The researchers studied air quality in Thessaloniki from 2004 to 2009 using data from three monitoring stations. They calculated different air quality indices based on pollutants like CO, SO2, O3, PM10, and NO2. Results showed that air quality in the city center was poor, with PM10 being the main issue. The indices also revealed that air quality was better in the suburbs and on Sundays. Overall, the study found that air quality in Thessaloniki is not good, especially in the city center.