Monitoring of Metal(loid)s Using Brachiaria decumbens Stapf Leaves along a Highway Located Close to an Urban Region: Health Risks for Tollbooth Workers

Abstract

Studies on tollbooth workers involving the concentration of metal(loid)s in highway dust are scarce. We aimed to assess the levels of metal(loid)s in soils and washed and unwashed leaves of Brachiaria decumbens on roadsides. Dust deposition and heavy metal content in the leaves are used to estimate the exposure of tollbooth workers to oral, inhalation, and dermal ingestion of metals in highway dust. The concentrations of aluminum (Al), arsenic (As), cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu), iron (Fe), nickel (Ni), zinc (Zn), and lead (Pb) in washed and unwashed soil and leaves were analyzed using inductively coupled plasma optical emission spectroscopy. The results showed that soils along highways had a high concentration of heavy metals. Concentrations of Cd, Cu, Cr, Ni, and Pb near the roundabout and tollbooth are higher than the concentrations at the points between them. The highest transfer factor values were determined for aluminum. In the case of the non-carcinogenic effect, the hazard index (HI < 1) of tollbooth workers due to oral exposure to street dust containing metal(loid)s is higher than dermal contact and inhalation. The Incremental Lifetime Cancer Risk showed a high potential carcinogenic risk for As and Cd.

1. Introduction

Tollbooth workers are exposed to significant levels of various pollutants, including metal(loid)s in dust. Dust containing metals (Cd, Cr, Cu, Ni, Pb, Zn, Fe, selenium (Se), strontium (Sr), barium (Ba), and titanium (Ti)) can accumulate on the ground [1], on moving vehicles, and especially on surfaces such as tollbooths where people work. In addition to power plants, industries, incinerators, and residential heating, natural sources such as wind dust, weathering of rocks and minerals, sea salt, and volcanic eruptions also contaminate the atmosphere [2,3]. It has been shown in [4] that the primary component of air pollution is road dust (RD), where the presence of Al in the dust is related to internal engine corrosion and exhaust gases. Furthermore, investigations of road dust sampling sites in the steel industrial areas of Pohang, South Korea, using magnetic separation show that there is an accumulation of twelve metals (vanadium (V), manganese (Mn), molybdenum (Mo), Co, Fe, Cr, Ni, Cu, Zn, Pb, and mercury (Hg)) in road dust [5]. Research indicates that different vehicles significantly contribute to soil and dust contamination through the release of Fe, Cu, Ni, Cd, Cu, Pb, Zn, and other pollutants [5,6,7,8].

Resuspended urban road dust can enter the human body through direct ingestion of dust, inhalation of dust particles through the mouth and nose, and dermal absorption [9]. Indeed, air pollution can cause respiratory irritation, inflammation, and cardiorespiratory effects [10]. Previous studies on the occurrence of respiratory and other health problems in highway tollbooth workers revealed that such professionals have a high occurrence of central nervous system complaints such as headaches, irritability, anxiety, and mucous membrane irritation (eye irritation, nasal congestion, and dry throat). In addition, the symptoms are reflective of the acute irritant and central nervous system effects of exposure to motor vehicle exhaust [11]. Biomarkers of lead exposure (blood lead, BPb) and effect (erythrocyte protoporhyrin, EP, and activity of delta-aminolevulinic acid dehydratase, ALAD) were measured in 68 male tollbooth operators on the Zagreb–Karlovac motorway, Croatia. According to Hursidić-Radulović and Cvitković [12], significant correlations were found between BPb or ALAD and both the smoking index and alcohol consumption. However, few studies have assessed the health risks of these professionals exposed to heavy metals from highway dust located close to urban areas.

Plants are widely used as biomonitors for monitoring heavy metal pollution in soil and air; thus, washed and unwashed leaves can be used to detect the deposition, accumulation, and distribution of heavy metal pollution [13]. Studies have been conducted on the use of parts of plants as biomonitors. The analyzed calcium (Ca), copper (Cu), and lithium (Li) levels in washed and unwashed needles, bark, and branches of blue spruce (Picea pungens) in the city of Ankara (Turkey) reveal significant variations based on organ type, washing status, and organ age [14]. The research indicates that the lowest concentrations of Ca and Cu were found in the bark, while Ca levels increased with the age of the organ. Additionally, in the study carried out in the city of Winterthur, Switzerland, the importance of washing plant samples (Picea abies) before analysis is highlighted, as it can significantly affect the determination of element concentrations [15]. Overall, these findings underscore the complexity of heavy metal accumulation in plant tissues and the necessity of considering multiple factors when assessing environmental pollution levels [16]. In this study, we will assume the health risk to toll operators occurs due to exposure to metals in highway dust by oral, inhalation, and dermal ingestion. Therefore, we will assume that the concentration of metal(loid)s in highway dust is obtained by the difference between washed leaves (WLs) and unwashed leaves (UWLs).

