Sulfur dioxide is considered immediately dangerous at?

• A. 400-500 ppm
• B. 100-200 ppm
• C. 800-900 ppm
• D. 1000-1200 ppm

Answer: (A)

Sulfur dioxide is a strong respiratory irritant, and at higher concentrations it can rapidly overwhelm the airways and hinder gas exchange. When exposure reaches roughly four to five hundred parts per million, the irritation becomes severe enough that life-threatening effects can occur within minutes, such as intense coughing, chest tightness, bronchospasm, and potential pulmonary edema. This combination of rapid onset and potential for serious harm makes that range the point at which the atmosphere is considered immediately dangerous to life or health.

Smaller elevations, like 100–200 ppm, still cause irritation and discomfort and may provoke symptoms in sensitive individuals, but they’re less likely to produce immediate life-threatening effects. Higher exposures (approaching or exceeding 800–1200 ppm) carry even greater risk, with a greater likelihood of rapid incapacitation, but the threshold at which immediate danger is declared is commonly cited around the four-to-five-hundred ppm range. In any case, detection, evacuation, and proper respiratory protection are essential when SO2 levels rise toward these ranges.