Flue gas desulfurization (FGD) is a technology set of techniques used to remove sulfur dioxide (SO2) from exhaust flue gases produced by fossil fuel power plants and other industrial processes. The primary function of FGD systems is to reduce environmental harm from SO2 discharges that can cause acid rain and respiratory problems in humans. Technologically, these systems usually entail the absorption of SO2 into a liquid or slurry -- with lime or limestone acting as the absorbent. The process turns the gas into solid by-products like gypsum, which are benign and can be safely disposed of or used to some other purpose. FGD technologies are indispensable for meeting air pollution regulations and are widely used in coal-fired power stations, cement kilns and metal smelters, helping to greatly reduce the volume of harmful pollutants that are released.