A cross-sectional study of sensory-motor neuropsychological function among sewage plant and sewage net workers exposed to hydrogen sulphide when handling wastewater

OBJECTIVES: Workers at sewage treatment plants are exposed to a complex mixture of toxins, including hydrogen sulphide (H(2)S). An issue of concern among sewage workers, is possible negative nervous system effects from low-level H(2)S exposure. Empirical neuropsychological evidence indicates both that low-dose exposure to H(2)S exposure affects the nervous system, and the contrary, that such exposure may facilitate nervous system function, since H(2)S is an endogenously produced central nervous system (CNS) gasotransmitter. The aim of this study is to describe a possible association between the H(2)S component of the total exposure and long-term effects on neuropsychological motor function among wastewater workers. METHODS: Workers (N = 138) treating wastewater in 6 sewage-treatment plants, or in the sewer net system participated in a cross-sectional study. H(2)S exposure was expressed in a dichotomous exposure variable defining currently H(2)S-exposed (N = 112) and unexposed referent workers (N = 26), and a variable defining a job-exposure matrix for long-term total typical workplace H(2)S exposure. The participants went through neuropsychological tests for hand coordination, reaction time (SRT), and balance, and completed questionnaires. Pearson chi-square test or independent samples t-test was used when comparing the currently H(2)S-exposed workers with the unexposed control group. Multiple linear regression was used to assess associations between the independent variables age, smoking and exposure variables, and the neuropsychological tests. RESULTS: The analyses indicate increased SRT in the currently H(2)S-exposed group compared to controls (mean [SD] = 225.8 [29.9] versus 210.7 [26.3] ms, P = 0.019), and an association between increased SRT and current H(2)S-exposure in the total study sample (β = 14.7, P = 0.026, R(2) = 0.06, P = 0.050). Blindfolded balance testing indicates a nonsignificant trend in the total study sample, of reduced balance in the highest versus lowest H(2)S total long-term exposure-index group (Sway area [mean {SD}, mm(2): 702 [410] versus 581 [278]), and a significant association between total long-term H(2)S exposure and reduced balance among smokers (Sway area, mm(2) [β = 38.7, P = 0.039], mean sway, mm [β = 0.3, P = 0.015]). CONCLUSION: The observed trends and associations may be due to exposure peaks in certain work operations and pinpoint the importance of minimizing and avoiding exposure peaks, also when H(2)S time-weighted average measurements do not exceed an occupational exposure limit of 5 ppm.