In several countries, a very common plant that is present on the sides of avenues, as well as being used for feeding cattle, is Brachiaria decumbens Stapf. This grass can grow in many environments, including areas affected by mining [17]. Thus, due to its great availability close to high-traffic avenues in the state of Mato Grosso do Sul, Central-West Brazil, this plant can be used in studies to investigate the deposition of heavy metals in its leaves.

Given the above, the objectives of this study were to (i) assess the concentration of Al, As, Cd, Co, Cr, Cu, Fe, Ni, Zn, and Pb in washed and unwashed leaves of B. decumbens and in the soil along one of the largest Brazilian highway and close to the tollbooths; (ii) compare soil concentration results with soils from China, Brazilian legislation (Conama Resolution No. 420/2009), and Brazilian states; (iii) calculate the transfer factor from soil to plants; (iv) calculate chronic oral, dermal, and inhaled daily intakes of tollbooth operators exposed to metals from highway dust; (v) assess the risk associated with the dust consumption for tollbooth operators according to the hazard quotients (HQs) for each metal and a hazard index (HI); and (vi) calculate the carcinogenic risk using the Incremental Lifetime Cancer Risk (ILCR). Thus, our results fill the gap related to the health risks of tollbooth workers caused by metal(loid)s in highway dust close to urban regions.

2. Materials and Methods

2.1. Schematic Drawing of the Study

The leaves of B. decumbens and soils were collected on the BR-163 highway in June 2021, between the city of Campo Grande (state of Mato Grosso do Sul) and Nova Alvorada do Sul, Brazil (coordinates: latitude –20570953; longitude –54551781). The BR-163 highway is 845.4 km long and crosses several Brazilian states. Vehicles remain stopped for 35 to 40 s in the toll booth, considering one direction Campo Grande/MS to São Paulo or Paraná (other Brazilian states). Traffic on the highway is usually intense during peak hours, reaching an average of 4000 vehicles per hour during peak times. Campo Grande is the capital and largest city of the Brazilian state of Mato Grosso do Sul in the Center-West region of Brazil. The economy of this city is partly derived from the farming of cattle that supplies local slaughterhouses, which in turn allows Campo Grande to export meat to other states in Brazil and abroad. The most important crops in the area are soy, rice, and cassava. In addition, the state is one of the largest producers of soybeans in Brazil. Avenue BR163 is one of the main routes used to transport its production. Soil and plant samples were collected by sampling in transects perpendicular to the highway with high vehicular traffic, as well as close to a toll booth and at distances of D = 15–45 m from the left and right edge of the highway (Figure 1). Five soil and plant collection sites were selected (P1, P2, P3, P4, and P5). The distance between P1 (0 km, established as the starting point of sample collection and next to the roundabout with high traffic of vehicles) and P2 was 8.5 km, the distance between P2 and P3 was 8.0 km, between P3 and P4 was 9.0 km, between P4 and P5 was 8.5 km, and between P5 and toll booth was 150 m (i.e., P5 is next to toll booth). The distance between the first site P1 and the toll booth is 34.36 km.

2.2. Sample Collection and Preparation

Fifty samples of the soils and the B. decumbens Stapf plant (botanical genus belonging to the family Poaceae) were selected. Samples were collected at each site on the same days under rainless conditions. The same places where the soil samples were collected also yielded samples of plant leaves. Sampling (soils and plants) was undertaken over five consecutive days, in the summer during times of drought. All collected soil and plant samples were stored in sealed polyethylene bags, labeled, and then transported to the laboratory. The leaf samples, as well as soils at each point, were mixed to obtain a representative sample. On both sides of the highway (i.e., left and right) at each point P1, P2, P3, P4, and P5, 1000 mg of B. decumbens leaves (a total of 5000 mg of leaves) were collected with a stainless steel knife based on the procedures described by Miclean et al. [18]. For each sampling point, small trenches were opened with the aid of a stainless steel blade, and soil samples of approximately 100 g were collected at depths of 0–20 cm (a total of 1 kg of soil at each point).

2.3. Soil Digestion

In the laboratory, the soil samples from each sampling site (P1, P2, P3, P4, and P5) were air-dried until a constant weight was obtained. They were then ground and sieved in a 2 mm sieve as per Miclean et al. [18]. Acid digestion was performed by placing about 0.5 g of soil samples in a Teflon DAP60^® vessels and digesting them with 9 mL of HCl (35%, Merck, Darmstadt, Germany) and 3 mL of H₂O₂ (65%, Merck, Darmstadt, Germany). In addition, samples with acids were allowed to stand for pre-digestion for 18 h with the DAP60^® container loosely capped to allow gases to escape. Soil samples that are digested using the 3051A digestion method can be analyzed using the guidelines of the United States Environmental Protection Agency (USEPA) [19]. After cooling, the samples were filtered, transferred into 25 mL volumetric flasks, and made up to the mark with ultrapure water. Each sample of soil was digested in triplicate for consistency of results. A blank was also run at the same time.

