## 4 Discussion  
During the pandemic, numerous studies conducted across countries worldwide examined the impact of restrictions on air pollution [46; 47; 48]. Most studies reported a decrease in pollutant concentrations [49], although some noted increases in pollution or a mixed effect [48]. However, the urban atmosphere is influenced by various factors, including traffic, industrial activities, construction, and dust-containing particulate matter that can travel both short and long distances. In the present study, we demonstrate that simply comparing different time periods is insufficient to fully understand the extent of pollution reduction. Meteorological factors can significantly influence atmospheric dilution and accumulation processes, complicating the assessment of pollution levels. Additionally, special events such as long-range transport can also impact local pollution. To minimize these influential factors, in the present study, the data series of PM\({}_{10}\) mass concentrations were analyzed to understand the impact of the COVID-19 pandemic restriction during 2020 on air pollution and human health using a long-time reference period before (2017-2019) and after (2021-2022) the pandemic. This approach leads to more convincing results, showing that the lockdown measures led to a significant decrease in air pollution levels in all three cities, thus highlighting the correlation between traffic and air quality in cities. A gradual increase in PM\({}_{10}\) concentrations was observed after the lockdown period once the traffic in cities returned to normal. Similarly, at the global level, significant reductions in PM\({}_{10}\) and PM\({}_{2.5}\) concentrations were observed during the lockdown period. For instance, [PERSON] et al. (2021) reported a 71% decrease in PM\({}_{2.5}\) in Salt Lake County [47], while they found reductions in PM\({}_{10}\) of 21% in Delhi and 15% in Sao Paulo compared with 2019 levels. Some previous studies hypothesized that small changes in PM\({}_{10}\) concentrations could be related to residential activity and an increase in domestic fuel consumption in residential areas [50]. However, the lack of detailed traffic data in cities can complicate the interpretation of these findings. The Google Community Mobility Report data were used to interpret the influence of human activities on air pollution by PM\({}_{10}\). Thus, the mobility in these three Romanian cities during the lockdown period decreased by about in Bucharest (50% Brasov, 45% Iasi). At the same time, the residential activities increased by approximately 22% in Bucharest (23% Brasov, 18% Iasi), indicating an increase in the number of people working from home. There was also a reduction of 80% in Bucharest (70% Brasov, 70% Iasi) in the visits to shops and establishments. The impact on park activities was uncertain because these activities also depend on other factors, such as weather conditions. Concerning the health risks associated with exposure to PM\({}_{10}\), the results using the hazard quotient method indicated that there is a long-term risk for cancer in these cities due to exposure to PM\({}_{10}\). Even if the PM\({}_{10}\) levels were reduced during 2020, this was not enough to reduce the chronic health risks during cold months in all three cities. In contrast, similar studies conducted over a 10-year period (2009-2019) in Cluj-Napoca, a major city in northern Romania, indicate that PM\({}_{10}\) concentrations pose significant concerns according to WHO recommendations. These exposure levels correspond to an HQ between 0.5 and 3.2, consistently above 1, with higher risks identified during the colder months [40]. However, long-range transport can also influence the health risk. The important event occurred on 27-28 March 2020, conducted to PM\({}_{10}\) levels exceeding the limits imposed by the European Commission's Directive 2008/50/EC in Bucharest, Brasov, and Iasi, with concentrations approximately four times higher than the permitted levels of 50 \(\upmu\)g m\({}^{-3}\) per day [51].