## 3 Results and Discussion
### Human Health Risk Assessment  
The exposure concentrations (\(EC\)) for the study periods, calculated based on monthly average PM\({}_{10}\) concentration, ranged from 7.1 ug/m\({}^{3}\) (May 2019) to 44.7 ug/m\({}^{3}\) (December 2009), while the \(EC\) calculated based on monthly 90 th percentile of PM\({}_{10}\) ranged from 9.3 ug/m\({}^{3}\) (May 2019) to 64.2 ug/m\({}^{3}\) (January 2010). This data shows that the EC gradually decreased from 2010 to 2019. The non-carcinogenic chronic \(HQ_{1}\) calculated based on the EU limit value obtained for the residential best-case scenario ranged between 0.2 and 1.1 in all studied years, indicating potential non-carcinogenic chronic health risk in the first years of the study (2009 and 2010), and no health risks for the other study years (2011-2019). However, for the worst-case scenario, the \(HQ_{1}\) values were higher (0.2-1.6), with monthly peaks with values above the safety benchmark being found in each year. The monthly variation of \(HQ_{1}\) in the best-case (Figure 4a) and worst-case scenarios (Figure 4c) was similar to that in the PM\({}_{10}\) concentration, being higher in the cold months than in the warm months. In the best-case scenario, non-carcinogenic health risk was identified only in December 2009, while in the worst-case scenario, non-carcinogenic health risk was identified in each year, mainly in the winter seasons. In both exposure scenarios, over the years, a clear decreasing trend from 2009 to 2019 was observed. Figure 4: Temporal variation of hazard quotient (\(HQ\)) calculated based on EU limit values for best-case (**a**) and worst-case (**b**) scenarios, and calculated based on World Health Organization air quality guideline for best-case (**c**) and worst-case (**d**) scenarios in Cluj-Napoca city for 2009–2019. The non-carcinogenic chronic \(HQ_{2}\) calculated based on the _WHO AQG_ obtained for the best-case scenario ranged between 0.4 and 2.1 in all studied years, indicating a potential non-carcinogenic chronic health risk between October and March in each year, while for the worst-case scenario, the \(HQ_{2}\) values were higher (0.5-3.2), exceeding the security benchmark in most of the months. The monthly \(HQ_{2}\) values, both in the best-case (Figure 4b) and worst-case scenarios (Figure 4d), were higher in the cold months than in the warm months, but slightly decreased from 2009 up to the present, without frequently falling below the security benchmark, indicating a non-carcinogenic risk for almost the whole period. The high \(HQ\) values suggest that environmental PM\({}_{10}\) levels, although below the legislative thresholds, are likely to induce various chronic pathologies. The differences between the human health risk measurements from the air quality monitoring stations for the period 2009-2019, in Cluj-Napoca city, were observed both for the best-case and worst-case scenarios (Figure 5). Thus, \(HQ_{1}\) values, calculated based on the EU limit values, did not exceeded the unity threshold at any of the monitoring sites, suggesting no potential health risk, but they were higher at the urban (CJ-1) and suburban (CJ-3) background monitoring stations than at the traffic (CJ-1) and industrial (CJ-4) stations. Similarly, in the case of \(HQ_{2}\), the health risk was higher at the urban and suburban stations than at those monitoring the traffic and industrial background, but in this case potential non-carcinogenic risk might possibly appear. A similar pattern was observed for the worst-case scenario, both for \(HQ_{1}\) and \(HQ_{2}\) when the unity threshold was above 1, indicating the presence of potential non-carcinogenic risk. Similar to Cluj-Napoca, a non-carcinogenic risk associated with PM\({}_{10}\) exposure was reported in Krakow, Poland and Shanghai, China [10,50]. Opposite to Cluj-Napoca, no human health risk associated with exposure to PM\({}_{10}\) for the general population, and a low risk for the sensitive population groups, were found for Pretoria West, South Africa [51].