2.4. Plant Digestion

The representative samples were divided into two portions for each plant for each study collection site (P1, P2, P3, P4, and P5). One portion of B. decumbens leaves (500 mg) were carefully washed for 5 min under tap ultrapure water to remove dust, dirt, and possible organisms, and soaked in 250 mL ultrapure water for 20 h, and then a 10 min ultrasonic shaking was applied. Ultrapure water (18 MΩcm, Milli-Q Millipore, Bedford, MA, USA) was used to wash the leaves, as it did not contain concentrations of minerals and was free of contaminants that could interfere with the analyses. The second portion of all samples (500 mg) was not washed with ultrapure water and was used to estimate heavy metal concentrations of particulate deposited on plant leaves [20]. Both dry samples were ground using a stainless mill (Thermomix, São Paulo, Brazil) and passed through a 200 μm granulometry sieve to obtain very fine particles [20,21,22].

Approximately 300 mg of washed and unwashed B. decumbens leaves were individually placed in a closed microwave vessel and digested with 5 mL HNO₃ (65%, Merck, Darmstadt, Germany) and 3 mL H₂O₂ (35%, Merck, Darmstadt, Germany). Digestion used microwave digestion equipment (Speedwavefour, Berghof, Eningen, Germany), with the heating program carried out in four successive stages (

Table 1. Operating conditions of a microwave digestion system.

Step	Temperature (°C)	Pressure (Bar)	Time (min)	Power (W)
1	150	30	5	710
2	190	35	15	1136
3	50	25	10	0
4	0	0	0	0

). Each sample of vegetation was digested in triplicate, and analytical blanks were also prepared following the same procedure used for the samples.

2.5. Elemental Measurement by Using ICP-OES

The concentration of Al, As, Cd, Co, Cr, Cu, Fe, Ni, Zn, and Pb in the soil and B. decumbens leaves were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) (iCAP 6300 Duo, Thermo Fisher Scientific, Bremen, Germany). The operating conditions employed for ICP-OES were 1250 W RF power, 0.35 L·mn⁻¹ sample flow rate, 12 L·mn⁻¹ plasma gas flow rate, 5 s integration time, 20 s stabilization time, and 20 psi pressure of nebulization. In addition, the axial view was used for metal determination, air gas was used with the axially viewed plasma. The following emission wavelengths (nm) were set and were used by the ICP OES for analysis of each of the elements: Al 309.271 nm, Pb 220.353 nm, As 189.042 nm, Cu 324.754 nm, Fe 259.940 nm, Ni 221.647 nm, Zn 213.856 nm, Cr 267.716 nm, Co 228.616 nm, and Cd 228.802 nm.

The calibration standard solutions were prepared by diluting stoke multi-elemental standard solution (SpecSol, Quimlab, Jacareí, Brazil) containing 1000 mg/L of each element (Al, As, Cd, Co, Cr, Cu, Fe, Ni, Zn, and Pb). For quantification of the investigated elements in the soils and leaves, external calibration curves were built on five different concentrations in the range of 0.01–5.0 mg/L. Optimal conditions were evaluated in terms of accuracy (by recovery test) and limit of detection. The spiking solution was made from a single multielement stock solution of 1000 ppm. Thus, a recovery test was performed, and the solutions were spiked with 1 ppm. The method had a recovery interval of 89–110%. The limits of detection (LOD) and limit of quantification (LOQ) were calculated according to Rosa (2021) and Rosa et al. (2022) [21,22]. Therefore, the range of all elements LOD was 0.02–0.3 µg/L, and the range of all elements LOQ was 0.06 to 10 µg/L. The range of the correlation coefficient (R²) was 0.9993–0.9999.

2.6. Transfer Factor

Transfer factor (TF) from soil to plants is calculated as the ratio of metal(loid) concentration in plants and metal concentration in soil as stated in equation [21,22]:

$$TF={\displaystyle \frac{C_p}{C_s}}$$

(1)

where C_p is the metal(loid)s concentration in B. decumbens leaves (mg·kg⁻¹·dw), and C_s is the metal(loid) concentration in soil (mg·kg⁻¹·dw). Higher TF values (>1) indicate higher absorption of metal from soil by the plant. Conversely, lower values indicate the plants had a poor response to metal absorption [23,24]. Transfer factor (TF) is calculated as a ratio of the concentration of a specific metal in plant tissue to the concentration of the same metal in soil.

