Pollution assessment and health risk implication of human exposure to heavy metals in road dusts from different functional areas in Jeddah , Saudi Arabia

Data dealing with the assessment of heavy metal pollution in road dusts in Jeddah, Saudi Arabia and its implication to human health risk of human exposure to heavy metals, are scarce. Road dusts were collected from five different functional areas (traffic areas TA, parking areas PA, residential areas RA, mixed residential commercial areas MCRA and suburban areas SA) in Jeddah and one in rural area (RUA) in Hada Al Sham. We aimed to measure the pollution levels of heavy metals and estimate their health risk of human exposure applying risk assessment models described by USEPA. Using geo-accumulation index (Igeo), the pollution level of heavy metals in urban road dusts was in the following order Cd > As > Pb > Zn > Cu > Ni > Cr > V > Mn > Co > Fe. Urban road dust was found to be moderately to heavily contaminated with As, Pb and Zn, and heavily to extremely contaminated with Cd. Calculation of enrichment factor (EF) revealed that heavy metals in TA had the highest values compared to that of the other functional areas. Cd, As, Pb, Zn and Cu were severely enriched, while Mn, V, Co, Ni and Cr were moderately enriched. Fe was consider as a natural element and consequently excluded. The concentrations of heavy metals in road dusts of functional areas were in the following order: TA > PA > MCRA > SA > RA > RUA. The study revealed that both children and adults in all studied areas having health quotient (HQ) < 1 are at negligible non-carcinogenic risk. The only exception was for children exposed to As in TA. They had an ingestion health quotient (HQing) 1.18 and a health index (HI) 1.19. The most prominent exposure route was ingestion. The cancer risk for children and adults from exposure to Pb, Cd, Co, Ni, and Cr was found to be negligible (< 1 x 10-6). Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 23 November 2017 doi:10.20944/preprints201711.0152.v1 © 2017 by the author(s). Distributed under a Creative Commons CC BY license. Peer-reviewed version available at Int. J. Environ. Res. Public Health 2018, 15, 36; doi:10.3390/ijerph15010036


Introduction
Due to rapid urbanization, population growth and increasing demand of land for development, urban areas are experiencing rapid change throughout the world including dramatic growth in both industrial and road traffic activity which placed great pressure on the local environment [1,2].Road dust, the accumulated particle on the ground road surfaces, is a heterogeneous mixture of different contaminants originating from natural and anthropogenic sources and also from the interaction of solid, liquid and gaseous pollutants which are derived from different sources [3][4][5].It is related to particulate content in the atmosphere through resuspension into and re-deposition from the atmosphere and it is chemically similar, in some respects, to the primary portion of atmospheric particulate [6,7].Therefore, road dust is a valuable medium for characterizing urban environmental quality [8] and its chemical composition is an indicator for environmental pollution [9].
Road dust is a main reservoir of metals in urban environment from surrounding areas [8,10].Metals in road dust result from traffic emissions (exhausts, oil lubricants, vehicle wear, brake lining, corroding building-material asphalts, automobile parts and yellow road paint degradation), industrial emission (smelters, incinerators, foundries and steel plants), as well as dry and wet deposition of atmospheric particulates [3,11,12].In urban areas, traffic-related metal pollution in road dust is affected by vehicle type, traffic volume and behavior, soil parameters and meteorological conditions [3,12,13].Recently, several studies investigated the contents, spatial distribution, source identification, contamination assessment and characterization of potentially toxic metals in road dust [8,[14][15][16].
Metals enriched in the accumulated dust due to the lack of bioavailability, biodegradability and persistence pose a great deal of risk to human health through direct and indirect human exposure [17].Ingestion and inhalation are the direct exposure pathways, while dermal contact and outfits are the indirect ones [18,19].Oral ingestion was identified as the most critical exposure route to street dust particles for humans, compared with dermal contact and inhalation [20][21][22][23][24]. Oral ingestion takes place inadvertently, with food and drink or via mucociliary clearance, and with respect to children, deliberately, through their hand to mouth activities [19,25,26].However, only the oral bio-accessible fraction of heavy metals that is soluble in the gastrointestinal tract available for absorption represents the actual health risks in ingested particles [27,28].
The accumulation of heavy metals in the human body increases with exposure to high levels and affects the central nervous system, circulatory system, the functioning of internal organs, malfunction the endocrine system [10,18,21,29] and acting as the secondary factor for other diseases such as growth retardation in children, kidney disease and cancer [18,[30][31][32][33].According to the calculated hazard indices, exposure to Hg, Pb, Zn, Cd and Mn in road dust was found to pose high potential ecological risk [2,34].
Recently, metal contamination of the road dusts has received much attention to assess the quality of the environment, identify pollution sources and investigate their adverse health effects [2,[34][35][36][37][38].However, data concerning evaluation, spatial distribution and health risk of heavy metals in road dust in different functional areas in Jeddah are scarce.Therefore, the main objectives of the current study were as follows: (1) to evaluate and compare the concentrations and spatial patterns of heavy metals in road dusts in different functional areas of Jeddah; (2) to assess the pollution level of the heavy metals in road dusts; and (3) to investigate carcinogenic and non-carcinogenic health risks due to heavy metals exposure in children and adults.

