Some effects of nitrogen dioxide on the lung.

Nitrogen dioxide (NO2) when inhaled in different concentrations and for varying times produces pulmonary injuries which are dependent on the anatomic site in the lung and the duration of exposure. Single exposures to high concentrations of NO2 for 5-6 hr produce an intense cellular proliferation which regresses within 48 hr in all lung regions except the terminal respiratory bronchiole region and the alveoli where the proliferation persists for 4-7 days. This same delayed response is also observed in more chronic exposures. Histologically, the lesion in the terminal and respiratory bronchioles may resemble an obliterative bronchiolitis, but the lesion clears if further exposure is terminated. Prolonged exposure to 2 ppm NO2, 20-22 hr per day for 7 days per wk, produces an increase in total pulmonary upstream resistance in animals killed immediately after exposure; this resistence returns to normal values within 3 mo after removal from exposure. The internal surface area (ISA) is decreased after 12 mo exposure to NO2, but this loss of surface progresses during the recovery period in air, suggesting an autonomous progression of the tissue destructive process. Exposure to 3.64 ppm NO2 with and without fly ash for periods of 12-14 mo causes no increase in pulmonary resistance and no alteration in lung surface. Lung phospholipids and protein synthesis appear to be depressed following exposure to NO2. Lecithin is significantly increased. The synthesis of proteases by alveolar macrophages is increased during NO2 exposure. Pigmented alveolar macrophages present in animals exposed to NO2 simulate those found in human lungs of young cigarette smokers in the terminal and respiratory bronchioles. The mechanism of tissue injury by oxidants such as NO2 may involve free radical formation, and peroxidation of lipids or proteins.