2.7. Risk Assessment Model

A risk assessment is a process to identify potential health hazards that could arise if a potential exposure concentration occurs. Exposure can occur via non-dietary ingestion of soil or dust on surfaces or objects that are contacted via hand-to-mouth or object-to-mouth activity [9,22]. In the present study, worker exposure of heavy metals to highway dust can occur naturally, considering the following facts: (a) direct ingestion of heavy metals through dust (CDI_oral); (b) inhalation of heavy metals via dust particulates resuspended particles through mouth and nose (CDI_inh); and (c) dermal absorption of trace elements in particles adhered to exposed skin (CDI_dermal) [9,25]. Therefore, the non-carcinogenic risk was estimated for Al, As, Cd, Co, Cr, Cu, Fe, Ni, Zn, and Pb. The CDI for each metal (chronic daily intake) was calculated using Equations (2)–(4).

$${CDI}_{oral}\left(mg/kgxday\right)=C\times {\displaystyle \frac{IR\times EF\times ED}{AT\times BW}}$$

(2)

$${CDI}_{inh}\left(mg/kgxday\right)=C\times {\displaystyle \frac{{IR}_{ar}\times EF\times ED}{AT\times BW\times PEF}}$$

(3)

$${ CDI}_{dermal}\left(mg/kgxday\right)=C\times {\displaystyle \frac{SL\times SA\times EF\times ED\times ABS}{AT\times BW}}$$

(4)

where C is the concentration of metal(loid)s in highway dust (mg/kg) obtained by the difference between unwashed leaves and washed leaves (concentration of metal(oids) in dust deposited under plants or in the air: C = UWL − WL). IR is the ingestion rate, which in this study, was 100 mg/day for adults. IR_ar is the inhalation rate (20 m³/day for adults). The EF exposure frequency for adults is 350 days/year. ED is exposure duration (30 years) [26]. SL is the skin adherence factor for dust, which in this study for adults is 0.07 mg/cm²∙day [24]. SA is the surface area of the skin in contact with dust (5700 cm²) [26,27]. AT is the average time in days: for non-carcinogens, AT = ED × 365 days; for carcinogens AT = 70 years × 365 days/years = 25,550 days. ABS is the dermal absorption factor (unitless), which in this study is 0.001 for all elements, except for arsenic which is 0.03. BW is the body weight (70 kg), and PEF is a particulate emission factor (1.30 × 109 m³/kg) [27,28].

2.8. Hazard Quotient (HQ) and Hazard Index (HI)

The non-carcinogenic health hazards through dust consumption were evaluated by the target hazard quotient (HQ) using Equation (5) [21,22].

$${ HQ}_{\beta }={\displaystyle \frac{CDI}{Rf{D}_{\beta }}}$$

(5)

Here, the CDI was obtained in Equations (2)–(4) for each route of exposure, and RfD_β is the reference dose. That is, in Equation (5) the subscript β corresponds to the oral reference dose (oral), inhalation reference dose (inh), and dermal reference dose (dermal). Therefore, this study considered the following values for reference doses (RfD_β):

-

Oral reference dose (RfD_oral): Al 1.0 mg/kg∙day, As 3.0 × 10⁻⁴ mg/kg∙day, Cd 1.0 × 10⁻⁴ mg/kg∙day, Co 3.0 × 10⁻⁴ mg/kg∙day, Cr 1.5 mg/kg∙day, Cu 4.0 × 10⁻² mg/kg∙day, Fe 7.0 × 10⁻¹ mg/kg∙day, Ni 2.0 × 10⁻² mg/kg∙day, Zn 3.0 × 10⁻¹ mg/kg∙day [29], and Pb 3.60 × 10⁻³ mg/kg∙day [30].

-

Innalatory reference dose (RfD_inh): Al 5.0 × 10⁻³ mg/kg∙day, As 1.5 × 10⁻⁵ mg/kg∙day, Cd 1.0 × 10⁻⁵ mg/kg∙day, Co 6.0 × 10⁻⁶ mg/kg∙day [29], Cr 2.86 × 10⁻⁵ mg/kg∙day [31], Cu 4.0 × 10⁻² mg/kg∙day [32], Fe not yet established, Ni 2.0 × 10⁻⁵ mg/kg∙day [29], Zn 3.0 × 10⁻¹ mg/kg∙day [32], and Pb 2.0 × 10⁻⁴ mg/kg∙day [33].

-

Dermal reference dose (RfD_dermal): Al 1.0 mg/kg∙day, As 3.0 × 10⁻⁴ mg/kg∙day, Cd 1.25 × 10⁻⁵ mg/kg∙day, Co 3.0 × 10⁻⁴ mg/kg∙day, Cr 1.95 × 10⁻² mg/kg∙day, Cu 4.0 × 10⁻² mg/kg∙day, Fe 7.0 × 10⁻¹ mg/kg∙day [34], Ni 5.4 × 10⁻³ mg/kg∙day [30], Zn 3.0 × 10⁻¹ mg/kg∙day, and Pb 4.0 × 10⁻² mg/kg∙day [34].

