Chronic low-level hydrogen sulfide exposure and potential effects on human health: a review of the epidemiological evidence.

The effects of exposure to high concentrations of hydrogen sulfide (H2S) on human health are well known. However, the potential human health hazards posed by low-level chronic environmental H2S exposure are being debated. Accordingly, we reviewed the literature regarding the effects of chronic, environmentally-relevant H2S exposures on human health. All human observational studies using an analytical study design (e.g. cohort, cross-sectional, case-control) to evaluate chronic-duration low-level H2S exposure (approximately ≤ 10 ppm on average, for 1 year or more), were evaluated for a range of health outcomes. Respiratory symptoms in both adults and children were the most consistently reported symptoms on the increase. When reported, such effects appear to be temporary, given that there is no consistent evidence of pulmonary function deficit in either age group, among those chronically exposed to low H2S concentrations. While sparse, some data also suggest potential ocular symptoms and disorders associated with chronic ambient level H2S exposure in adults (not children), but the limited data on H2S exposures, co-exposures and/or strong odor stimulus of H2S, temper interpretation. Neurological symptoms and deficits have been reported in some studies, but the highest quality evidence, obtained using objective outcome measures and a reasonably detailed assessment of exposure, does not support a neurological-related risk in adults (only one study in children). For the other endpoints assessed (cardiovascular, reproductive and developmental, and carcinogenicity), the results were mixed and/or conflicting, but did not indicate a potential health hazard, although this literature has several major limitations, particularly with regard to exposure estimation and the ability to assess exposure-response.