Burning coal is a primary source of sulfur dioxide (SO2) in the atmosphere, and so it was one of the first causes of air pollution, arising during the Industrial Age. Sulfur in bituminous and other forms of coal, when burned, bonds with oxygen to form this pollutant, which irritates eyes and the respiratory system. It is also a source of acid rain. Technologies have been developed that scrub sulfur dioxide emissions from smoke stacks, and have done much to improve air quality. While the United States has dramatically lowered these emissions, other developing nations, such as China and India, do not impose vigorous restrictions on sulfur dioxide from factories and power plants.