A hazard quotient less than or equal to 1 indicates that adverse effects are not likely to occur. However, if HQ > 1 in the exposed population, health risks may occur [21,22]. Another very important concept related to HQ is the hazard index (HI). The HI is obtained as the sum of all HQs of each metal(loid)s [11], and was calculated using Equation (6):

$$HI=\sum HQ=\sum {{HQ}_{oral}{+HQ}_{inh}+HQ}_{dermal}$$

(6)

If HI < 1, exposures are unlikely to result in non-cancer adverse health effects during the lifetime of exposure; however, when HI > 1, exposure may pose a health risk.

According to Equation (6), the hazard index recorded for adults was obtained as follows: HI = HQAl + HQAs + HQCd + HQCo + HQCr + HQCu + HQFe + HQNi + HQZn + HQPb, for each different pathway (oral, inalatory, and dermal). However, since more than one pathway is present, we can consider HI_pathways = HQ_oral + HQ_inh + HQ_dermal [35].

2.9. Carcinogenic Analysis

The possibility of cancer risks in the studied dust through intake of carcinogenic metal(loid)s was estimated using the Incremental Lifetime Cancer Risk (ILCR). The equation for calculating the lifetime cancer risk is:

$${ ILCR}_{pathway}=CDI\times CSF$$

(7)

where the subscript CDI corresponds to CDI_oral, CDI_inh, and CDI_dermal (Equations (2)–(4)), while CSF is the cancer slope factor (mg/kg/day)⁻¹ [36]. The following values were considered for CSF: oral CSF: Cd 6.3, Cr 0.5, Pb 0.0085, and As 1.5; inhalation CSF: As (1.5), Pb (4 × 10⁻²), Cd (6.30), Cr (4.1), and Co (9.8); dermal CSF: As (1.5) [28,37]. The permissible limits are considered to be 10⁻⁶ and <10⁻⁴ for a single carcinogenic element and multi-element carcinogens. The carcinogenic risk level was classified as Oral Risk < 10⁻⁶ estimated as a very low level, 10⁻⁶–10⁻⁵—estimated as a low level, 10⁻⁵–10⁻⁴—the medium level, 10⁻⁴–10⁻³—high level and >10⁻³—estimated as a very high level [38].

The total excess lifetime cancer risk for an individual is finally calculated from the average contribution of the individual heavy metals for all the pathways using the following equation:

$${ ILCR}_{total}={{ { ILCR}_{oral}+ILCR}_{inhl}+ILCR}_{dermal}$$

(8)

2.10. Statistical Analysis

Data were processed using the Origin 9.0 software (OriginLab Corporation, Northampton, MA, USA). Concentrations were expressed as mean ± standard deviation. One-way analysis of variance (ANOVA) and Principal Component Analysis (PCA) were used to test for differences in element levels in grass and soil samples at different collection sites.

3. Results

3.1. Concentration of Meta(loids) in Soil

The results of the total concentrations of metal(loid)s studied in soil samples at sampling sites P1, P2, P3, P4, and P5 are presented in

Table 2. Concentrations of elements in soil (median and standard deviation) compared to Conama, Brazil [39], soils with agricultural activities from China [40], and forested soils of the State of Pará, Brazilian Amazon [41].

Element	Concentrations of Elements in Soil: (Sampling Sites to Highway) (mg/kg·dw)
	P1 (0 km)	P2 (8.5 km)	P3 (16.5 km)	P4 (25.5 km)	P5 (34.46 km)	Conama/Brazil (mg/kg)	China (mg/kg)	State of Pará Brazil (mg/kg)
Al	75.04 ± 1.59	62.48 ± 1.41	65.19 ± 1.13	65.42 ± 2.18	88.37 ± 2.47	*	**	8.2 × 103
As	16.51± 1.28	15.67±1.47	13.10 ± 1.95	13.98 ± 1.82	19.86 ± 1.28	15	12.207	0.8
Cd	34.25 ± 2.47	28.19 ± 1.62	25.88 ± 1.05	29.15 ± 1.49	32.88 ± 1.65	1.3	1.497	0.1
Co	13.19 ± 1.18	9.43 ± 1.86	8.25 ± 1.62	10.45 ± 1.05	14.46 ± 0.79	35	**	1.6
Cr	36.21 ± 2.24	31.68 ± 1.78	33.11 ± 1.68	39.25 ± 1.52	40.45 ± 0.98	*	70.09	14.3
Cu	52.16 ± 2.51	49.16 ± 2.32	50.65 ± 2.14	52.42 ± 3.26	53.68 ± 2.42	200	44.60	6.0
Fe	165.48 ± 2.2	110.76 ± 3.84	118.43 ± 1.58	100.56 ±1.92	152.15 ± 2.56	*	**	9.3 × 103
Ni	38.5 ± 1.53	28.42 ± 2.76	30.43 ± 2.53	32.75 ± 3.12	37.53 ± 2.45	30	41.968	1.4
Zn	119.72 ± 1.2	119.3 ± 1.22	99.42 ± 0.07	110.49 ± 2.53	121.23 ± 1.59	300	154.203	7.0
Pb	35.44 ± 0.51	28.08 ± 1.33	28.64 ± 0.45	22.01 ± 1.28	33.19 ± 2.46	72	55.14	10.4

