Sulfur Dioxide

Sulfur dioxide is a colorless gas with a sharp, pungent odor, like that of a burning match. Most people are able to detect this highly characteristic odor at concentrations of about 0.5 ppm or greater.

Sulfur dioxide is an irritant to the eyes, the respiratory system, and, in some cases, the skin. One factor in producing this effect is the interaction of sulfur dioxide with water to form sulfuric acid, a strong biological irritant.

At concentrations normally found in ambient air, these effects are annoying, but not particularly dangerous for most people. But for some individuals, such as those who have respiratory disorders, the young, or the elderly, even these low levels may pose a risk. Such individuals may experience more serious breathing problems that require medical attention. People who are constantly exposed to relatively high concentrations of sulfur dioxide (such as smelter workers) may experience more serious long-term health problems, such as asthma, chronic bronchitis, lung disease, or emphysema.

In addition to its effects on human health, sulfur dioxide has some important consequences for the physical and biological environment caused by the gas’s contribution to the formation of acid rain.

Sulfur dioxide is produced by both natural and anthropogenic sources. The most important of the natural sources are volcanic eruptions, which account for about 40 percent of all natural emissions of the gas. Since volcanic eruptions are episodic events, the amount of sulfur dioxide attributable to this source in any one year varies widely.

Other natural sources of the gas are forest fires and other natural burns, biological decay, and certain metabolic processes carried out by living organisms, especially marine plankton and bacteria. Natural sources release about 27.5 million short tons (25 million metric tons) of sulfur dioxide per year.

The most important anthropogenic source of sulfur dioxide are power generating plants, which account for about 70 percent of all the gas produced. Less than 5 percent of the gas is produced by motor vehicles and other forms of transportation.

Sulfur dioxide is released during these operations when either coal or oil is burned. Coal and oil both naturally contain compounds of sulfur that, when burned, produce sulfur dioxide. The sulfur content of both coal and oil ranges from less than 1 percent to more than 7 percent, although refined petroleum tends to have lower concentrations of sulfur than do most forms of coal.

Natural gas, by contrast, tends to have very low sulfur concentrations, usually less than 0.001 percent.