Governing equations for health risk assessment

According to the USEPA, “a human health risk assessment is the process to estimate the nature and probability of adverse health effects in humans who may be exposed to chemicals in contaminated environmental media, now or in the future”[31]. Pollutants such as Cd, Cr, Cu, Fe, Mn, and Zn were selected in this study. The health risk assessment of each heavy metal contaminant in water is based on the estimation of the risk level. The methods delineated by the USEPA [32,33] have been used to estimate the average chronic daily intake (CDI) from direct ingestion and dermal absorption routes, respectively. The governing equations are as follows:

where CDI_{ingestion/dermal} is expressed in μg/kg/day; C_w is the measured concentration of chemicals in water (μg/L); IR is the ingestion rate (3.5 L per day for adults [34]; 1.32 L per day for children [35]); EF represents the exposure frequency (EF = 365 days per year); ED is the exposure duration (ED = 70 years for carcinogenic risk and 30 years for non-carcinogenic risk for adults; 6 years for non-carcinogenic risk for children [32, 33]); BW is the average body weight (57.5 kg for adults [36] and 15 kg for children [32,33]); AT refers to the average time representing the period over which exposure is averaged (AT = 25,550 days for carcinogenic risk and 10950 days for non-carcinogenic risk for adults; 2190 days for children [32,33]); SA is exposed skin area available for contact (18000 cm² for adults; 6600 cm² for children [33]); ET is exposure time (0.58 h/day for adults; 1 h/day for children [33]); CF is unit conversion factor (0.001 L/cm³ [33]); and Kp is dermal permeability coefficient (cm/h) of heavy metal in water [33].

The hazard quotient (HQ) is an estimate of the toxicity potential posed by an element from direct ingestion or dermal absorption routes, which can be calculated using the relation:

where RfD_{ingestion/dermal} represents oral/dermal reference dose (μg/kg/day) [32,33]) of each contaminant under consideration. In general, RfD is an approximate estimate of daily exposure to the human (including sensitive subgroups) that is likely to have any noticeable risk of harmful effects during a lifetime [32,33]. The RfD estimate may have an uncertainty spanning perhaps an order of magnitude.

The hazard index (HI) is the overall potential for non-carcinogenic effects posed by more than one contaminants via ingestion or dermal pathway, which can be estimated from the relation:

Carcinogenic risk (CR) associated with the ingestion pathway can be estimated using the formula:

where CSF_ingestion represents the cancer slope factor. Of the selected six heavy metals, Cd and Cr have significant cancer risk.