Sulfur Dioxide

A noxious, poisonous gas with a strongly acrid smell, sulfur dioxide is a chemical compound that can originate from both natural and (human-made) anthropogenic sources. One of the sulfur oxides (SOx) formed by the interaction of sulfur and oxygen, sulfur dioxide is also referred to as sulfurous anhydride and has the formula SO2. It may eventually be converted to the highly corrosive sulfuric acid (with a formula of H2SO4).

A colorless gas, SO2 is an air pollutant with health risks for both humans and animals. Sulfur dioxide in the atmosphere contributes to smog formation, as well as acid rain. Additionally, it can negatively affect vegetation and damage structures. Therefore, efforts have been made to reduce the amount of SO2 in the atmosphere. Despite the adverse effects associated with SO2, the gas also has beneficial uses in industry, though not without controversy. It is, for example, used in the food and beverage industries, where it is utilized as a preservative for fruit, specifically to prevent fruits from becoming discolored and brown. The beer and wine industries also employ the use of SO2 as a preservative. Additionally, it can be used to bleach materials such as paper and fabrics. It has also been used in wastewater treatment.

Sulfur dioxide impairs breathing function in humans. Upon inhalation, it acts to constrict the bronchi (passages that convey air into the lungs), resulting in chest tightness and shortness of breath. It may also form particulates (discrete matter in liquid or solid form) when it interacts with other atmospheric components. Such small particulates (< 2.5 μm) present increased health risks. They may be inhaled and, due to their minute size, become lodged deeply inside lungs, where they restrict lung function.

While everyone is subject to the dangers of inhaling both SO2 and sulfur-based particulate matter, some populations are especially vulnerable. Asthmatics, sufferers of bronchitis, emphysema, and heart disease, along with young children and the elderly, are particularly sensitive to damage from SO2. Lung function in the presence of SO2 is made worse when breathing is predominantly done through the mouth, as often occurs during exercise.

Various organizations, such as the U.S. Environmental Protection Agency (EPA) and the World Health Organization (WHO), have issued guidelines reflecting unsafe concentrations of SO2 in the atmosphere. These limits are used to indicate air quality by describing when air pollution (by constituent) can be assumed to result in health consequences.

The WHO revised air pollution guidelines, including those for SO2. Concentrations should not exceed 20 μg/m3 based on a 24-mean (or 500 μg/m3 based on a 10-minute mean), reflecting new information on the health impacts of SO2 inhalation. The EPA, based on the Clean Air Act, sets levels of permissible concentrations for a given set of atmospheric pollutants; these include SO2, which is defined as a primary standard (because of its damaging effects on human health). The National Ambient Air Quality Standards (NAAQS) lists this level as 75 parts per billion by volume (ppb) for a one-hour averaging period. A secondary standard of 0.5 parts per million by volume (ppm) is designed [Page 1283]to protect humans indirectly (by protecting flora, fauna, and built structures); this standard utilizes a three-hour averaging time.

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