## 5 Conclusions  
The air quality index and health hazards were determined based on data provided by the EPA reports for the last 11 years (period of 2011-2021), for three pollutants (PM\({}_{2.5}\), CO, and As). For each city, three monitoring stations for daily measurements were considered. Statistical analyses were performed using the minimum and maximum concentrations measured within each year for each city, while the air quality index and health hazards considered the annual average measured concentrations. The results prove that Iasi is the most polluted city in the case of fine particulate matter, with the highest measured value in 2017, 44% higher compared to 2016. Carbon monoxide concentrations remained below the alert threshold, registering variations with a maximum increase in 2017, increasing by 75% compared to 2016. The arsenic concentrations were recorded close to the alert threshold value in Timisoara, for each year. In this case, the highest increase was observed in 2020, increasing by 90% compared to 2019. The air quality situation in Brasov indicates a decreasing trend of the concentrations of the three pollutants starting in 2018. Regarding the city of Cluj-Napoca, for PM\({}_{2.5}\) and carbon monoxide, a decreasing trend of pollutant concentrations was recorded. A more unusual situation was reported for arsenic because no concentrations, or insignificant ones, were measured until 2020, the COVID-19 pandemic year, when a value of 2.59 [ng/m\({}^{3}\)] was recorded, but in 2021, it decreased to 0 again. Regarding the variation in the maximum concentrations recorded on certain days of each year, compared to the value of the alert threshold, it was found that the threshold was exceeded for PM\({}_{2.5}\) in the case of all four cities.  
Since the values of the air pollution index and health hazard index are within the regular range, the immediate perspective of this study includes a more complex statistical analysis. This also includes data from low-cost sensors for intercomparison and to create a degree of confidence that could be considered, for instance, in smart city applications. The results of the aggregate index in the case of the four cities considering the three pollutants for the period 2011-2021 show that the most polluted city is Iasi, followed by Cluj-Napoca, Brasov, and Timisoara. The air pollution index and the health hazard index showed that, even though their values are within normal limits, there is still a risk for human health since the values are almost triple for children compared to adults. This work offers certain perspectives, being a starting point for a much more detailed future analysis of the data and the correlation of possible increases or exceedances of the normal limits depending on the season, or even other parameters, because it is known that one of the factors that influence pollutant concentration variations is the clime of each season.  
Conceptualization, B.S. and I.T.; methodology, B.S.; software, I.T. and M.C.; validation, M.C. and B.S.; formal analysis, I.T.; investigation, I.T.; resources, I.T.; data curation, I.T and M.C.; writing--original draft preparation, I.T.; writing--review and editing, B.S. and M.C.; visualization, M.C.; supervision and project administration, B.S. All authors have read and agreed to the published version of the manuscript.  
This work was supported by a grant from the Romanian Ministry of Education and Research, CNCS-UEFISCDI, project number PN-III-P1-1.1-TE-2019-1921, within PNCDI III.