Pollutant means any substance present in ambient air, which could have adverse effects on human health and/or environment as a whole.

          Regulated Pollutants are the pollutants taken into account in assessing the ambient air quality through the Law no. 104 from 15/06/2011 (Romania), Clean Air Law and Ordinance no.12 from 15/07/2010 (Bulgaria). These laws represent the transposition of the Directive 2008/50/EC of the European Parliament of the Council into the national legislation - Romanian and Bulgarian.

          Regulated pollutants:

   1. Sulfur dioxide SO₂

   2. Nitrogen oxides NO_X (NO/NO₂)

   3. Ozone O₃

   4. Carbon monoxide CO

   5. Benzene C₆H₆

   6. Particulate matter PM10 and PM2.5

   7. Toxic metals Pb, Cd, As and Hg

   8. Polycyclic Aromatic Hydrocarbons PAH

          Specific index for air quality is system for coding the concentrations recorded for a monitored pollutant.

The specific indexes are represented by integers between 1 and 6, each number corresponding to a color and a concentration range for the monitored pollutant.

The specific index for air quality is a system for coding the concentrations recorded for each of the following monitored pollutants: sulfur dioxide (SO₂), nitrogen dioxide (NO₂), ozone (O₃), carbon monoxide (CO), particulate matter (PM10)

The General Index is established for each of the automatic stations as the highest one from the specific indexes corresponding to the monitored pollutants.