Nitrogen dioxide is formed during burning processes. Main sources are the motor vehicles (MV), thermal electric power stations (TEPS), some industrial enterprises, tobacco smoking. Under the influence of intensive sunlight and in the presence of volatile organic substances in the atmospheric air nitrogen dioxide undergoes chemical reaction as a result of which the secondary pollutant ozone is formed.

Impact on human health
Nitrogen dioxide penetrates the human organism through breathing. Major part of the nitrogen dioxide is absorbed by the organism, and a substantial portion thereof may be retained for a long time in the lungs.
Continuous exposure to concentrations above the LV may cause structural lung changes. This pollutant has harmful effect mainly on the respiratory function.
Adverse effect is witnessed with chronic patients suffering from respiratory infections, and the asthmatic patients are particularly sensitive to nitrogen dioxide level rise. It is established that in the case of short-term exposure (in the course of 1 hour) the lowest concentration influencing the asthmatic patients is 560 mg/m3. This serves as a basis for determining the admissible air pollution limits.

The following limit values for nitrogen dioxide are set by Regulation No. 9 (State Gazette No. 46/1999, amended and supplemented, SG No. 86/2005): - 1-hour LV (for 1 hour exposure) - 200 µg/m3;
- average annual LV (for 1 year exposure) - 40 µg/m3;
- alarm threshold - 400 µg/m3, measured during three consecutive hours by the monitoring stations, which are representative for the air quality in a certain whole region or agglomeration.
If the alarm threshold is exceeded measures should be taken for limitation of the adverse effect of the pollutant.