This indicator shows the frequency of exceeding of the hourly human health limit value for NO₂ concentration, mean annual NO₂ concentrations that are also important for human health (annual limit value 40 μg/m³) and NO_x concentration (plant protection critical value 30 µg/m³) at measuring sites throughout Slovenia in the period 1992–2016. As monitoring of the plant protection critical value is relevant in the natural environment, the Zavodnje, Kovk and Vnajnarje measuring sites are representative for its monitoring.

Nitrogen oxides (the sum of nitrogen monoxide and nitrogen dioxide concentrations) play an important role in the impact of polluted air on the environment, especially through acidification and eutrophication (excessive amount of nutrients in water bodies) as well as formation of photochemical smog. The main sources of nitrogen oxides are traffic, small heating units and thermal power plants using fossil fuels. Nitrogen oxides are formed when nitrogen combines with oxygen at high temperatures. The percentage of nitric oxide (NO) in exhaust gases is over 90%. In the atmosphere, it gradually oxidises into NO₂. The rate of transport-related NO oxidation into higher oxides increases with its distance from the emission source. The presence of ozone in outdoor air greatly accelerates the oxidation reaction. Participation of ozone molecules in the oxidation reaction may reduce the concentration of ozone harmful to humans. Therefore, concentrations of ozone are usually lower along busy roads. In addition to traffic, which causes an increase in NO₂ concentrations at traffic measuring sites during morning and evening hours and on workdays, meteorological conditions (especially solar radiation, temperatures and seasons of the year) also affect NO₂ concentrations.

High concentrations of nitrogen oxide are mostly limited to a narrow band along busy roads and streets. The highest NO₂ concentrations are measured at the central Ljubljana traffic measuring site, where the measured values almost reach or exceed the annual limit value. From the perspective of human health, the Ljubljana centre, Maribor centre, Nova Gorica and Celje measuring sites are the most problematic.

Since 1994, NO₂ concentration has not been exceeding the legally prescribed hourly limit value for human health protection. Also the annual limit value of 40 µg/m³ has not been exceeded anywhere in Slovenia since 2014.

The annual limit NO_x concentration for the protection of vegetation and allowable in the natural environment has not been exceeded in a long time.

As a rule, NO₂ concentrations are higher in wintertime, which is largely a result of unfavourable weather conditions (temperature inversion). They are lower during the summer since NO₂ is used for the formation of ozone and because wind conditions are more favourable. Elevated NO₂ concentrations are mostly measured along busy roads. The NO₂ concentration decreases the further away you get from busy roads. Thanks to the introduction of catalytic converters in cars, NO₂ emissions from transport have decreased; however, the decrease was not substantial, due to the permanently increasing volume of traffic. Large thermal power plants that use fossil fuels are another source of nitrogen oxides; however, increased concentrations have not been observed in the vicinity of large thermal power plants in recent years.