Risks Assessment of Copper (Cu), Lead (Pb), Mercury (Hg) and Zinc (Zn): A case study of Tilapia guineensis in Lagos Lagoon

This study determined the daily intake of copper (Cu), lead (Pb), zinc (Zn) and mercury (Hg) in order to assess both carcinogenic and non-carcinogenic risks caused by the heavy metals in Tilapia guineensis of Lagos Lagoon. The heavy metals were determined using flame atomic absorption spectrophotometer. The metal distribution was Zn > Cu > Pb > Hg with values of 0.0248±0.04, 0.0093±0.01, 0.0005±0.00 and 0.000±0.00 mg/kg respectively. Daily Intake of Metal was in the order of Zn (0.02) > Cu (0.01) > Pb (0.00) ≥ Hg (0.00), while Target Hazard Quotient decreased in the order of Pb (3.82×10 -4 ) > Cu (9.54×10 -5 ) > Zn (2.49×10 -6 ) > Hg (0.00). The values for Health Risk Index were 0.016 (Cu), 0.00 (Hg), 0.006 (Zn) and 0.034 (Pb), while that for the Health Quotients were 0.400 (Pb), 0.187 (Cu), 0.066 (Zn) and 0.00 (Hg). Similarly, all Hazard Index were less than one. This showed that the consumption of the fish from the study area had no non-carcinogenic health implication of Hg, Zn, Cu and Pb. However, the Target Cancer Risk for Pb (1.38×10 -8 ) indicated minimum cancer risks for the consumers. The ecological risks quotients also revealed that T. guineensis of Lagos Lagoon constituted no ecological risk to the environment since the ERQs were less than one (i.e. ERQ < 1).


Introduction
The high contents of essential materials for growth and development (e.g. protein, lipid etc.) contained in fishes makes them important components of meals across the globe (Olawusi-Peters & Adejugbagbe, 2020). However, increased aquatic pollution as results of agricultural runoffs, domestic wastes, industrial discharges, accidental oil spills and mine drainages indicate that consumption of fish could seriously expose man to heavy metals as well as other hazardous substances. Most metals in the aquatic environment are bio-accumulative in nature (Orosun et al., 2016) either passively from water or by facilitated uptake. Metals cause great danger to the various components of the food chain in any environment (Abubakar et al., 2014). Generally, bio-accumulation of heavy metals in fish depends on physico-chemical properties of water, metabolism and feeding patterns of fish, time of exposure, environmental or ecological conditions and intrinsic factors such as bioavailability of each metal in the environment, the process of storage, uptake and excretion mechanisms (Ajibare et al., 2018).
Fish that have bio-accumulated heavy metals may pose chronic health problems to its consumers. For example, there is a link between Lead (Pb), cardiovascular, muscular, and renal problems. Lead may also cause reproductive disorders in adults as well as developmental and cognitive deficiencies in children (Omobepade et al., 2020). Copper (Cu) on the other hand is an essential mineral needed for various body activities and functions in humans, but may cause kidney, liver and brain disorders when concentration is above the recommended limit (Ajibare et al., 2018). Due to various health effects associated with heavy metals and other toxic substances stated above, global research interest has increased in the area of food safety with respect to accumulation of these hazardous substances in food including fish.
Different studies have shown that fish is a major source of dietary heavy metals and that its consumers may be exposed to adverse health as a result of the consumption (Orosun et al., 2016). However, fish consumption does not always represent health risk. Thus, this study determined the Daily Metal Intake (DIM), Health Risk Index (HRI), Hazard Index (HI), Health Quotient (HQ), Target Hazard Quotient (THQ) and Target Cancer Risk (TR) of Zn, Hg, Cu, and Pb via consumption of Tilapia guineensis of Lagos lagoon in order to evaluate the health risks posed by the metals to its consumers.