Study area
Jeddah lies on the Red Sea coast in the western part of Saudi Arabia and is surrounded by mountains from north-eastern, eastern and south-eastern sides (latitude 29.2 North and longitude 39.7 East).It is the largest city in Saudi Arabia, with a land area of 1765 km 2 , and represents a very important commercial, in addition of being the crossroads between East and West to Asia, Africa and Europe, with a population of ca.3.6 million.Jeddah receives approximately 2 million visitors during pilgrimage season each year.In particular, with increasing developmental activity, environmental concerns are increasing in Saudi Arabia [39].
Road traffic and stationary sources are the main sources emission of air pollutants in Jeddah.
Jeddah experiences a huge traffic congestion due to increasing population and growing number of commuters.More than 1.40 million vehicles/ day are running in the streets of Jeddah city [40].These vehicles use mainly unleaded gasoline and diesel fuels.Oil refinery, seaport activities, desalination plant, power-generation plant and industrial activities in the south are the main stationary sources in the city.Jeddah has an arid climate, warm and humid or moderate in winter, and characterized by high temperature, humidity, and solar radiation in summer.
Rainfall is generally sparse.

Sampling collection
Road dusts samples were collected from five different functional areas in Jeddah and one in rural area in Hada Al Sham.The sampling locations (Figure 1) were distributed over the areas that represent various functional categories to reveal the pollution impacts from various human activities; including residential areas (RA), suburban areas (SA), mixed commercial/residential areas (MCRA), parking areas (PA), traffic areas (TR), and one rural area (RUA).The traffic areas included in this study cover a major highway, roundabouts and crossroads.The RUA was located at Hada Al Sham, about 60 km east of the city of Jeddah.
Road dust samples were collected on the driest month of the year (September 2016).Samples at each sampling locations (approximately 200 g each) were carried on by gentle sweeping motion of an appropriate area from the pavement on both sides of the roads using a soft polyethylene brush and dustpan.The collected samples were stored in labeled sealed polypropylene bags and transported to the lab.

Sample preparation and analysis
In the laboratory, the samples were air-dried at room temperature and the coarse impurities of the samples were removed using 1.0 mm mesh nylon.The rest was homogenized and sieved through 63 μm sieve size and stored in small self-sealing plastic bags for analysis.Only, dust with particle size < 63 µm diameter was selected to determine its metal concentration in this study because: (1) metal concentration decreases with increase particle size of dust [41,42], (2) the represent high health risks [8,24] and (3) are easily transported and remain airborne for considerable durations [2,35,43].

