# Temporal Variations of Heavy Metal Sources in Agricultural Soils in Malta
## 2 Materials and Methods
### Health Risk Assessment  
Exposure to heavy metals and other chemicals present in the environment such as arsenic, cadmium, chromium, cobalt, copper, lead, mercury, molybdenum, selenium, nickel, and zinc [22] may pose risks to human health through inhalation, dermal contact, and ingestion. Impacts of the content of various heavy metals in soil samples are assessed by calculating the average daily doses (ADD) by ingestion and by dermal contact as shown in the below equations. \[\text{ADD}_{\text{Ingestion}}=\frac{\text{C}_{\text{s}}\times\text{IR}_{ \text{ingest}}\times\text{EF}\times\text{ED}\times\text{CF}}{\text{BW}\times \text{AT}} \tag{1}\]  
\[\text{ADD}_{\text{dermal}}=\frac{\text{C}_{\text{s}}\times\text{SA}\times \text{AF}\times\text{ABS}\times\text{EF}\times\text{ED}\times\text{CF}}{\text{ BW}\times\text{AT}} \tag{2}\]  
where C\({}_{\text{s}}\) is the average heavy metal concentration in the sample; IR\({}_{\text{ingest}}\) is the ingestion rate of heavy metals; EF is the frequency of exposure; ED is the duration of exposure; BW is the average body weight for adults; AT is the averaging time for carcinogens and non-carcinogens; CF is a unity conversion factor; SA is the estimated average exposed skin area; AF is the adherence factor; and ABS is the dermal absorption fraction [22]. Typical values used to compute ADD\({}_{\text{ingestion}}\) and ADD\({}_{\text{dermal}}\) are included in Table 1.  
The Hazard Quotient (HQ) shows the tendency of non-carcinogenic toxicity which may come about from a long-time exposure of humans to a non-carcinogenic heavy metal. The HQ and the Hazard Index, HI, for all metallic elements in the soil samples analyzed are calculated using:  
\[\text{HQ}=\frac{\text{ADD}}{\text{RFD}} \tag{3}\]  
\[\text{HI}=\sum_{i=0}^{\text{n}}\text{HQ}_{i} \tag{4}\]  
where RFD is the reference dose for each heavy metal that an individual is exposed to per day either by ingestion or dermal contact throughout his entire life without any harm; \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Parameter** & **Symbol** & **Unit** & **Adult Value** & **Children Value** \\ \hline Ingestion Rate & IR\({}_{\text{ingest}}\) & mg/day & 100 & 200 \\ Exposure Frequency & EF & days/year & 350 & 350 \\ Exposure Duration & ED & years & 30 & 6 \\ Average Body Weight & BW & kg & 70 & 15 \\ Non-Carcinogens Averaging Time & AT = ED \(\times\) 365 & days & 10,950 & 2190 \\ Carcinogens Averaging Time & AT = 70 years \(\times\) 365 & days & 25,550 & 25,550 \\ Unity Conversion Factor & CF & kg/mg & 1 \(\times\) 10\({}^{-6}\) & 1 \(\times\) 10\({}^{-6}\) \\ Exposed Skin Area & SA & cm\({}^{2}\) & 5700 & 2800 \\ Adherence Factor & AF & mg/cm\({}^{3}\) & 0.07 & 0.07 \\ Dermal Absorption Fraction & ABS & / & 0.001 & 0.001 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Parameter values used to compute average daily dose by ingestion and dermal contact [22; 23]. HI is the overall toxic risk and n is the total number of metals under consideration as stated in the United States Environmental Protection Agency IRIS (USEPA IRIS) (2011), and referenced by [PERSON], et al. (2020), [PERSON] (2010), and [PERSON], et al. (2017) [22; 24; 25; 26] shown in Table 2. If HQ \(>\) 1, it represents adverse non-carcinogenic effects and if HQ \(<\) 1, it shows an acceptable level of these concentrations without any concerns. If the Hazard Index, HI \(<\) 1, the non-carcinogenic adverse effects are assumed to be negligible [24; 25]. Cancer risk (CR) is the incremental risk of a person having cancer over a lifetime by means of exposure to carcinogenic substance (like heavy metals) because of the exposure of twenty-four hours per day for almost a full year for a lifetime i.e., seventy years [27].