Study Area
The Lagos lagoon (Latitude N 06 o 31. 048' and Longitude E 003 o 24. 473') is an open, shallow and tidal lagoon, with a surface area of about 208 km 2 . The lagoon is located in the South-Western region of Nigeria (Onyema, 2008). The lagoon experiences more discernable brackish condition in the dry season, while low brackish/freshwater conditions are observed in the wet season due to increased river inflow (Onyema, 2008).
Collection and Identification of Samples 36 samples of Tilapia guineensis were collected fortnightly from June to August, 2011, using cast net. The samples were identified according to Olaosebikan & Raji (2013) and transported in ice chest to the Marine Science Laboratory of the University of Lagos. The samples were refrigerated at -4 o C before the determination of heavy metals.

Determination of heavy metals
The thawed fish samples were oven dried at 105°C for 72 hours to achieve constant dry weight. They were pulverized, homogenized and digested according to Javed et al. (2015). Dried tissue (1.0 g) of the sample was weighed into a digestion vessel. About 10 ml acidic mixture of HNO 3 /HClO 4 in a ratio of 2:1 was then added to the sample. It was stirred with a glass rod so that it would be evenly distributed in the acid. The beaker was then placed on the digestion block in a fume cupboard for 2 hours at 150°C for digestion. The digested samples were then filtered into a 25 ml volumetric flask and made to mark with deionized water. The determination of heavy metals (Cu, Pb, Zn and Hg) in the muscles of the fish was done with flame atomic absorption spectrophotometer (FAAS) according to Javed et al. (2015).
For quality control/assurance, double distilled water was used throughout the study. In addition, glass wares were thoroughly prewashed with detergent, soaked in 20% nitric acid and were further rinsed using double distilled water. Samples were run in triplicates in order to ensure the accuracy of all the instruments. Moreover, to check instrumental drift, a reagent blank and a standard were done after every three samples.

Statistical Analysis
The concentration of Cu, Pb, Zn and Hg obtained in this study was subjected to descriptive statistics to determine the means and standard deviations using SPSS 20.0. The ecological and potential health risks of heavy metal consumption through T. guineensis were assessed based on the Ecological Risk Quotient (ERQ), Health Risk Index (HRI), Health Quotient (HQ), Hazard Index (HI), Target Hazard Quotient (THQ) and Target Cancer Risk (TR) for individual adult. Ecological Risk Quotient (ERQ) -which numerically evaluates the associated risks in quantification and interpretation of the concentration of chemicals in aquatic environment was calculated as: Ogungbile et al., 2022) NOTE: Values of ERQ below 1 are unlikely to result in any negative ecological effects and would normally be considered as acceptable. Health Risk Index (HRI) -which gives quantitative information on risk posed by each contaminant to the health of the fish consumers was calculated as: Where, (Isibor & Imoobe, 2017) M is the concentration of metal in sample (mg/kg), Conversion Factor is 0.085, daily fish intake of 48 g/day was estimated as the fish consumption rate in Nigeria (Omobepade et al., 2020), while the average body weight of consumers was 60 kg (Olawusi-Peters & Adejugbagbe, 2020). Reference Oral Doses (RfD) was 0.040, 0.300, 0.004, and 0.0003 mg/kg/day for Cu, Zn, Pb and Hg respectively (USEPA, 2011; Olawusi-Peters & Adejugbagbe, 2020). Health Quotient -which estimates the hazard heavy metal could have on the human population in their later life) was determined as: (Omobepade et al., 2020) Where, W is the dry weight of the fish consumed per/day, M is concentration of metal in the edible parts of fish (mg/ kg), RfD is the reference oral dose. Target Hazard Quotient (THQ) -which is a dimensionless quantity that defines the exposure duration and the non-carcinogenic risk within the period was calculated as: (Orosun et al., 2016) Where FIR is the fish ingestion rate (48 g/person/ day); EF is the metal Exposure Frequency (350 days/year); ED is the metal Exposure Duration (54 years, equivalent to the average life expectancy of the Nigerian population) (Omobepade et al., 2020); BW is the average Body Weight (60 kg) (Olawusi-Peters et al., 2019); and ATn is the average Exposure Time for non-carcinogens (19710). If the THQ value is greater than 1, the exposure is likely to cause obvious adverse effects (USEPA, 2011). Hazard Index (HI) -which is expressed as the sum of the target hazard quotients was calculated as: ( Núñez et al., 2018) (NYSDOH, 2007), the TR values are classified as TR ≥ 10 −1 = 'Very High'; 10 −3 to 10 −1 = 'High'; 10 −4 to 10 −3 = 'Moderate'; TR ≤ 10 −6 = 'Low'.

