Abstract In recent years, more and more studies have been published on the impact of air pollution on the increase in the incidence of type 1 diabetes mellitus (T1DM) in children and adolescents. To confirm this, we attempted to show whether there are differences between the impact of air pollution in 2017–2018 compared to the impact of air pollution during the lockdown period, i.e., 2020–2021, and its potential relationship with the incidence rates of new cases of T1DM. Methods: We obtained the number of new cases of T1DM in 2017–2018 and 2020–2021 in the Lublin Voivodeship. Data on the annual average concentrations of nitrogen dioxide (NO2), nitric oxides (NOx), sulphur dioxide (SO2) and particulate matter (PM10 and PM2.5) were obtained from Annual Air Quality Assessment reports from 2017–2018 and 2020–2021, made available by the Provincial Inspectorate of Environmental Protection (WIOS) in Lublin. Results: In 2017–2018, air pollution in the entire Lublin Voivodeship was higher than during the lockdown period, i.e., 2020–2021. Moreover, in 2017 and 2018 in the Lublin Voivodeship, strong statistically significant positive correlations were found between NO2 and PM2.5 concentrations and the occurrence of T1DM in children. Conclusions: The research results indicate that air pollution is one of the factors that may suggest a potential association with the development of T1DM. Therefore, every effort should be made to minimize air pollution, which will reduce the risk of developing T1DM and other diseases. Keywords: children and adolescents; type 1 diabetes mellitus; air pollution; particulate matter; nitrogen dioxide; nitric oxides; sulphur dioxide; oxidative stress; lockdown