Objectives To investigate the short-term effects of lowlevel hydrogen sulfide (H 2 S) exposure on oil field workers. Materials and methods Observational study included 34 patients who work at an oil field. All patients were males with age range of 22-60 years (mean 37 years). The data were collected by systematic questionnaire about symptoms. The inclusion criteria of patients were symptoms related to inhalation of H 2 S gas in the oil field. The complaints should be frequent and relapsed after each gas exposure and disappeared when there was no gas exposure. Exclusion criteria were the symptoms which experienced with or without H 2 S exposure. The presence of H 2 S gas was confirmed by valid gas detector devices. Results The most frequent presenting symptom was nasal bleeding. It was revealed in 18 patients (52.9 %). This followed by pharyngeal bleeding, gum bleeding, and bloody saliva (mouth bleeding) which were encountered in five cases for each complaint (14.7 %). Other less frequent presenting symptoms were tongue bleeding, bloody sputum, headache, abdominal colic, pharyngeal soreness, fatigue, and sleepiness. Conclusions Nasal mucosa was the most vulnerable part to H 2 S effect. Inhalation of H 2 S produced upper respiratory tract epithelial damage that led to bleeding from nose, pharynx, gum, tongue, trachea, and bronchi. There were no complaints of asthmatic attack upon exposure to low level of H 2 S. Sunlight had a significant role in reduction of ambient air H 2 S level.