Most people have heard of acid rain, widely discussed as a threat to forests
downwind from coal-fired power plants. Acid rain is primarily caused by
sulfur dioxide (SO2), a byproduct of burning coal, oil or gas that is tinged with
sulfur. Because sulfur is a commonly occurring element, it is virtually impossible
to find deposits of these fossil fuels that do not contain sulfur. When sulfur dioxide
is emitted as these fuels are burned, it enters the atmosphere and reacts with
water. The outcome of this reaction is sulfuric acid (H2SO4); it is this acid that
gives the rain its name.

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