* Values not determined by Conama/Brazil; ** Values not determined by China.

. In addition, the values of concentrations of elements obtained in soil from the highway were compared to the threshold values of the Brazil National Council of Environment (Conama/Brazil) determined from human health-based risk analysis [39], and compared to the concentration of elements in soils with agricultural activities from China [40] and forested soils of the state of Pará, Brazilian Amazon [41].

The PCA indicated that the elements in PC-1 showed higher trends and proportions in the distance of P5 and smaller trends in P1. In fact, the concentrations of elements such as Al, As, Cd, Co, Cr, Cu, Fe, Ni, Zn, and Pb in the soil collection sites at P1 and P5 from the highway are higher than those at P3 and P4 (Table 2 and Figure 2).

3.2. Concentration of Meta(loids) in Unwashed (UWL) and Washed (WL) Leaves of B. decumbens

The levels of metal(loid)s in the washed leaf samples were lower than those in the unwashed samples in this study (

Table 3. Levels (mg/kg) of metal(loid)s in the washed (WL) and unwashed (UWL) leaves of B. decumbens in different sampling sites.

Element	P1 (0 km)		P2 (8.5 km)		P3 (16.5 km)		P4 (25.5 km)		P5 (34.46 km)
	Washed	Unwashed	Washed	Unwashed	Washed	Unwashed	Washed	Unwashed	Washed	Unwashed
Al	91.15 ± 2.75	122.45 ± 3.92	92.82 ± 2.98	116.72 ± 3.59	95.17 ± 1.72	115.86 ± 2.86	96.45 ± 2.01	120.42 ± 2.98	89.72 ± 2.38	132.38 ± 2.97
As	0.61 ± 0.12	1.98 ± 0.79	0.56 ± 0.16	1.75 ± 0.65	0.53 ± 0.19	1.88 ± 0.64	0.62 ± 0.26	1.72 ± 0.53	0.72 ± 0.21	1.91 ± 0.65
Cd	0.78 ± 0.25	1.86 ± 0.48	0.82 ± 0.33	1.62 ± 0.72	0.86 ± 0.18	1.71 ± 0.49	0.91 ± 0.24	1.76 ± 0.42	0.98 ±0.36	2.01 ± 0.96
Co	0.59 ± 0.11	0.79 ± 0.27	0.56 ± 0.16	0.86 ± 0.32	0.48 ± 0.12	0.91 ± 0.28	0.49 ± 0.19	1.02 ± 0.31	0.56 ± 0.15	1.50 ± 0.56
Cr	2.76 ± 0.49	8.39 ± 0.98	2.89 ± 0.76	6.48 ± 1.28	3.58 ± 0.45	7.41 ± 0.35	4.54 ± 0.49	9.48 ± 1.97	6.12 ± 0.47	11.78 ± 1.84
Cu	4.48 ± 0. 20	10.88 ± 0.79	3.65 ± 0.34	9.78 ± 0.57	4.01 ± 0.54	10.29 ± 0.48	5.02 ± 0.57	10.54 ± 0.59	6.07 ± 0.75	12.84 ± 0.87
Fe	65.23 ± 1.89	125.48 ± 1.42	31.53 ± 0.75	68.52 ± 1.70	34.72 ± 0.86	73.27 ± 1.07	31.72 ± 0.78	65.19 ± 1.19	62.48 ± 0.81	112.15 ± 1.58
Ni	1.32 ± 0.52	4.87 ± 0.43	1.22 ± 0.72	3.69 ± 0.34	1.45 ± 0.53	4.40 ± 0.22	1.68 ± 0.75	4.50 ± 0.48	1.74 ± 0.68	5.78 ± 0.58
Zn	40.58 ± 1.52	52.87 ± 1.02	35.15 ± 1.35	49.49 ± 1.47	41.82 ± 0.98	50.49 ± 0.67	54.75 ± 0.84	60.65 ± 0.85	50.23 ± 0.65	62.86 ± 0.94
Pb	9.56 ± 0.45	20.80 ± 0.37	6.78 ± 0.86	13.48 ± 0.65	6.95 ± 0.91	13.64 ± 0.45	4.80 ± 0.26	7.80 ± 0.62	5.24 ± 0.78	8.45 ± 0.56

and Figure 3). Thus, high percentages of metals and metalloids were removed from all plants.

