Formed when fossil fuels (notably oil, coal, and natural gas) and biomass (plant matter, wood chips, and landfill gas) are burned at high temperatures. NOx contributes to acid rain and smog. Health effects associated with smog include damage to lung tissue, increased asthma attacks, and respiratory illness in children with frequent high-level exposure. When NOx causes acid rain, it contributes to pollution of lakes and coastal waters and the degradation of sensitive forests. This pollution is destructive to fish and other animal life. Making electricity is responsible for 25% of all the NOx pollution in the U.S., over 6 million tons each year.