## 2 Materials and Methods
### Human Health Risk Assessment  
#### 2.4.1 Health Risk Assessment Model  
The Dose-Response Model recommended by the US EPA was conducted to determine the health risks posed by MSW heavy metals. According to the model theory, the three primary heavy metals exposure pathways to MSW workers are hand-mouth ingestion, dermal absorption, and air inhalation. The average daily intake (ADI, mg/kg/day) of heavy metal via the three exposure pathways can becalculated using Equations (1)-(3) [25]. Hazard quotient (HQ) of non-carcinogens can be determined subsequently by dividing the ADI of each exposure pathway by the corresponding reference dose (RfD, mg/kg-day) [25]. Furthermore, the hazard index (HI) is defined by adding HQs of each non-carcinogens or each exposure pathway together to assess the combination non-carcinogenic risks posed by multiple non-carcinogens and/or more than one exposure pathways (refer to Equation (4)) [25]. Thus, the total exposure hazard index (TEHI) is the sum of HIs to estimate the overall potential non-carcinogenic hazard posed by all of non-carcinogens to the MSW workers through all exposure pathways (refer to Equation (5)) [25]. Whereas, the level of carcinogenic risk caused by carcinogens is calculated through multiplying the ADI by the corresponding slope factor (SF, per mg/kg-day). Similarly, the total carcinogenic risk index (TCRI) is the sum of carcinogenic risk of each carcinogen across all exposure pathways (refer to Equation (6)) [25].  
\[\text{ADI}_{\text{ing}}=\text{C}_{\text{MSW}}\times\frac{\text{IngR}\times \text{EF}\times\text{ED}}{\text{BW}\times\text{AT}}\times 10^{-6} \tag{1}\]  
\[\text{ADI}_{\text{dermal}}=\text{C}_{\text{MSW}}\times\frac{\text{SA}\times \text{AF}\times\text{ABS}\times\text{EF}\times\text{ED}}{\text{BW}\times \text{AT}}\times 10^{-6} \tag{2}\]  
\[\text{ADI}_{\text{inh}}=\text{C}_{\text{MSW}}\times\frac{\text{InhR}\times \text{EF}\times\text{ED}}{\text{PEF}\times\text{BW}\times\text{AT}} \tag{3}\]  
\[\text{HI}=\sum\text{HQ}_{i}=\sum\frac{\text{ADI}_{i}}{\text{RfD}_{i}} \tag{4}\]  
\[\text{TEHI}=\sum\text{HI}=\sum_{j=1}^{N}\frac{\text{ADI}_{\text{ing}}^{j}}{ \text{RfD}_{\text{ing}}^{j}}+\sum_{j=1}^{N}\frac{\text{ADI}_{\text{dermal}}^{j}}{\text{RfD}_{\text{dermal}}^{j}}+\sum_{j=1}^{N}\frac{\text{ADI}_{\text{inh}}^{j}}{ \text{RfD}_{\text{inh}}^{j}} \tag{5}\]  
\[\text{TCRI}=\sum_{j=1}^{N}\Bigl{(}\text{ADI}_{\text{ing}}^{j}\times\text{SF} _{\text{ing}}^{j}\Bigr{)}+\sum_{j=1}^{N}\Bigl{(}\text{ADI}_{\text{dermal}}^{j} \times\text{SF}_{\text{dermal}}^{j}\Bigr{)}+\sum_{j=1}^{N}\Bigl{(}\text{ADI}_ {\text{inh}}^{j}\times\text{SF}_{\text{inh}}^{j}\Bigr{)} \tag{6}\]  
where \(\text{ADI}_{\text{ing}}\), \(\text{ADI}_{\text{dermal}}\), and \(\text{ADI}_{\text{inh}}\) are the average daily intake from hand-mouth ingestion, dermal absorption and air inhalation, respectively (mg/kg\(\cdot\)day); \(\text{C}_{\text{MSW}}\) is the heavy metal concentration of MSW (mg/kg); IngR is the rate of hand-mouth intake (mg/day); InhR is the rate of breath inhalation (m\({}^{3}\)/day); EF is the exposure frequency (day/year); ED is the exposure duration (year); BW is the body weight of the exposed individual (kg); AT is the time period over which the dose is averaged (day); PEF is the emission factor(kg/m\({}^{3}\)); SA is the exposed skin surface area (cm\({}^{2}\)); AF is the adherence factor (kg/cm\({}^{2}\)-day); ABS is the dermal absorption factor (unitless); RfD is a reference dose (mg/(kg\(\cdot\)day)); HQ is the non-carcinogenic risk quotient; SF is the carcinogenic slope factor; HI is the non-carcinogenic risk index; TEHI is the total non-carcinogenic index; TCRI is the carcinogenic risk index; \(i\) indicates the number of heavy metals or exposure pathways; N is the number of heavy metals.