In addition, the C concentration of metals(loid)s in the dust deposited under the plants obtained by the difference in the average concentration of metals between washed (WL) and unwashed (UWL) leaves is shown in

Table 4. Concentration (mg/kg) of metal(loid)s in the dust deposited under the leaves of plants at sampling sites along the highway.

Element	P1 (0 km)	P2 (8.5 km)	P3 (16.5 km)	P4 (25.5 km)	P5 (34.46 km)
	C = UWL − WL	C = UWL − WL	C = UWL − WL	C = UWL − WL	C = UWL − WL
Al	31.30	23.90	20.69	23.97	42.66
As	1.37	1.19	1.35	1.10	1.19
Cd	1.08	0.90	0.93	0.95	1.10
Co	0.20	0.30	0.43	0.41	0.94
Cr	5.63	3.59	3.83	4.94	5.66
Cu	6.40	6.13	6.28	5.52	6.77
Fe	60.25	36.99	38.55	33.47	49.67
Ni	3.55	2.47	2.95	2.82	4.04
Zn	12.29	14.34	8.67	5.60	12.63
Pb	11.24	6.70	6.69	3.0	3.21

C = UWL − WL; unwashed (UWL) and washed (WL) leaves of B. decumbens.

.

According to the PCA, the greatest tendency for accumulation of heavy metals occurs at sites P1 = 0 km and P5 = 34.46 km (Figure 4).

3.3. Transfer Factor

The transfer factor (TF) of metal(oid)s concentrations in soils for washed and unwashed B. decumbens leaves are presented in

Table 5. Transfer factor of element concentrations in soil (collected at points P1, P2, P3, P5, and P5) for washed and unwashed B. decumbens leaves.

	Transfer Factor (TF)
Element	P1 (0 km)		P2 (8.5 km)		P3 (16.5 km)		P4 (25.5 km)		P5 (34.46 km)
	Washed	Unwashed	Washed	Unwashed	Washed	Unwashed	Washed	Unwashed	Washed	Unwashed
Al	1.2140	1.631	1.4855	1.8681	1.459	1.777	1.474	1.840	1.0151	1.4970
As	0.0369	0.1199	0.0357	0.1116	0.0404	0.1435	0.0443	0.1230	0.0362	0.0961
Cd	0.0227	0.0543	0.0290	0.0574	0.0332	0.0660	0.031	0.0603	0.0298	0.0611
Co	0.0447	0.0598	0.0593	0.0911	0.0581	0.1103	0.0468	0.0976	0.0387	0.1037
Cr	0.0762	0.2317	0.0912	0.2045	0.1088	0.2237	0.1156	0.2415	0.1512	0.2912
Cu	0.0858	0.2085	0.0742	0.1989	0.0791	0.2029	0.0957	0.2010	0.1130	0.2391
Fe	0.3941	0.758	0.2846	0.6186	0.2931	0.6186	0.0315	0.6482	0.4106	0.7371
Ni	0.0342	0.1264	0.0429	0.1298	0.0476	0.1445	0.0512	0.1374	0.0463	0.1540
Zn	0.3380	0.4416	0.2946	0.4148	0.4206	0.5078	0.4955	0.5489	0.4143	0.5185
Pb	0.2690	0.586	0.2414	0.4800	0.2426	0.4762	0.2180	0.3543	0.157	0.2545

. Here, the mean concentrations of the elements in the soil and in the leaves of the plants were considered in the TF calculations. For all sampling sites, the transfer factors of Al from soil to washed and unwashed leaves were greater than 1 (Figure 5). However, in other cases, the TFs were below 1.

3.4. Risk Assessment Model

The results on the ingestion of heavy metals through dust (CDI_oral, Equation (2)), as well as inhalation of metal(loid)s via dust particulates (CDI_inh, Equation (3)), and dermal absorption of metal(loid)s in particles adhered to exposed skin (CDI_dermal, Equation (4)) are presented in

Table 6. Chronic daily intake (CDI) (mg/kg/day) for heavy metals through different pathways.

