The largest single source of nitrogen oxides in the atmosphere is bacterial action in soil. The nitrogen acids are then washed from the atmosphere by rain or some other form of deposition, which is harmful to aquatic lives in rivers and lakes.
Nitrogen oxides exert their effects on the environment both in the form of oxides and as acids.
Oxides damage leaves, flowers, fruits and other plant structures. Nitric acid attacks metal, marble, limestone, and other building materials. High concentrations of nitrogen oxides are also harmful to animals and humans. Individuals exposed to concentrations of 100 ppm or more in an hour or less may experience nasal irritation, breathing discomfort, respiratory distress, pulmonary edema, and even death. One reason is that the conversion of SO 2 to SO 3 occurs only slowly.