Hazard surveillance for industrial magnetic fields: II. Field characteristics from waveform measurements

Abstract

Magnetic field characteristics have been surveyed systematically in six factories with the Multiwave® II waveform capture instrument. These six facilities manufactured plastics, pharmaceuticals, cement, liquid air products, aluminum parts, and aluminum-framed filters. The study goals were to survey the physical characteristics of magnetic fields that may be related to biological effects under various interaction mechanisms and to relate those characteristics to the field's sources. From 59 waveform measurements at worker locations near sources, we calculated the extremely low frequency (ELF) and static field magnitudes, their frequency characteristics, and spatial characteristics of the 60Hz component. The RMS vector magnitude of the ELF magnetic field (the usual exposure metric in most studies) had medians ranging from 0.53 to 12.83μT in the six factories. The static magnetic field magnitudes had medians of 24.2–46.2μT, which is well below the geomagnetic reference field of 55.0μT because of shielding from steel structures. The maximum static field was 128.6μT near a DC motor. The frequency spectra of the most common fields is dominated by 60Hz, and has a median total harmonic distortion equal to 14.8%. The most common higher frequencies are the third, fifth, and second harmonics of 60Hz. However, magnetic fields in these workplaces had many other 60Hz harmonics and non-harmonic frequencies due particularly to electric motors and computer monitors. The 60Hz component magnetic fields have elliptical polarization with median axial ratio of 25.4%. The average proportion of the 60Hz component parallel to the static field vector was 51.5±3.0%, which indicates a significant trend towards perpendicular orientation between these two field components. In this survey of only six factories, the Multiwave® II measurements documented a wide diversity of complex magnetic field characteristics and non-sinusoidal waveforms. Although these characteristics are important to the various mechanisms postulated to explain biological effects, they are overlooked by the popular exposure assessment methods which only measure the ELF magnitude. Therefore, spot measurements with the Multiwave® II or similar waveform capture instruments are necessary for a complete magnetic field exposure assessment.