Hazard surveillance for industrial magnetic fields: I. Walkthrough survey of ambient fields and sources.

A walkthrough survey method was developed for measuring ambient magnetic fields (MFs) in industrial facilities as the first stage in hazard surveillance. This survey was designed to measure the mean and peak MF magnitudes at extremely low frequencies (ELFs), so that factories could be ranked by MF levels and prioritized for subsequent personal exposure monitoring. Sixty-two facilities from 13 Standard Industrial Classifications (SICs) with the highest monthly electric power usage were surveyed. To measure ambient MFs, a structured walkthrough survey with a special emphasis on workstations was conducted with an EMDEX-II meter in continuous operation, while MF sources were noted. The broadband MF data (40-800Hz) for each facility were summarized with the geometric mean (GM) and the average of the five highest readings (Hi-5). The range of the GM magnetic field magnitude was 0.04-1.61microT, where the maximum was measured at a steel mill operating large electric furnaces. Maximum values for specific sources were highly variable across and within facilities (Hi-5 range: 1.0-530 microT). Chemical and Allied Products (SIC 28) and Primary Metal Products (SIC 33) had facilities with GM and Hi-5 magnetic fields greater than any of the other industrial categories. However, the SIC categories were found to be poor predictors of the ambient MF in this sample of factories. A weak relationship was found between the facility-specific monthly electric power consumption and the GM magnetic field magnitude, but confidence limits were too broad to make meaningful exposure predictions from electric power data. Overall, 89% of the GMs were at or below 0.4 microT, consistent with most other studies that collected industrial MF exposure data. The walkthrough survey is a practical way of measuring ambient MFs in a large number of workplaces, and should be evaluated with personal measurements as a screening method for hazard surveillance.