Sample digestion and analysis
To measure heavy metal concentration, accurately weighed road dust samples (1 g) were digested with nitric acid (HNO3) and hydrochloric acid (HCl) mixture on a hot plate as described by Hassan and Khoder [44].The digested solutions were filtered through Whatman filter paper (No. 42) using deionized water and diluted to 100 mL.They were stored at 4 o C in pre-cleaned polyethylene bottle until analysis.Inductively Coupled Plasma Optical Emission Spectrometry ICP-OES-5100 was used to determine the concentrations of heavy metals (Fe, Mn, Zn, Pb, Cd, V, Co, Ni, As, Cr and Cu).The quality of data was ensured using standard material between samples.Precision of measured metals, determined from the standard deviation of repeated measurements of standards, was less than 2.5%.The concentration of metals in laboratory blanks, filter blanks and reagent blanks were measured by the same method described above in order to evaluate external metal contamination from analytical procedures.No contamination was detected.

Geo-accumulation index (Igeo)
The geo-accumulation index (Igeo) was used to evaluate the contamination levels of metal in road dust [45].This index is widely applied to assess the heavy metal pollution of urban road dusts [17,46].It assesses the metal pollution in terms of seven enrichment classes ranging from (0-6), starting from normal background value to very heavily polluted [17,45].
The seven different classes for Igeo values are given in Table 1.The Igeo was computed from the following equation [48]: where Igeo is the geo-accumulation index for different metals, Cn the measured concentration of the metals in road dust samples.The constant 1.5 is used to minimize the effect of possible variations in the background values.Bn refers to the metal background value in the earth's crust [49].

Enrichment factor
The enrichment factor (EF) was used to differentiate between the anthropogenic sources of trace metals and their natural origin in road dust, as well as, to evaluate the degree of the anthropogenic contribution and metal contamination.It was calculated using the following equation [50,51]: Where EF is the enrichment factor, Cx the concentration of the target metal, and CReference the concentration of the reference metal.In this study, Fe was chosen as a reference metal and was used for EF calculation.The earth crust composition was taken from Taylor [49] and Taylor and McLennan [52].An EF values < 2 indicate deficiency to minimal enrichment [53].EF values between 2 and 10 refer to moderate enrichment, whereas EF values > 10 show severe enrichment [54].

Health risk assessment model
Health risk assessment models were used to quantify the health risk (carcinogenic and non-carcinogenic) for children and adults exposed to heavy metals in road dust.They are based on those developed by the United States Environmental Protection Agency (USEPA) [55,56].
Local residents are exposed to metals in road dust through three main exposure pathways: direct ingestion, inhalation through mouth and nose, and dermal absorption.The total noncarcinogenic risk was calculated for each metal in road dust by the summation of the individual risks calculated for the three exposure pathways [55,57].
The average daily dose (ADD) (mg kg -1 day -1 ) for heavy metals in road dust through the three exposure pathways was calculated according to Exposure Factors Handbook [58] and the Technical Report of USEPA [59] using the following equations:

LADD =
Where the ADDing, ADDinh and ADDdermal are the average daily dose (mg kg -1 day -1 ) exposure to metals through ingestion, inhalation and dermal contact, respectively.LADD is the lifetime average daily dose exposure to metals (mg kg -1 day -1 ) for cancer risk, CR is the contact frequency and is the same IngR used in the calculation of ADDing [59][60][61].The detailed description about the values of exposure factors for children and adults that were applied to the above models (Eqs.[3][4][5][6] in the present study are given in Table 2.In order to evaluate the human health risk of heavy metal exposure from road dusts in Jeddah, the HQ (hazard quotient), HI (hazards index), and CRA (carcinogenic risk assessment) were applied.The potential risk of carcinogenic and non-carcinogenic hazards for individual metals were calculated using the following equations [67,73]: RfD and SF are the values of reference dose (mg kg -1 day -1 ) and slope factor [36,60,61,74,75].RfD is an estimation of maximum permissible risks to human population through daily exposure by considering sensitive group (children) during a lifetime [17].
The carcinogenic risk is the probability of an individual developing any type of cancer from lifetime exposure to carcinogenic hazards [18,21,55,60,61] recommended that the value of CRA < 1 x 10 -6 can be regarded as negligible, whereas CRA > 1 x 10 -4 is likely to be harmful to human beings.The acceptable or tolerable risk for regulatory purposes is in the range of 1 x 10 −6 ~ 1 x 10 −4 .There are no adverse health effects when the value of HQ ≤ 1, whereas adverse health effects occur when HQ > 1 [55].HI value show the sum of the value of the HQ for different substance through different pathways [18,76] and refers to total risk of noncarcinogenic for a single metal.The value of HI ≤ 1 refers that no significant risk of noncarcinogenic effects is occur.On the other hand, there is a chance that non-carcinogenic effects may occur when HI >1, and the probability increase with increasing the value of HI [60,61].