Concentration of heavy metals
The mean concentrations of Mercury (Hg), Zinc (Zn), Copper (Cu), and Lead (Pb) in T. guineensis of Lagos lagoon, Nigeria are presented in Table 1. Metal distribution in the fish was in the order of Zn > Cu > Pb > Hg. The concentrations of Zn ranged from 0.006±0.00 to 0.0575±0.07 mg/kg with a mean of 0.0248±0.04 mg/kg. Cu content in the fish ranged from 0.0070±0.00 to 0.0135±0.01 mg/ kg with mean of 0.0093±0.01 mg/kg. The average concentration of Pb was 0.0005±0.00 mg/kg with a range of 0.000±0.00 mg/kg to 0.0015±0.00 mg/kg, while Hg was not detected (ND) and was regarded as 0.000 ± 0.00 mg/kg throughout the study.
The Daily Intake of Metals (DIM), Health Risk Index (HRI), Health Quotient (HQ), Target Hazard Quotient (THQ) and Target Cancer Risk (TR) of Zn, Hg, Pb, and Cu through consumption of T. guineensis in Lagos lagoon is presented in Table 2. The trend of the DIM was in the order of Zn > Cu > Pb ≥ Hg with values of 0.02, 0.01, 0.00 and 0.00 respectively. The HRI of the fish species analysed for the metals were all less than 1 (HRI < 1) with values of 0.016, 0.00, 0.006 and 0.034 for Cu, Hg, Zn and Pb respectively. This indicated that no negative health effect would arise as a result of the consumption of the fish. The health quotient (HQ) for Pb (0.400), Cu (0.187), Zn (0.066) and Hg (0.00) indicated that there would be no health risk in the later life of the consumers.
The trend of THQ values for consumers decreased in the order of Pb > Cu > Zn > Hg with values of 3.82×10 -4 , 9.54×10 -5 , 2.49×10 -6 and 0.00 respectively. This also showed that no health implication may arise due to consumption of the fish. The target cancer risk for Pb was 1.38×10 -8 which indicated low cancer risks for the consumers of the fish. The ecology risk quotient (ERQ) in this study revealed no ecological risk from the bio-accumulation of Cu, Hg, Zn and Pb since the values were lesser than 1 (ERQ < 1) for Cu (0.003), Hg (0.000), Zn (0.001) and Pb (0.000). The Hazard Index (HI) of heavy metals in T. guineensis of Lagos lagoon as presented in Figure 1 shows that no systemic adverse health risk would be experienced by the consumers of the fish since all values were less than 1 (HI < 1). The daily intake of metals (DIMs) gives information on the relative bio-availability of metal but does not consider possible metabolic ejection of the metals. It however tells the probable ingestion rate of a given metal (Olawusi-Peters et al., 2019). This study revealed that Daily Intake of Metals through fish for Cu (0.001), Zn (0.02), Pb (0.00) and Hg (0.00) were significantly lower than the recommended Oral Reference Dose (RfD) for the metals (USEPA, 2011). According to New York State Department of Health (NYSDOH, 2007), if the ratio of DIM of heavy metal to its RfD was equal to or less than the RfD, then there is minimum risk. However, if it is greater than up to five times the RfD, then there is low risk.
Also, if it is greater than up to 10 times the RfD, then there is moderate risk. But, if it is greater than 10 times the RfD, it indicates high risk. Therefore, since the concentrations of the observed metals were lower than the RfD, there was minimum or no potential health hazard to the consumers (NYSDOH, 2007). Similarly, the HRI for Cu, Hg, Zn and Pb in Tilapia guineensis were all less than 1, indicating that the consumption of Tilapia guineensis from Lagos lagoon posed minimum or negligible health risk to the consumers. This The results of Hazard Quotient (HQ) showed values below 1 for all the recorded metals. This observation was similar to the findings of Omobepade et