trations result from many natural and anthropogenic sources, including combustion of fossil fuels for heating and transportation, power generation, industrial processes, solid-waste disposal, and forest fires. In forested and rural areas of the United States, ambient nitrogen dioxide concentrations average less than 0.10 ppm (parts per million), whereas in urban areas peak levels may exceed 0.2 ppm, particularly in the late afternoon and evening (EPA 1993). As a major component of smog, nitrogen dioxide has been measured at concentrations of between 0.1 and 0.8 ppm (maximum hourly average) with short-term peaks of 1.27 ppm (Mohsenin 1994). Indoor air also can contain nitrogen dioxide at peak concentrations of 2–4 ppm as a result of the use of gas-burning appliances or kerosene heaters. Firefighters can encounter concentrations of up to 1 ppm, but rarely higher (Gold et al. 1978).