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Epidemiologic Notes and Reports Occupational Fatality Following Exposure to Hydrogen Sulfide -- Nebraska

Hydrogen sulfide (H((2))S) is a potential hazard for workers in wastewater-treatment plants. Investigation of an occupational fatality resulting from exposure to H((2))S in such a plant illustrates the hazards associated with this agent.

On September 3, 1983, a worker at a wastewater-treatment plant in Omaha, Nebraska, was found unconscious after he had gone to collect samples in the building where wastewater enters the plant. He died later that day from acute respiratory distress syndrome. A review of hospital records and the autopsy report showed the pattern of his fatal illness was compatible with exposure to H((2))S.

On September 6, engineers of the City of Omaha requested that the National Institute for Occupational Safety and Health (NIOSH) evaluate working conditions and help develop a health and safety plan for the plant (1). NIOSH investigators collected 40 personal-breathing-zone* and 26 long-term area air samples for H((2))S in all areas of the plant. Concentrations of H((2))S in the personal air samples ranged from none detected to 2.2 parts per million (ppm); results from the long-term area air samples ranged from none detected to 56.0 ppm. The highest concentrations were found in the area near where the worker was apparently fatally overcome. Instantaneous area air samples for H((2))S were also collected in this area. These concentrations ranged from 50 ppm to 200 ppm (the maximum reading on the instrument used) when one of the supply fans in the building malfunctioned.

During the week of October 17, 54 (83%) of the 65 workers in the plant responded to a self-administered questionnaire. Forty-one (76%) respondents indicated that, during the previous 2 weeks, they had experienced at least three of the symptoms known to be associated with H((2))S exposure, most commonly cough (61%), eye irritation (57%), and nose irritation (54%). However, no clear association between frequency of symptoms and estimated exposure was found.

The exact circumstances resulting in the worker's death may never be known. NIOSH investigators concluded that the factors contributing to the death included: (1) the summer temperature and the long transit time of the sewage entering the plant (resulting in high concentrations of dissolved H((2))S); (2) inappropriate design of the ventilation system; and (3) inadequate safety procedures for workers entering potentially dangerous areas. Based on these factors, NIOSH investigators provided recommendations to prevent any future fatal incidents. Reported by NIOSH Region VII, Hazard Evaluations and Technical Assistance Br, Div of Surveillance, Hazard Evaluations, and Field Studies, National Institute for Occupational Safety and Health, CDC.

Editorial Note

Editorial Note: At room temperature, H((2))S is a colorless gas and has a characteristic rotten-egg odor. Although it has a rather low odor threshold (0.13 ppm), it can cause olfactory fatigue at 100 ppm in 2-15 minutes. It is a rapid-acting systemic poison that causes respiratory paralysis with consequent asphyxia at high concentrations (1,000-2,000 ppm). Inhalation of high concentrations may cause coma after a single breath and may be rapidly fatal. Prolonged exposure to 250 ppm H((2))S may cause pulmonary edema. Exposure to concentrations above 50 ppm for 1 hour may produce acute conjunctivitis with pain, lacrimation, and photophobia; in severe form, this may progress to keratoconjunctivitis and vesiculation of the corneal epithelium. Prolonged exposure to concentrations as low as 50 ppm H((2))S may cause rhinitis, pharyngitis, bronchitis, and pneumonitis. In low concentrations, H((2))S may cause headache, fatigue, irritability, insomnia, eye and respiratory irritation, and gastrointestinal disturbances; in somewhat higher concentrations, it affects the central nervous system, causing excitement and dizziness (2,3).

The Occupational Safety and Health Administration (OSHA) has established a one-time, 10-minute exposure limit of 50 ppm during a work shift (4). NIOSH recommends that the concentration for a 10-minute sample not exceed 10 ppm and also that the area be evacuated if the concentration of H((2))S exceeds 50 ppm (2).

The recommendations resulting from the Nebraska investigation provided a basis for preventing recurrence of the problem. The nature of the sewage (i.e., high concentration of H((2))S) entering the plant probably contributed to the death of this worker. The NIOSH investigators recommended the plant retain a consulting firm to evaluate adding an aeration system or chemicals along the sewage-transit line to prevent the growth of bacteria that cause the production of H((2))S. The average flow time through more than 25 miles of sewer pipe to the plant is approximately 8 hours. At all times, but especially during times of low flow and warmer water temperatures, the sewage becomes anaerobic, facilitating the production of H((2))S by certain bacteria. The presence of H((2))S had been a recurring problem at this plant. During the last stages of plant construction, a worker died in the main sewer that enters the plant; sewer gas was listed as the probable cause of death.

A second factor was the ventilation system in the mezzanine, bar screen, and wet-well areas. This system was designed to keep the entire area under positive pressure so the exhausted air could be filtered to avoid community odor problems. When the ventilation system failed during a power outage, an H((2))S level of 200 ppm was measured at the doorway to the mezzanine area before the ventilation system was turned back on. Based on this figure, NIOSH investigators estimated the level of H((2))S to be in the 1,000-2,000 ppm range in the area where the incident occurred. This is considerably above the value of 300 ppm that NIOSH considers immediately dangerous (5). As an experiment, NIOSH investigators and plant maintenance personnel reversed the fan in an effort to correct the airflow to the wet-well area; the H((2))S concentration dropped from 125 ppm to 7 ppm in 2 hours. The NIOSH investigators recommended that all ventilation systems throughout the plant be evaluated and deficiencies be corrected.

A third probable contribution to the death was the lack of specific procedures to ensure safe entry into areas containing potentially hazardous gases. The implementation of carefully written and enforced procedures can help prevent the same potentially hazardous conditions that existed for this fatality.


  1. National Institute for Occupational Safety and Health. Health hazard evaluation report no. HETA 83-440-1537. Cincinnati, Ohio: National Institute for Occupational Safety and Health, 1984.

  2. National Institute for Occupational Safety and Health. Criteria for a recommended standard.....occupational exposure to hydrogen sulfide. Cincinnati, Ohio: National Institute for Occupational Safety and Health, 1977. DHEW publication no. (NIOSH) 77-158.

  3. International Labour Office. Encyclopaedia of occupational health and safety. Vol I/a-k. Geneva: International Labour Office, 1983.

  4. Occupational Safety and Health Administration. OSHA safety and health standards. 29 CFR 1910.1000. Occupational Safety and Health Administration, U.S. Department of Labor, revised 1983.

  5. National Institute for Occupational Safety and Health. NIOSH/OSHA pocket guide to chemical hazards. Cincinnati, Ohio: National Institute for Occupational Safety and Health, 1985. DHHS (NIOSH) publication no. 78-210. *Personal air samples are collected in the worker's breathing zone. Long-term area air samples are collected in the work area over an entire work shift. Instantaneous air samples are measured by a direct reading instrument.

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