Human health risk assessment of the levels of dioxin-like polychlorinated biphenyls (PCBs) in soils from mechanic workshops within Nekede mechanic village, Imo State, Nigeria

ABSTRACT Dioxin-like polychlorinated biphenyls (DL-PCBs) are a class of persistent toxic substances with a high potential to accumulate in the soil as an organic pollutant and bioaccumulate in humans. The study determined the level and human health risk of twelve (12) DL-PCBs in soils from mechanic workshops within Nekede mechanic village (NMV), Imo State. In this study, soil samples (0–15 cm depth) were collected at four different sampling points within Nekede mechanic village. At each sampling point, composite samples consisting of 10 sub-samples were collected with pre-cleaned pet bottles using soil auger and were labelled SA, SB, SC, SD. The control sample was taken 500 metres away from the mechanic village where there was no form of auto-mechanic activities. Sample analysis was conducted using Agilent GC 6890 N coupled with Agilent MS 5970B to determine the levels of DL-PCBs. The data were analysed using SPSS version 23. The results revealed that the mean concentration of 12 DL-PCB congeners (Σ12PCBs) in soil ranged from 0.31–16.31 µg kg−1, which was significantly higher than that in the control sample (0.006–0.087 µg kg−1). PCB-126 was the major contributor (58.99%) while the least contributor was PCB-157 (0.0040%) of the Σ12 DL-PCBs TEQ. The sum total LCR (adults 1.29 × 101, children 4.42 × 101) and HQs (adults 6.68 × 102, children 2.29 × 103) were above the acceptable risk limit (LCR = 10−4, HQ = 1); indicating a very high adverse effect of DL-PCBs on humans especially children.


Introduction
The impact of wastes from auto-mechanic workshops on health and the environment has been an issue in developing countries in recent years [1][2][3][4].Auto-mechanic workshops are sources of environmental pollution through which persistent organic pollutants (POPs) such as polychlorinated biphenyls (PCBs), Hexachlorobenzene and polyaromatic hydrocarbons (PAHs) are introduced into the environment through soil and groundwater at levels that are considered to be above their threshold limit [5][6][7][8].Polychlorinated Biphenyls has been in existence since 1864 although they were first used globally in 1929 [4,7,9].Even though the use of PCBs has been heavily restricted and are no longer manufactured, they can still be found in the environment.Also, in the environment, PCBs bioaccumulate within the food chain due to their high affinity for organic materials [10].Besides, they are introduced into the blood, human tissues, and breast milk through the intake of milk, fish and meat [11].Also, due to the toxic characteristics of PCBs in the soil, they have been linked to chronic effects in humans including immune system damage, decreased pulmonary function, bronchitis, and interferences with hormones leading to carcinogenic and non-carcinogenic health risks to the populace [10].The International Agency for Research on Cancer (IARC) and the Environmental Protection Agency (EPA) classify PCBs as probable human carcinogens.Furthermore, the possibility of PCBs to cause cancer in humans was affirmed by the National Toxicology Program (NTP) [11][12][13].Moreover, the effect of PCBs on human health is dependent upon the concentrations of PCBs, type of chlorinated homology and the extent of exposure [4,[14][15][16][17].
In Nigeria, it has been established that industrialisation and urbanisation have led to the migration of humans from rural to urban areas in search of greener pastures, and this has led to an increase in the means of transporting people from one point to another [6,18].One common mode of transportation is the use of vehicles and in effect, this has led to an upsurge in the number of vehicles available and used in urban areas.At some point, these vehicles require servicing, repairing and replacing of damaged parts; which are carried out in auto-mechanic workshops by auto-mechanic artisans [8,19,20].In most cities in Nigeria, vehicular repair locations often referred to as 'auto-mechanic villages' are allocated for automechanic works in other to prevent traffic congestion on the highways [8,18,19].The automechanic villages are mainly sited in neighbourhoods of cities and adjourning towns [6,21].In such areas, the spills from lubricants, gasoline and diesel oil constitute the major contaminants of the soil and groundwater within the area [20].Additionally, malfunctioning vehicular parts are discarded and abandoned on the surface of the soil, which leads to increased soil and groundwater contamination through the leaching of environmental pollutants such as PCBs, PAHs and potentially toxic elements (heavy metals) [7,8,16,22,23].The need for this study became necessary owing to the paucity of PCB data in the study area.Many studies have been carried out by Nigerian researchers on the impact of auto-mechanic activities on surface soil and groundwater quality concerning potentially toxic elements [4,5,8,19,21,24].There are few documented studies by Nigerian researchers on persistent organic pollutants [8,20,23,25].However, there was no reference to dioxin-like PCBs, and this study is aimed at filling the knowledge gap.This is the first report on levels and human health risks posed by dioxin-like PCBs in soils from Nekede mechanic village, Imo State, Nigeria.

