Nocturnal 6-hydroxymelatonin sulfate excretion in female workers exposed to magnetic fields

The objective of this study was to determine whether daytime occupational e xposure to extremely low frequency magnetic fields (MFs) suppresses nocturn al melatonin production. Sixty female volunteers were recruited. Thirty-nin e worked in a garment factory, and 21 office workers served as a reference group. Exposure assessment was based on the type of sewing machine used and MF measurements around each type of machine. Eye-level MF flux density was used to classify the operators to higher (> 1 mu T) and lower (0.3-1 mu T) exposure categories. A third group of factory workers had diverse MF expos ures from other sources. The reference group had average exposure of about 0.15 mu T. Urine samples were collected on Friday and Monday for three cons ecutive weeks. Melatonin production was assessed as urinary 6-hydroxymelato nin sulfate (6-OHMS) excretion. The ratio of Friday morning/Monday morning 6-OHMS was used to test the hypothesis that melatonin production is suppres sed after 4 days of occupational MF exposure with significant recovery duri ng the weekend. Possible chronic suppression of melatonin production was ev aluated by studying exposure-related differences in the Friday values by mu ltivariate regression analysis. The Monday/Friday ratios were close to 1.0, suggesting that there is no increase in melatonin production over the week end. The average 6-OHMS excretion on Friday was lower among the factory wor kers than in the reference group, but no monotonous dose-response was obser ved. Multivariate regression analysis identified MF exposure, smoking, and age as significant explanatory variables associated with decreased 6-OHMS e xcretion.