Heavy metals concentration in urban road dusts
The average concentrations heavy metals in urban road dusts collected from Jeddah are shown in Figure 2. The mean concentrations of heavy metal in descending order were Fe > Mn metal pollution in urban road dusts might be derived mainly from anthropogenic sources [8,14,17].Fe concentration was lower in urban than rural dusts, while Mn concentration in both urban and rural dusts were nearly similar.The maximum permissible concentration (MPC) for Pb, Cu, Mn, Zn, Co and Cd in soil are 100, 100, 1500, 300, 30, and 3 mg/kg, respectively [77], in the present study, the concentration of Zn, Pb, Cd and Cu only were higher than the MPC.
Comparison of heavy metals concentrations in road dust of Jeddah with those in other cities in the world is shown in Table 3.In general, the concentrations of heavy metals in road dust from Jeddah were lower/higher or similar to those reported in other cities of the world.
These variations between the different cities of the world might be referred to the difference in The spatial variations of heavy metals concentrations in road dusts from different functional areas are shown in Table 4.The concentrations of all the heavy metals (except Fe in all sites and Mn in RA and SA) in RA, SA, MCRA, PA and TA dusts were higher than rural values, assuming that the heavy metals in urban road dusts might be contaminated by anthropogenic activities like vehicular traffic, building construction and demolition activities and waste disposal [36].Fe concentration in urban dusts was lower than that in RUA, supporting that it mostly comes from natural sources.Based on the total heavy metals (Mn, Zn, Pb, Cd, V, Co, Ni, As, Cr and Cu) concentrations, functional areas in Jeddah could be classified as follows: TA > PA > MCRA > SA > RA.The observed high concentrations of the total heavy metals in the road dusts of TA suggests that the TA areas may be a reservoir of heavy metals in this urban environment.Traffic area (TA) cover major highway, roundabout and crossroads with the highest traffic volumes and traffic jams in Jeddah.Therefore, the vehicular-related deposition of particles is responsible for higher concentrations of metals in road dusts of TA.
These particles come from vehicle exhaust particles, lubricating oil residues, tire wear particles, brake lining wear particles, particles from atmospheric deposition, plant matter, and materials produced by the erosion of the adjacent soil [69,[84][85][86][87]. Generally, the urban area is an assembly of different land use types with typical local and diffuse pollution sources.So, the widely variations of heavy metals concentrations between different functional areas may be attributed to the distinctive artificial activities in each functional area that release different kinds of heavy metals content and deposited in the street surface [78,88].