Study area
The Nekede mechanic village (NMV) is situated in Nekede, Owerri West Local Government Area of Imo State, and located geographically on longitude 7°2ʹ15" -7°3ʹ00"E and latitude 5°27ʹ80" -5°28ʹ00"N (Figure 1).It is about 130 acres and was set up in the year 1983 to repair and maintain automobile vehicles used in the transportation of humans, animals and goods [2,25,26].The soils of this area are drained by Nworie and Otamiri Rivers and underlain by the sandy Benin formation of Miocene to recent age, and consists of coastal plain sands (about 0.05-2.0mm in size) with minor clay beds; this type of soil has good drainage and is well aerated, causing it to dry out quickly [14,18,26].The topography of NMV is relatively a level ground but has been sculpted by erosion, forming deep gullies towards the Otamiri River.The climate of the area is tropical rainy, with two distinct seasons, i.e. rainy season and dry season with mean annual rainfall (2500-4000 mm) and daytime temperature that ranges from 18 to 34°C [15,23,25].

Samples collection and pre-treatment
Soil samples (0-15 cm depth) were collected at four different sampling points within Nekede mechanic village.At each sampling point, composite samples consisting of 10 subsamples were collected with pre-cleaned pet bottles using soil auger and were labelled SA, SB, SC, SD.Samples from the same point were air-dried, debris removed, and then homogenised to form a representative sample.A control sample was taken 500 metres away from the mechanic village where there was no form of auto-mechanic activities.The samples were immediately transported to the laboratory for analysis.The coordinates of the sampling points were referenced with the Garmin GPSMAP 76, a handheld global positioning system (GPS) unit, and was used in the map generation (Figure 1).The soil samples were collected in June 2021.In all, a total of forty sub-samples were collected for this study.Agilent MS 5970B.10.0 g of the sample was mixed with 30 ml of acetone/hexane mixture (1:1 v/v) and ultrasonicated at 50°C for 30 mins.The mixtures were filtered through a 0.45 μm pore size filter and the sonication process was repeated three times with fresh portions of the acetone/hexane mixture.The extracts were then combined and concentrated to 2 ml with a rotatory vacuum evaporator and then purified with a multi-layer silica gel/alumina glass column.The clean-up was carried out using a multi-layer column loaded from bottom to top with neutral silica gel (4 g, 5% deactivated), neutral alumina (2.0 g, 6% deactivated) and anhydrous sodium sulphate (5.0 g).The PCBs were finally eluted with 20 ml of the acetone/hexane mixture (1:1 v/v) and concentrated to approximately 1 ml under a gentle stream of high purity nitrogen gas.

Quality assurance and quality control (QA/QC)
Quality assurance and control measures were ensured by using high purity analytical reagents and solvents.Calibration standards were utilised in checking the instruments.The analysis of a certified reference material (CRM), procedure blanks and triplicate analysis were utilised for the validation of the analytical methods.Also, each organic pollutant in the soil sample was determined using the method limit of detection (LoD), repeatability, reproducibility, accuracy, and precision.The PCB congeners were quantified using calibration curves that cover the dynamic range in which the compounds of interest are expected to be present.Internal standards ranging from 90.20% to 106% were added to the samples before extraction and recoveries and were very good according to the Chinese standard HJ 743-2015.

Data analysis
Data analyses were performed using the statistical software package SPSS version 23.0 and Ms Excel 2007.Test of significance for variability in the distribution of PCB congeners among different sampling points was carried out using one-way analysis of variance (ANOVA).

Calculation of toxicity equivalent quotient (TEQ)
In this study, the procedure described by the World Health Organization [11] was adopted for risk assessment of the dioxin-like PCBs (DL-PCBs) contamination in soils from mechanic workshops within Nekede mechanic village, Imo State, Nigeria.The toxicity equivalence factors (TEF) with reference to 2,3,7,8,-TCDD and toxicity equivalent quotient (TEQ) were used to expedite the monitoring and toxicity risk assessment of the DL-PCBs (Table 1).
Calculation of TEQ values of DL-PCBs was done by multiplying the concentration of each congener with their corresponding WHO toxicity equivalence factors [11].The TEQ was calculated using Equation (1).
Where C i is the concentration of the dioxin-like PCB congener (μg kg −1 ) in the soil sample, and TEF i is the TEQ factor assigned to each dioxin-like PCB congener.

