*2.4. Health Risk Assessment*

In this study, health risks associated with O3 and PM2.5 via the inhalation route were calculated according to the recommended methods of the U.S. EPA and previously reported studies [27,28]. Exposure concentrations (EC) of O3 and PM2.5 were calculated according to Equation (5).

$$\text{EC} = (\text{CA} \times \text{EF} \times \text{ET} \times \text{ED}) / \text{AT} \tag{5}$$

In this expression, CA is the concentration of O3 and PM2.5; ET is the exposure time (3 h/day); EF is the exposure frequency (300 d/year); ED is the exposure duration (25 years); and AT is the average time (non-carcinogenic risk: ED × 365 d/year × 24 h/day, carcinogenic risk: 70 years × 365 d/year × 24 h/day). The hazard quotient (HQ) method was used to estimate the non-carcinogenic risk. The carcinogenic risk CR was calculated from the Inhalation Unit Risk (IUR). The HQ and CRs are expressed by Equations (6) and (7).

$$\text{HQ} = \text{EC} / \text{REL} \tag{6}$$

$$\text{CR} = \text{EC} \times \text{IUR} \tag{7}$$

In the above equation, REL is the reference exposure level. According to the NAAQS, the second upper limit standards for O3 and PM2.5 were 160 μg/m<sup>3</sup> and 75 μg/m3, respectively. The IUR value per μg/m<sup>3</sup> of PM2.5 was 0.008 [29]. The non-carcinogenic and carcinogenic risks of PM2.5 were calculated. In contrast, due to the lack of available data on exposure parameters, only non-carcinogenic risks were determined for O3 exposure. An HQ value less than 1 indicates no significant risk. A CR value less than 1 × 10−<sup>6</sup> indicates a negligible cancer risk, a CR value between 1 × 10−<sup>6</sup> and 1 × 10−<sup>4</sup> indicates a potential cancer risk, and a CR value greater than 1 × 10−<sup>4</sup> indicates a high potential cancer risk.
