After a brief introduction about: the different gaseous pollutants present in the atmosphere; the cause of their concentration rise: their consequences on human health; and the different devices used to monitor each gas, two types of semiconductor gas sensors are presented. The first of them is a simple InP-based resistive sensor. The sensitive layer is a thin n-type InP epitaxial layer grown on a semi-insulating InP substrate. The electrical resistance, increasing in the presence of oxidising gases, is measured between two ohmic contacts. The second sensor is a thin film of copper phthalocyanine deposited by sublimation on an alumina substrate. The conductivity of the phthalocyanine layer increases in the presence of oxidising gases. Influence of NO2 and O-3, especially at low concentration, is investigated. Influence of other parameters like operating temperature or interfering gases is also studied. In conclusion, optimisation of sensor characteristics to improve the evaluation of oxidising air pollutants concentrations is discussed.