a gas that results from the burning of fossil fuels like coal and natural gas. Sulfur dioxide is a component of acid rain and is also associated with certain respiratory illnesses.

A heavy, smelly gas that can be condensed into a clear liquid; used to make sulfuric acid, bleaching agents, preservatives and refrigerants; a major source of air pollution in industrial areas.

a corrosive gas produced both by nature and technology in nearly equal amounts. Burning fuels, such as coal and oil, that contain sulfur produces SO. It is also produced from sea spray, organic decomposition and volcanic eruptions. When combined with water in the air, it produces a weak, corrosive sulfuric acid -- an ingredient of "acid rain." sustainable - The Worldwatch Institute defines "sustainable" as "meeting present needs without compromising the ability of future generations to meet their own needs." For example, wood harvested from a "sustainable forest" means that the wood is derived from a forest managed in a manner so that the trees harvested will be replaced at a rate that matches the rate of removal. In this regard, the forest will continue producing wood for successive generations.