitude (TWA field). Relatively few studies have evaluated 'alternative' exposure metrics related to field characteristics such as temporal variability, frequency harmonics, vector polarisation, spatial orientation, static fields, high frequency transients, or induced electric fields. These field attributes fall into three major categories: (1) temporal characteristics of exposure intensity and timing, (2) frequency-domain characteristics, (3) spatial characteristics. The first category describes the magnitude and time history of exposure, including the TWA field metric, which most often is the focus of MF exposure assessment. The second category depicts the waveform characteristic (harmonic content), which has been relatively poorly described in most studies. The third category describes the field vector's time-space orientation and relation to static fields. Some examples of 'alternative metrics' that have been proposed based on hiological mechanisms and potential measurement techniques are examined. The limited correlation of some alternative metrics with the TWA field metric in available data suggests that substantial exposure misclassification could occur if measurement protocols only focus on average field levels.