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Epidemiologic Notes and Reports Acute Occupational Exposure to Sulfur Dioxide -- Missouri

On March 18, 1982, workers at a manufacturing facility in St. Louis, Missouri, were acutely exposed to sulfur dioxide following an industrial accident (1). In an attempt to clean a dip tank containing chromic acid solution, a cleaning contractor added sodium bisulfite. The resulting reaction liberated gaseous sulfur dioxide, which spread rapidly in the plant, causing evacuation of the plant's 148 workers.

Forty employees who worked near the tank were evaluated at a local hospital or by a company physician. The principal symptoms reported were cough (78%), chest discomfort (78%), and irritation of the throat (78%) and eyes (50%). These symptoms were still present 2 weeks after the incident: cough (8%), chest discomfort (30%), and irritation of the throat (18%) and eyes (1%). Such symptoms are consistent with acute exposure to sulfur dioxide. In 28 cases, chest radiographs were taken within 2 weeks of the incident; 19 (68%) of these x-ray reports were interpreted by investigators from the National Institute for Occupational Safety and Health (NIOSH), and none showed radiographic evidence of chemical pneumonitis or bronchopneumonia. Results of pulmonary function tests indicated one worker had obstruction of the airways (2). However, because no results of similar tests done before the incident were available for comparison, it was not possible to determine whether this abnormality was associated with the exposure to sulfur dioxide. The results of other laboratory tests showed no consistent patterns of abnormalities.

The day after the accident, an industrial hygienist sampled air at the plant to measure concentrations of sulfur dioxide and total and hexavalent chromium; airborne concentrations of these contaminants were below the analytic limits of detection. On March 25, additional air samples were collected to measure sulfur dioxide. Samples taken at various locations near the tank showed no detectable sulfur dioxide, but two air samples taken inside the covered dip tank revealed sulfur dioxide levels of 5 and 7 parts per million (ppm).

The dip tank was covered and vented 2 weeks after the incident. During the weekend after the visit by NIOSH investigators, the company made another attempt to clean the dip tank; the operation was completed without incident. Reported by 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: Sulfur dioxide is a colorless, water-soluble gas that forms sulfurous acid (H((2))SO((3))) on contact with moisture. This reaction may occur on mucous membranes lining the respiratory tract after the gas is inhaled. Acute inhalation causes immediate irritation of the respiratory tract and can cause bronchial constriction, which produces symptoms such as cough, breathlessness, a choking sensation, and tightness or discomfort of the chest. These acute effects are usually reversible if the individual is rapidly removed from exposure.

Although atmospheric levels of sulfur dioxide of 1-3 ppm have been reported to cause severe narrowing of the bronchioles (3), sensitivity to the broncho-constricting effects of sulfur dioxide varies among different individuals. Repeated exposures to 10 ppm have caused nosebleeds among exposed workers (4), and acute overexposures to sulfur dioxide may result in death from asphyxia (5). Survivors of such acute exposures may suffer chemical bronchopneumonia and bronchiolitis obliterans (narrowing and inflammation of the small airways), which can be fatal after a few days; delayed chemical pneumonitis and bronchial asthma can also result (6). Delayed chemical pneumonitis has been described following acute exposure to other noxious gases, such as ozone, nitrogen dioxide, and phosgene (3,5).

Sulfur dioxide and other air pollutants have been reported to contribute to or aggravate acute, nonspecific diseases of the upper respiratory tract, chronic bronchitis, emphysema, and lung cancer (3). These effects usually result from long-term chronic exposure rather than a single, acute exposure.

Although the current permissible exposure limit promulgated by the Occupational Safety and Health Administration standard is 5 ppm for an 8-hour time-weighted average (TWA) exposure (7), based on available scientific information, NIOSH recommends that occupational exposures to sulfur dioxide not exceed an 8-hour TWA of 0.5 ppm (4).

Information obtained during this investigation indicates that sodium bisulfite was added to a chromic acid solution to convert all hexavalent chromium to the more stable and acceptably disposable trivalent form. The sulfur dioxide formed usually dissolves readily; the release of sulfur dioxide into the air in this accident was probably caused by too rapid addition of bisulfite and/or inadequate mixing of the solution during the reaction. Either could have resulted in localized heating of the solution, and rapid release of gaseous sulfur dioxide.

To prevent such accidents in the future, NIOSH recommends that if sodium bisulfite is to be used for cleaning dip tanks, it should be added to the tank slowly, with adequate mixing during the process. Only trained personnel, aware of the potentially hazardous chemical reaction and the necessary precautions to prevent it, should participate in such procedures.


  1. National Institute for Occupational Safety and Health. Health hazard evaluation report no. HETA 82-191-1267. Cincinnati, Ohio: National Institute for Occupational Safety and Health, 1983.

  2. National Institute for Occupational Safety and Health. Manual of spirometry in occupational medicine. Cincinnati, Ohio: National Institute for Occupational Safety and Health, Division of Training and Manpower Development, 1981.

  3. Doull J, Klaassen CD, Amdur MO, eds. Casarett and Doull's toxicology: the basic science of poisons. 2nd ed. New York: Macmillan Publishing Co., Inc., 1980.

  4. National Institute for Occupational Safety and Health. NIOSH/OSHA occupational health guidelines for chemical hazards. Cincinnati, Ohio: National Institute for Occupational Safety and Health, 1981. (DHHS (NIOSH) publication no. 81-123).

  5. National Institute for Occupational Safety and Health. Occupational diseases: a guide to their recognition. Revised ed. Cincinnati, Ohio: National Institute for Occupational Safety and Health, 1977. (DHEW (NIOSH) publication no. 77-181).

  6. National Institute for Occupational Safety and Health. Criteria for a recommended standard: occupational exposure to sulfur dioxide. Cincinnati, Ohio: National Institute for Occupational Safety and Health, 1974. (DHEW publication no. (NIOSH) 74-111).

  7. Occupational Safety and Health Administration. OSHA safety and health standards. 29 CFR 1910.1000. Occupational Safety and Health Administration, revised 1980.

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