Impurity	Designation
Ammonia	NH3
Benzo(a)pyrene	BP
Suspended solids	SS
Nitrogen dioxide	NO2
Sulphur dioxide	SO2
Ozone	O3
Carbon monoxide	CO
Formaldehyde	F
Hydrogen sulphide	H2S
Particles 10 microns	PM10
Particles 2.5 microns	PM25

The following characteristics are used in order to determine the level of atmospheric pollution:
• q_ср(q average) – average concentration of pollutant (impurity), mg/m3 or µg/m3
• q_м (q maximum) - maximum single measurement concentration of impurity, mg/m3 or µg/m3

The degree of air pollution is assessed through comparison of actual concentrations with MPC.
MPC – maximum permissible concentration of impurities for residential areas, defined by the Chief sanitary inspector of the Russian Federation.

Average concentrations are compared with the average daily MPC (MPC daily average) and annual (MPC annual), while maximum single measurement concentrations - with the maximum single MPC (MPC maximum single measurement).
The following indicators of air quality are used:
• API – comprehensive index of atmospheric pollution, incorporating several impurities. The API value is calculated based on the values of annual average concentrations. The indicator reflects the level of chronic, long-term air pollution
• SI - standard index, the highest measured single measurement impurity concentration, divided by MPC maximum single measurement. It is determined based on the observation data at the station for one impurity, or at all stations within the examined territory for all impurities for a month or a year
• HF - the highest frequency of occurrence, % of excess of MPC maximum single measurement according to the observation data for a single impurity at all town stations over the year
In accordance with existing methods of assessing, the level of air pollution is considered elevated if API is between 5 and 6, SI < 5, HF < 20%, high if API is between 7 and 13, SI - between 5 and 10, HF from 20% to 50% and very high if API is greater than or equal to 14, SI > 10, HF > 50%.

Information source:
1. Yearbook of the state of the atmosphere in cities on the territory of Russia for 2015 – St. Petersburg: IPCPU, 2016
2. A review of the status and pollution of environment in Russian Federation in 2015 – Moscow: Roshydromet, 2016