Five oxides of nitrogen are known: N2O, NO, N2O3, NO2, and N2O5. Environmental scientists usually refer to only two of these, nitric oxide and nitrogen dioxide, when they use the term nitrogen oxides. The term may also be used, however, for any other combination of the five compounds. Nitric oxide and nitrogen dioxide are produced during the combustion of fossil fuels in automobiles, jet aircraft, industrial processes, and electrical power production. The gases have a number of deleterious effects on human health, including irritation of the eyes, skin, respiratory tract, and lungs; chronic and acute bronchitis ; and heart and lung damage.