Oxides of nitrogen (NO_x), such as nitric oxide (NO), dinitrogen oxide (N₂O), and nitrogen dioxide (NO₂), which are pollutants contributing to acid rain. Nitrogen oxides are expelled in the emissions from car exhausts, aircraft, and factories. Nitric oxide is biologically important, with various roles in the body. High concentrations of nitrogen dioxide are irritating to the respiratory tract and can aggravate conditions such as asthma, while NO_x in general contribute to the formation of ground-level ozone. See also air pollution.