## 4 Discussion
In Bucharest and Brasov, a maximum concentration of 171 \(\upmu\)g m\({}^{-3}\) was recorded, while Iasi registered a maximum concentration of 91 \(\upmu\)g m\({}^{-3}\) during this period. Identification of dust intrusions over Romania and other European countries can be performed using the DREAMS-assim model, a method currently used in the air quality field to identify and forecast these large-scale events [52] (Supplementary Material, Figure S3). The output of the DREAMS-assim model [53] for surface dust concentration showed a high dust particle intrusion over Romania, with an estimated concentration greater than 200 \(\upmu\)g m\({}^{-3}\). Previous studies on this event have associated the dust intrusion source with the Arallum desert from Kazakhstan [54]. The event under discussion is a unique and significant incursion of mineral dust that originated on 27 March 2020. This event period was characterized by a number of atmospheric anomalies that were not all related, including an intense, prolonged, and widespread eastern circulation that was relatively uncommon at these latitudes due to fundamental dynamic constraints and the SARS-CoV-2 lockdown [49]. Furthermore, atmospheric dynamics affect dust activity in central Asia both directly and indirectly [55]. This special event of dust intrusion in March 2020 significantly impacted HQ values, leading to unusually high levels. In Bucharest, the HQ2 ranged from 1.39 to 2.56, recording the highest values, while in Brasov and Iasi, HQ2 fluctuated between 1.18 and 1.47, all exceeding the benchmark limit of 1. In particular, there is evidence of an increasing number of events with dust and smoke coming from long distances over Romania that lead, therefore, to high peaks of PM [56]. In this light, when such pollution episodes span over a longer time, it is very likely the hazard quotient will increase. [PERSON] et al. (2022) performed a study of eight large regions in Romania, calculating the health risk for all-cause mortality associated with short-term PM\({}_{10}\) exposure [57]. They used a methodology different from the present study, based on averages of PM\({}_{10}\) mass concentrations by region. They showed the higher excess risk is in the Bucharest region, while the lowest excess risk was found in the western regions, in areas without industrial pollution. Because people living in densely populated cities like Bucharest, Brasov, and Iasi are more exposed to higher levels of PM\({}_{10}\) than the regional average, they are, therefore, more likely exposed to a higher risk, as the hazard quotient (HQ1 and HQ2) in the present study indicates. The cancer risk (CR), the hazard quotient (HQ) from particle-bound metal concentrations and the epidemiology-based excess risk (ER) were comparatively estimated by [PERSON] et al. (2019) in three European cities (Athens, Kuopio, Lisbon) [58]. They found that HQ, due to the inhalation of particle-bound metals, was lower than the acceptable level (<1), but ER and attributable fraction for PM\({}_{10}\) and PM\({}_{2.5}\) were higher in Lisbon compared with Athens and Kuopio due to higher ambient concentrations measured in this city. The cancer risk was higher than acceptable in some situations. To investigate these aspects in Romanian cities, further analyses are necessary, and they will be carried out in the future. This study shows the importance of implementing efficient and sustainable measurements for reducing air pollution and thus reducing the impact on human health. The COVID-19 pandemic shed light on how reducing activities such as traffic can improve air quality in cities. Even if this strict traffic restriction cannot be applied in normal conditions, there are other measures that can be implemented to improve air quality. These measures include the use of electric vehicles, the enhancement of public transportation systems and the creation of more green spaces in cities.