Risk Characteristics of Hydrogen Sulphide Exposure in Wastewater Collection and Treatment Related Occupations

Water and wastewater workers can be exposed to hydrogen sulphide (H2S), with an unpredictable exposure pattern, dominated by sharp peaks. These peaks can often be high above the ceiling value (CV) at 10 ppm.

We have analyzed self-administrated H2S exposure data among 60 wastewater workers in the Municipality of Trondheim, Norway, from 2015 till 2021. The detection range of the personal alarm equipment used was 1.6 to 100 ppm H2S. The workers were divided in four similar exposed groups (SEGs): wastewater collection net, wastewater treatment plants, wastewater pumping stations and water distribution net.

We identified measurements from 7083 different workdays, approximately 10% of the workdays between 2015 and 2021. Within these, 1295 days had readings above 1.6 ppm H2S, and 424 (33%) of these days had readings that exceeded the CV of 10 ppm H2S. This percentage was similar across the SEGs. Only one workday had a time weighted average (TWA) exceeding the occupational exposure limit (OEL) of 5 ppm H2S, and only 14 days exceeded 0.5 ppm H2S, 1/10th of the OEL.

Wastewater workers in this study are regularly exposed to short peaks of H2S, but even high peaks do not influence the 8-h TWA values significantly. A preliminary measurement program over 3 days according to EN 689 to evaluate the need for further measurements would probably not find TWA values greater than 1/10 of OEL; the EN 689 standard is not made for evaluation to peak exposures. Exceedances of CV at 10 ppm H2S occur in 6% of workdays, and in 33% of days with exposure above 1.6 ppm. The toxicity and exposure profile of H2S makes continuous exposure monitoring necessary for alarm purposes. Reliance on the 8-h occupational exposure limit as has been the normal in Norway, will not be adequately protective for wastewater workers. H2S alarm equipment should continue to be used.