al. (2020) who observed HQ values of less than 1 in Nematopalaemon hastatus of coastal waters of Ondo state, Nigeria and the authors concluded that the metals would not pose any serious health hazards on the consumers in the later life. Moreover, the Target Hazard Quotient (THQ) is commonly used to assess the non-carcinogenic risks associated with prolonged exposure to dietary metals (Orosun et al., 2016). THQ does not measure risk, but it shows the extent of concern and its value should not exceed 1. THQ values greater than 1 means potential (non-carcinogenic) risks to the exposed population. Also, NYSDOH (2007) reported that if THQ is greater than one but less than five (i.e. 1 < THQ < 5), the risk is low; if THQ is greater than five but less than 10 (5 < THQ < 10), the risk is moderate; however, if THQ is greater than 10 (THQ > 10), the risk is high. The THQs observed for the metals in this study were all lesser than 1, and this suggests that health effects associated with the metals were unlikely to occur.
The Hazard Index (which is the numerical sum of THQs) values of Zn, Hg, Pb and Cu ranged from 2.64×10 -5 to 3.92×10 -3 , suggesting no significant health risks for the consumers since the recorded values were all less than 1. According to Abubakar et al. (2014), HI should not exceed 1 otherwise it implied significant health risks to consumers. Similarly, carcinogenic risks describe the incremental probability of an individual to develop cancer during a life-time exposure to potential carcinogens of any level. Therefore, the Target Cancer Risks (TR) of less or equal to 10 -6 (TR ≤ 10 -6 ) as observed for Pb in this study is considered as low or no carcinogenic risk (NYSDOH, 2007).
Also, the results of ecological risk quotient showed that no observed metal posed ecological risk to the environment. However, given that Cu, Zn, and Pb were found in the fish throughout the study, there may be a potential ecological risk if indiscriminate effluent discharge is not controlled. Prolonged exposure to these heavy metals can have detrimental ecological effects such as high mortality of juvenile fish, reduced breeding potential of adult fish, discomfort, disorientation, inability to find food, decrease in organism's fitness, imbalance, and failure of multi-system organs, among others. Additionally, the ecological impact could include reproductive consequences such as spawning blockage, deformities in newly-hatched fry, decreased egg production per female, decreased young survivability, and other effects at low levels of Cu, as previously observed by Ajibare et al. (2018).

Conclusion and Recommendation
This study has revealed that the Daily Intake (DIM) of Zn, Hg, Pb and Cu through consumption of T. guineensis in Lagos lagoon, Nigeria, was within the recommended limits. The Target Hazard Quotient (THQ), Hazard Index (HI), Health Quotient (HQ), and Health Risk Index (HRI) all showed that there was no probable non-carcinogenic health risk for fish consumers. The consumption of T. guineensis in the study area was not related with any carcinogenic risk, according to the Target Cancer Risk (TR), which quantifies the degree of exposure to carcinogenic risk. The Ecological Risk Quotient (ERQ) indicated that T. guineensis was not ecologically risky to the environment; consequently, its consumption does not pose any carcinogenic and non-carcinogenic risk to any population. In conclusion, since Zn, Cu, and Pb were found in the studied fish species, there is need for routine monitoring of heavy metals in the Lagos lagoon in order to ensure food security and safety in and around the study area.