Because of the sulfur dioxide (SO₂) emissions when coal is burned, sulfur levels in coal are important. Coal supplies 32% of the world's energy. In 1995, United States electric utilities used coal to generate 55% of the nation's net electric power, and this approach is projected to remain virtually the same through 2005. In the United States, the utility and industrial sectors are the largest emitters of sulfur oxides, and in 1985 accounted for 22.3 million tons, or 96%, of the total SO₂ emitted. In the presence of particulate matter, sulfur oxides have been related to irritation of the human respiratory system, reduced visibility, materials corrosion, and varying effects on vegetation. The reaction of sulfur oxides with moisture in the atmosphere contributes to acid rain.