Assessment urban road dusts quality
The Igeo values for heavy metals in road dust from different function areas are presented in Table 5 [47,89].Shi et al. [57] and Garcia-Martinez and Poleto [90] revealed that average Igeo value of Pb was high in urban areas.
The highest level of pollution was found for As having Igeo values of more than six in metropolitan area of Hefei, China.[36].The EF for each heavy metals in road dusts from different functional areas are shown in Table 5. EF values lower than 2 were found for V and Co in RA, Co in SA and Mn, Cr, Cu, V, Co and Ni in RUA, indicating that these metals originate from natural sources such as crustal erosion and wind-blown soil minerals.The EF values of Mn, Ni, Cr, and Cu in RA, Mn, V, Ni, Cr, and Cu in SA, Mn, V, Co, Ni, and Cr in MCRA, Mn, V, Co, Ni, and Cr in PA, Mn, V, Co, Ni, and Cr in TA and Zn, Pb and As in RUA were between 2 and 10.Furthermore, the EF values for Zn, Pb, Cd and As in RA and SA, Zn, Pb, Cd, As and Cu in MCRA, PA and TA were more than 10.Generally, the mean EF values in the urban road dusts of Jeddah displayed the following decreasing trend: Cd > As > Pb > Zn > Cu > Ni > Cr > V > Mn >Co.The mean EF values of Mn, V, Co, Ni and Cr were between 2 and 10, indicating that they were moderately enriched.For Cd, As, Pb, Zn and Cu were more than 10, indicating that they were severely enriched.Cu, Pb and Zn are reported to be multi-source related and their accumulation is commonly found to be anthropogenic and from traffic related materials (brake dust, tires tread and yellow paint) [91].Moreover, high atmospheric temperature and exposure to weather may accelerate corrosion processes, causing wear of the wares, walls, lamps and railings that often contained the heavy metals such as Zn, Cu, Cd and Cr, then resulting in the release of the metals to the urban environment and accumulation in urban street dust [14,78,92].Although the legal usage of leaded gasoline was phased out in Saudi Arabia in 2001 [93], elevated Pb in urban road dusts of Jeddah may be attributed to historical Pb contamination and the long half-life of Pb in soils [94].

Human health risk assessment
Human health risk assessment of heavy metals in the road dusts of different functional areas through possible exposure pathways (ingestion, inhalation, and dermal contact) was performed for children and adults (Tables 6 and 7).Based on the calculated HQ values for the ingestion (HQing) and dermal (HQdermal) pathways for children and adults exposed to heavy metals in road dusts, the rank order of functional areas was TA > PA > MCRA > SA > RA > RUA.While the rank order for inhalation (HQinh) pathway was RUA > TA > SA =RA > PA > MCRA.
Concerning the heavy metals, As, Pb and Cr, Fe, Mn and Cr, and Cr, V and Cd displayed high HQing, HQinh and HQdermal, respectively, for children and adults compared with other elements in the study areas, except in RUA where HQing and HQdermal were the highest for Cr, Mn and As, and Cr, V and Mn, respectively.Results reveal that no non-carcinogenic significant risk was found in the study areas for all measured heavy metals, since the HQs and HI values were < 1 [56].The only exception is for HQing and HI values for children exposed to As in TA (1.18 and 1.19, respectively).Therefore, it can be deduced that there is no serious non-carcinogenic risk from heavy metals in road dusts via different exposure routes.However, the possibility that these metals can cause serious health effect by their accumulation in body tissues persists [97][98][99].Special attention should be paid for children exposure.HQing and HQinh values for children was higher than adults.The HQ for children through ingestion was averaged 9.33 times higher than that for adults, with inhalation of 2.79 times, indicating that children faced more potential harmful health through both ingestion and inhalation pathways from the heavy metals in road dusts from Jeddah city.Children are more vulnerable to dust exposure because of their playing habits (ingestion of dust through mouth, hand licking, toys and other household objects) [19,99].
Cancer risk (CRA) for some selected heavy metals (Pb, Cd, Co, Ni, As and Cr) was estimated using inhalation mode of exposure.The CRA values for children and adults exposed to these heavy metals in road dusts from different functional areas are presented in Tables 8   and 9.All CRA values for both the populations were equal to or lower than 1 x 10 -6 , with higher values in children, suggesting that the carcinogenic risk from exposure to these metals is negligible.These results are similar to those reported in previous studies [8,34,36,100].

Conclusions
This study aimed to find out the concentration, spatial variation, pollution level and health risk implication of human exposure to heavy metals (Fe, Mn, Zn, Pb, Cd, V, Co, Ni, As, Cr and Cu) in road dusts from five different functional areas in Jeddah and one in rural  Jeddah was found to be within the safe limits for children and adults, suggesting no potential harm from exposure to these metals in road dusts.Again, CRA values of heavy metals were higher in children than adults.

