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What Is Sulfur Dioxide?

Sulfur dioxide is familiar to anyone who has lived in a polluted city, since it is a component of air pollution. In addition, this gas has been identified as a major contributor to the environmental problem of acid rain. Once in the air, it reacts with other airborne chemicals to create corrosive sulfuric acid, which results in corrosive precipitation. It does, however, have some useful applications as well.

Basics of SO2
- Sulfur is a chemical substance which also goes by the name of sulfurous oxide or simply sulfur oxide. It has the molecular formula SO2, and each molecule is made of one sulfur atom with two oxygen atoms on either side. The molecule is polar, meaning there is an imbalance in the distribution of electrons around it. The result is that the sulfur is slightly positive and the two oxygen atoms slightly negative. Since it does not contain carbon, sulfur dioxide is in the category of inorganic compounds.
Physical Properties
- SO2 is a gas at room temperature and is colorless, although it has a strong, acidic odor. It freezes at -104 degrees Fahrenheit and boils at 14 degrees Fahrenheit. Its vapor density is more than twice that of air, so its vapors are heavier than air. It is normally shipped and used as a compressed liquefied gas. It is nonflammable but can react with other chemicals --- including metals such as brass and zinc --- under some conditions. It can also react with water and oxygen to make highly corrosive acid.
Health Hazards
- Contact with SO2, whether by skin, eye or inhalation, can have serious health consequences. At low levels, it produces irritation of the eyes or throat and lungs. At higher concentrations, it can produce chemical burns which are especially dangerous if the gas in inhaled, and can lead to serious lung damage. A concentration in air of 2,520 parts per million (ppm) was found to be lethal in animal testing. The U.S. Occupational Safety and Health Administration (OSHA) has set an exposure limit of 5 ppm SO2 in air for the workplace.
Occurrence and Applications
- Small amounts of SO2 enter the atmosphere due to natural phenomena such as volcanic eruptions. However, the vast majority of this gas is produced by human activity. Most of this SO2 is the result of burning fossil fuels such as coal for the generation of power. Sulfur dioxide also has a number of useful applications. It is used to bleach wood pulp in making paper and also is a common food preservative.

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