Sulfur dioxide (SO2) is a colorless, highly water-soluble, pungent irritant gas that is heavier than air. It is widely used in industry as a byproduct of sulfur mines, papermaking, and the combustion of mineral fuels. It is also a common pollutant in the atmosphere. Sulfur dioxide can be detected at a concentration of 1.5 mg/m3. At a concentration of 1.8 to 3.0 mg/m3, it irritates the nasal cavity and throat. Slightly higher concentrations cause obvious upper respiratory tract discomfort and persistent coughing. Acute poisoning is rare and can cause clinical symptoms similar to chlorine poisoning. Relatively low concentrations of sulfur dioxide (3.0 to 150 mg/m3) are mostly deposited in the nasopharynx and throat. Because of its high water solubility, exposure to 18 to 30 mg/m3 of sulfur dioxide can cause severe irritation of the conjunctiva and upper respiratory tract mucosa. Exposure to higher concentrations of sulfur dioxide can cause damage to the larynx, trachea, distal airways, and alveoli, stimulate mucosal secretions, cause bronchospasm, and even pulmonary edema.