Long-term exposures to NO 2 may lead to increased susceptibility to respiratory infection and may cause alterations in the lung. Nitrogen oxides also contribute to the formation of acid rain. Between 1996 and 1997, national average annual mean NO 2 concentrations remain unchanged. In the last 10 years, NOx emissions levels have remained relatively constant. However, over the longer term since 1970, total NOx emissions have increased 11 percent and NOx emissions from coal-fired power plants have increased 44 percent. .
Bold line indicates national air standard.