Objectives: The current work aimed at investigating the cognitive functions impairment among workers of sewer networks due to exposure to H2S and the relation of this impairment ,if any, to the level of H2S exposure biomarker “urinary thiosulfate". Besides, the validity of using Mini Mental State Examination (MMSE) as screening test for cognitive impairment among the exposed workers was tested.Subjects: The work was conducted among 33 sewage network maintenance male workers and a matched unexposed control group (n= 30).Methods: The participants were subjected to clinical neurological history, estimation of urinary thiosulfate, and assessment of cognitive dysfunction by using neurophysiological (simple reaction time, P300 test) and neuropsychological tests (Wechsler MemoryScale) and frontal executive functions tests. Results: Clinical neurological history revealed significantly higher neurological symptoms
(headache, memory defects, lack of concentration) among exposed workers compared to their controls (P< 0.05). Exposed workers had significantly prolonged simple reaction time and delayed P300 latency and showed poor performance of most of neuropsychological tests. Marked elevation of urinary thiosulfate was observed among the
exposed workers (P< 0.001) but this elevation was not correlated with the duration of exposure or any of the other measured parameters. Exposed workers had significantly lower mean value of MMSE scoring than that of the controls (P<0.001). Conclusion: Exposure to H2S among sewer network workers is associated with cognitive impairment which can be screened by applying MMSE as a simple rapid test for hydrogen sulfide occupationally exposed workers.