Occupational exposure to low frequency magnetic fields in health care facilities

A characterization of health care workers' exposures to magnetic fields in the 40-1000 Hz range was performed in a multistory research hospital and single-story outpatient surgery unit. Area survey, source, and personal measurements of broadband and frequency-specific magnetic flux densities were performed with frequency analyzer dosimeters; the background static field was measured with a portable magnetometer. Dosimetry measurements were cross-referenced to workers' time/activity patterns. Spot measurements of magnetic flux density in the research hospital varied between 0.8 mG (.08 microT) and 65 mG (6.5 microT). Workers' time-weighted average exposures ranged from 1.2 mG (0.12 microT) to 10.4 mG (1.04 microT), with peak exposures ranging from 8.9 mG (0.89 microT) to 103.7 mG (10.37 microT). Frequency spectra of the exposures were variable and included harmonic bands as well as 60 Hz. The background static magnetic field in the hospital varied between 250 mG (25 microT) and 480 mG (48 microT) suggesting shielding of the geomagnetic field from building materials. These results indicate that health care workers are exposed to 40-1000 Hz magnetic fields during routine patient care. The fields are characterized by a switching (on/off, in/out) pattern of occurrence, variable frequencies that include 60 Hz and harmonic bands, and peak excursions that may occur at regular intervals. These characteristics vary both within and among job categories, suggesting that job title may be a poor surrogate for exposure classification. Although limited in scope, the results indicate that spot measurement of broadband or 60-Hz magnetic fields, or summary indices such as time-weighted average, mean or median, are insufficient to characterize occupational exposures.