Nitrogen dioxide and nitrogen oxides are released into the air from the exhaust of motor vehicles and the burning of coal, oil or natural gas. Nitrogen dioxide has a strong odor and is a liquid at room temperature, becoming a reddish-brown gas above 70°F.
Exposure to high levels of nitrogen oxides can damage the respiratory airways and contact with skin or eyes can cause burns. In addition to its own noxious qualities, nitrogen dioxide contributes to particle pollution and to the chemical reactions that make ozone. Nitrogen oxides are one of six widespread air pollutants that have national air quality standards to limit their levels in the outdoor air.
More information: U.S. Environmental Protection Agency