Figure 1 .
Figure 1.Sampling site distruibution in the different functional areas of Jeddah area at Hada Al Sham, located about 60 km from the city of Jeddah.The average concentrations of Zn, Pb, Cd, V, Co, Ni, As, Cr and Cu in urban road dusts were higher than that of rural area, indicating that this pollution may result from anthropogenic sources.Among the five urban areas, the highest levels of Mn, Zn, Pb, Cd, V, Co, Ni, As, Cr and Cu were found in TA and the lowest in RA.Based on Geo-accumulation Index (Igeo), urban road dusts of Jeddah was uncontaminated by Ni, Cr, V, Mn, Co and Fe, uncontaminated to moderately contaminated by Cu, moderately to heavily contaminated by As, Pb and Zn, and heavily to extremely contaminated by Cd.The order for the average Igeo values was Cd > As > Pb > Zn > Cu > Ni > Cr > V > Mn > Co > Fe in urban street dusts.The mean EF values in the urban road dusts of Jeddah displaying the following decreasing trend: Cd > As > Pb > Zn > Cu > Ni > Cr > V > Mn > Co. Cd, As, Pb, Zn and Cu in road dusts were severe enriched, whereas Mn, V, Co, Ni and Cr were moderately enriched.EF values of heavy metals in urban dusts were higher in TA than other functional areas.The HQs and HI values for the different exposure pathways of measured heavy metals in children and adults decreased in the following order: ingestion > dermal contact > inhalation.These values for all heavy metals in all functional areas were below the safe level (< 1) indicating that no significant potential health risk is posed to inhabitants (children and adults) from exposure to heavy metals in road dusts, except for As with HQing value of 1.18 and HI value of 1.19 for children in TA.The HQing, HQinh and HI values were higher in children than adults.The carcinogenic risk (CRA) for heavy metals in

Table 2 .
Values of exposure factors for heavy metals doses for children and adults.

Table 3 .
Heavy metals concentrations (mg/kg) in urban road dusts of different cities around the world [2] traffic density, intensity of human activities, land use patterns, technologies employed, and local weather conditions[2].For example, the mean concentration of Cu in Jeddah road dust is almost similar to those of Iran (Shuraz) and UK, lower than Colombia, Iran (Tahran, Asfhan), Jordan and China (Guangzhou), and higher than China (Chengdu, Beijing, Baoji, Nanjing, Xian), USA, Turkey and Greece.Pb content in Jeddah road dust and Turkey are similar, higher than USA, Iran (Shuraz) and China (Chengdu, Beijing and Nanjing) but lower than Colombia,

Table 4 .
Concentrations (mg/kg) of heavy metals in road dusts of different functional areas Notes: RA, residential area; SA, suburban area; MCRA, mixed commercial residential area; PA, parking area; TR, traffic area; RUA, rural area; Min, minimum; Max, maximum; SD, standard deviation

Table 5 .
The EF and Igeo of heavy metals in road dusts of different functional areas.

Table 6 .
Hazard quotient and hazard index of each heavy metal for children population living in different functional areas

Table 7 .
[4,8,17,18,19,35,36,57,95,96]dex of each heavy metal for adults population living in different functional areasWhen the mean HQs of the five urban areas was calculated (Table8), the average hazard quotient values of heavy metals for children and adults were in the order of As > Pb> Cr > Mn >V > Cd > Cu > Ni > Zn >Fe > Co for HQing, Fe > Mn > Cr > Co >As > Pb > V > Cd > Cu > Ni > Zn for HQinh and Cr > V > Cd > Mn > Pb > Fe > As > Cu > Ni > Zn > Co for HQdermal.The HQ values for the different exposure pathways of measured heavy metals in children and adults decreased in the following order: ingestion > dermal contact > inhalation.The contribution of the HQing, HQinh and HQdermal to the HI (the total risk of non-cacinogenic exposure) were 94.86 %, 1.52 % and 3.62 % for children and 56.13 %, 3.01 % and 40.86 % for adults, respectively.This result indicates that ingestion was the main pathway exposure to measured heavy metals in urban road dusts of Jeddah city in the two population groups.These results are consistent with other studies[4,8,17,18,19,35,36,57,95,96].

Table 9 .
Carcinogenic risk (CRA) of each heavy metal for children and adults population living in different functional areas