Big urban centres are the epicentres of business and cultural life. However, the dense population, the high number of industrial facilities and major roads in their vicinity have brought into question the air quality in their area. Large amounts of pollutants are released into the air, which have a negative influence on human and ecosystem health. To control air pollution, but also for the purpose of informing the public in an easily understandable way about air quality in a certain area, the air quality index (AQI) was created. In this thesis, analyses of AQI values at measuring stations in Paris and London were carried out, taking into account the frequency of extreme events over the years and comparing data before and during the COVID-19 lockdown. A boxplot analysis was also performed between AQI and population density and number of inhabitants. The results showed that over the years, the worst air quality was recorded in the colder periods of the year, and at stations near major roads. A comparative analysis of AQI from the period of the COVID-19 pandemic and the year preceding it showed a significant decrease in \(NO_{2}\) concentrations and an increase in \(O_{3}\) concentrations in the air during the lockdown.