Health risk assessment
Humans (adults and children) may be exposed to PCB contaminated soils via ingestion, inhalation and dermal exposure routes.Recently, studies have reported the ingestion route as the major intake route of PCBs in humans [4,5,17,22].PCBs are carcinogenic in nature and are associated with several health implications when ingested in the human body [4,5].The lifetime average daily dose (LADD) of DL-PCBs via ingestion route for adults and children were calculated.This was followed by the lifetime cancer risk (LCR) and non-cancer risk (hazard quotient, HQ) for adults and children.Equations ( 2)-( 4) were used for estimating LADD, LCR and HQ respectively.
Where, C s is the pollutant concentration in soil (μg kg −1 ), IR is the soil ingestion rate (mg kg −1 day −1 ), CF is the unit conversion factor (10 −6 mg kg −1 ), EF is exposure frequency (days/year), ED is the lifetime exposure duration (years), BW is the body weight (kg), and AT is the averaging time for carcinogens (EF×ED days).CSF oral is an oral cancer slope factor (mg kg −1 day −1 ).RfD is the reference dose of individual DL-PCBs (mg kg −1 day −1 ).The exposure parameters for human health risk assessment are outlined in Table 2.

Concentration of dioxin-like polychlorinated biphenyls (DL-PCBs) in soils
The concentrations (μg kg −1 ) of the dioxin-like PCB congeners in soil samples from Nekede mechanic village are represented in Table 3. Twelve DL-PCBs were detected in all the samples, which showed no significant differences (p > 0.05).The mean concentration of 12 DL-PCB congeners (Σ12PCBs) in soil ranged from 0.31-16.31µg kg −1 , which was much higher than that in the control sample (0.006-0.087 µg kg −1 ).The highest Σ12PCBs was observed in sample SC while the lowest was observed in sample SB.This was attributed to poor management in the handling of lubricants, gasoline and diesel oil,  and malfunctioning vehicular parts discarded on the surface of the soil containing DL-PCBs.In comparison to other studies [4,11,17,27], the mean concentration of 12 DL-PCB congeners reported in this study was higher.In a study conducted within a research station by Hyun et al. [28], the high concentration of chlorinated homologues was attributed to local pollution activities within the area.
It is of importance to note that the Σ12 DL-PCBs TEQ values (0.399 μg TEQ kg −1 ) exceeded the Canadian soil quality acceptable limit for dioxins (0.004 μg TEQ kg −1 ).Furthermore, Ma et al. [22], reported TEQ for humans and mammals which were below that of the present study.

Conclusion
Dioxin-like PCB congeners were found in all the soil samples in this study.The mean concentration of 12 DL-PCB congeners (Σ12PCBs) in soil ranged from 0.31-16.31µg kg −1 .The highest Σ12PCBs was observed in sample SC while the lowest was observed in sample SB.PCB-126 was the major contributor (58.99%) while PCB-157 was the least contributor (0.0040%) of the Σ12 DL-PCBs TEQ.Worthy of note is that the Σ12 DL-PCBs TEQ values (3.99 × 10 −1 μg TEQ kg −1 ) in this study exceeded the Canadian soil quality acceptable limit (4.0 × 10 −3 μg TEQ kg −1 ) for soils for the protection of human and environmental health.Furthermore, the sum total lifetime cancer risks (adults 1.29 × 10 1 , children 4.42 × 10 1 ) and hazard quotients (adults 6.68 × 10 2 , children 2.29 × 10 3 ) for 12 DL-PCBs were higher than the recommended tolerable limits; indicating a very high harmful effect of DL-PCBs on humans.Finally, this quantitative evidence has demonstrated the need for remediation of soils within mechanic villages to protect residents, especially children from persistent organic pollutants such as PCBs in the environment.

Figure 1 .
Figure 1.Map of study area showing the sampling points.

Figure 2 .
Figure 2. Chemical structures of the dioxin-like polychlorinated biphenyls.

Figure 3 .
Figure 3. Toxicity equivalent quotients for 12 DL-PCBs at the different sampling points.

Table 1 .
TEF, CSF and RfD values for human health risk assessment.

Table 2 .
Exposure parameters for human health risk assessment.

Table 4 .
Estimated lifetime average daily dose (LADD), lifetime cancer risk (LCR) and hazard quotient (HQ) for adults and children due to DL-PCBs exposure.