Reduced excretion of a melatonin metabolite in workers exposed to 60 Hz magnetic fields

The effects of occupational 60 Hz magnetic field and ambient light exposures on the pineal hormone, melatonin, were studied in 142 male electric utility workers in Colorado, 1995-1996. Melatonin was assessed by radioimmunoassay of its metabolite, 6-hydroxymelatonin sulfate (6-OHMS), in post-work shift urine samples. Personal magnetic field and light exposures were measured over 3 consecutive days using EMDEX C meters adapted with light sensors. Two independent components of magnetic field exposure, intensity (geometric time weighted average) and temporal stability (standardized rate of change metric or RCMS), were analyzed for their effects on creatinine-adjusted 6-OHMS concentrations (6-OHMS/cr) after adjustment for age, month, and light exposure. Geometric mean magnetic field exposures were not associated with 6-OHMS/cr excretion. Men in the highest quartile of temporally stable magnetic field exposure had lower 6-OHMS/cr concentrations on the second and third days compared with those in the lowest quartile. Light exposure modified the magnetic field effect. A progressive decrease in mean 6-OHMS/cr concentrations in response to temporally stable magnetic fields was observed in subjects with low workplace light exposures (predominantly office workers), whereas those with high ambient light exposure showed negligible magnetic field effects. Melatonin suppression may be useful for understanding human biologic responses to magnetic field exposures.