In the ambient atmosphere, the major sources of NO₂ are the combustion of fossil fuels and motor-vehicle emissions. Indoor sources include such appliances as gas stoves, water heaters, and kerosene space heaters. In the workplace, exposures to NO₂ have been reported in such occupations as electroplating, acetylene welding, agriculture, space exploration, detonation of explosives, certain military activities, and burning of nitrogen-containing propellants (Mohsenin 1994). In such situations, exposure concentrations can be very high. For example, in armored vehicles during live-fire tests, peak concentrations of NO₂ have been measured at over 2,000 parts per million (ppm). That decreases to about 500 ppm after 1 min and decreases to about 20 ppm within 5 min (Mayorga 1994).

Of concern to the Air Force is the presence of NO_x/HNO₃ emissions from rockets that use liquid propellants composed of nitrogen-based compounds. For normal launches, the nitrogen-associated emissions at ground level are negligible. However, in the case of a catastrophic abort, large quantities of nitrogen tetroxide can be released. That gas is rapidly converted to NO₂ and HNO₃, possibly resulting in production of quantities of NO_x/HNO₃ as high as 200,000 lb (see Appendix A).

When inhaled, NO₂ reacts with the moisture in the respiratory tract, resulting in the formation of nitric acid (HNO₃). The nitric acid dissoci-