Toxic Effects of Sulfur Dioxide

Although sulfur dioxide is not a particularly toxic material, episodes have occurred in which increased deaths have been associated with high levels of sulfur dioxide in the atmosphere. The most notable such incident occurred in London in December 1952 when a stagnant layer of dense fog and black coal smoke accompanied by high levels of sulfur dioxide descended on the city for days, a period during which thousands of excess deaths were recorded. Because of particles and fog in the atmosphere, visibility descended to near zero, and the fine particulate pollution was so bad that even the underwear of exposed people was soiled by it. 

Sulfur dioxide is an irritant that affects eyes, the respiratory system, mucous membranes, and skin. Because of its water solubility, it is largely removed in the upper respiratory tract, which may be irritated by exposure to the sulfur dioxide, causing bronchioconstriction and increased airflow resistance. Some people are hypersensitive to sulfur dioxide and suffer allergic reactions to it. Included among such people are some asthmatics who are especially sensitive to exposure to the sulfur dioxide. 

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Among sulfur-containing compounds, the toxic effects of sulfur dioxide have importance in practice. Sulfur dioxide belongs to the group of major air pollutants, and emissions from industry and ocean-going ships are the main sources of its input into the environment. 

Bisulfites are oxidised in the body by sulfite oxidase (sulfite ferricytochrom c oxidoreductase) to sulfates, which are excreted in the urine. The enzyme activity is individual, and toxic effects of sulfur dioxide and sulfites are therefore variable. Some individuals tolerate amounts of up to 50 mg/kg, while in sensitive individuals such concentrations will cause headaches, nausea and diarrhoea. In asthmatics receiving steroids, sulfites may cause allergic reactions (ADI = 0.7 mg/kg bodyweight). 

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