Nitrogen dioxide is toxic by inhalation. Symptoms of poisoning (lung edema) tend to appear several hours after one has inhaled a low but potentially fatal dose. Also, low concentrations (4 ppm) will anesthetize the nose, thus creating a potential for overexposure. Nitrogen dioxide is formed in most combustion processes using air as the oxidant. The most important sources of NO2 are internal combustion engines, thermal power stations and, to a lesser extent, pulp mills.