Along with sulfur trioxide, one of the two common oxides of sulfur existing in the atmosphere. Sulfur dioxide is produced naturally in the atmosphere by the oxidation of hydrogen sulfide, a gas released from volcanoes and hot springs. About 22 million tons (20 million metric tons) of sulfur dioxide is released annually into the atmosphere by human activities, primarily during the combustion of fossil fuels. The amount of anthropogenic sulfur dioxide is roughly equal to that formed naturally from hydrogen sulfide. Sulfur dioxide reacts with water in clouds to form acid rain, causes damage in plants, and is responsible for a variety of respiratory problems in humans.