Air quality indices: a review

National directives on air quality oblige nations to monitor and report on their air quality, otherwise require the public to be informed on the state of the ambient pollution. The last is the reason for the always increasing interest, in recent years, in air quality/pollution indices. The interest is demonstrated by the number of publications on this topic. In this paper we will give an overview about the Air Pollution Indices proposed in literature and/or adopted by Governments, trying also to categorize them into homogeneous groups. For the classification different approaches can be followed. Since in real life exposure to mixtures of chemicals occurs, with additive, synergistic or antagonistic effects, here we will distinguish between indices that consider the conjoint effect of pollutants and indices that are only based on the actual most dangerous pollutant.