Element/CDI	P1 (0 km)	P2 (8.5 km)	P3 (16.5 km)	P4 (25.5 km)	P5 (34.46 km)
Al
CDIoral	4.287 × 10−5	3.273 × 10−5	2.834 × 10−5	3.283 × 10−5	5.843 × 10−5
CDIinh	6.305 × 10−9	4.814 × 10−9	4.168 × 10−9	4.828 × 10−9	8.593 × 10−9
CDIdermal	1.710 × 10−7	1.306 × 10−7	1.130 × 10−7	1.310 × 10−7	2.331 × 10−7
As
CDIoral	1.876 × 10−6	1.630 × 10−6	1.849 × 10−6	1.506 × 10−6	1.630 × 10−6
CDIinh	2.759 × 10−10	2.397 × 10−10	2.719 × 10−10	2.215 × 10−10	2.397 × 10−10
CDIdermal	2.246 × 10−7	1.951 × 10−7	2.214 × 10−7	1.804 × 10−7	1.902 × 10−7
Cd
CDIoral	1.479 × 10−6	1.232 × 10−6	1.273 × 10−6	1.301 × 10−6	1.50 × 10−6
CDIinh	2.175 × 10−10	1.813 × 10−10	1.873 × 10−10	1.913 × 10−10	2.215 × 10−10
CDIdermal	5.903 × 10−9	4.919 × 10−9	5.083 × 10−9	5.192 × 10−9	6.012 × 10−9
Co
CDIoral	2.739 × 10−7	4.109 × 10−7	5.890 × 10−7	5.616 × 10−7	1.287 × 10−6
CDIinh	4.029 × 10−11	6.043 × 10−11	8.662 × 10−11	8.259 × 10−11	1.893 × 10−10
CDIdermal	1.093 × 10−9	1.639 × 10−9	2.350 × 10−9	2.240 × 10−9	5.137 × 10−9
Cr
CDIoral	7.712 × 10−6	4.917 × 10−6	5.246 × 10−6	6.767 × 10−6	7.753 × 10−6
CDIinh	1.134 × 10−9	7.232 × 10−10	7.715 × 10−10	9.951 × 10−10	1.140 × 10−9
CDIdermal	3.077 × 10−8	1.962 × 10−8	2.093 × 10−8	2.700 × 10−8	3093 × 10−8
Cu
CDIoral	8.767 × 10−6	8.397 × 10−6	8.602 × 10−6	7.561 × 10−6	9.273 × 10−6
CDIinh	1.289 × 10−9	1.234 × 10−9	1.265 × 10−9	1.112 × 10−9	1.363 × 10−9
CDIdermal	3.498 × 10−8	3.350 × 10−8	3.432 × 10−8	3.017 × 10−8	3.700 × 10−8
Fe
CDIoral	8.253 × 10−5	5.067 × 10−5	5.280 × 10−5	4.584 × 10−5	6.804 × 10−5
CDIinh	1.213 × 10−8	7.451 × 10−9	7.765 × 10−9	6.742 × 10−9	1.00 × 10−8
CDIdermal	3.293 × 10−7	2.021 × 10−7	2.107 × 10−7	1.829 × 10−7	2.714 × 10−7
Ni
CDIoral	4.863 × 10−6	3.383 × 10−6	4.041 × 10−6	3.863 × 10−6	5.534 × 10−6
CDIinh	7.151 × 10−10	4.975 × 10−10	5.942 × 10−10	5.680 × 10−10	8.138 × 10−10
CDIdermal	1.940 × 10−8	1.350 × 10−8	1.612 × 10−8	1.541 × 10−8	2.208 × 10−8
Zn
CDIoral	1.683 × 10−5	1.964 × 10−5	1.187 × 10−5	7.671 × 10−6	1.730 × 10−5
CDIinh	2.475 × 10−9	2.888 × 10−9	1.746 × 10−9	1.128 × 10−9	2.544 × 10−9
CDIdermal	6.717 × 10−8	7.837 × 10−8	4.738 × 10−8	3.06 × 10−8	6.903 × 10−8
Pb
CDIoral	1.539 × 10−5	9.178 × 10−6	9.164 × 10−6	4.109 × 10−6	4.397 × 10−6
CDIinh	2.264 × 10−9	1.349 × 10−9	1.347 × 10−9	6.043 × 10−10	6.466 × 10−10
CDIdermal	6.143 × 10−8	3.662 × 10−8	3.656 × 10−8	1.639 × 10−8	1.754 × 10−8

. The chronic daily intake (CDI) was calculated using Equations (2)–(4) for an adult person with an age of 30 years and 70 kg body weight (full details are in Section 2.5) and considering values of C obtained in Table 4.

The hazard quotients (HQ) of the assessed metal(loid)s are presented in

Table 7. Hazard quotient (HQ) for heavy metals through different pathways (Equation (5)).

Element/ HQ	P1 (0 km)	P2 (8.5 km)	P3 (16.5 km)	P4 (25.5 km)	P5 (34.46 km)
Al
HQoral	4.287 × 10−5	3.273 × 10−5	2.834 × 10−5	3.283 × 10−5	5.843 × 10−5
HQinh	1.261 × 10−3	9.628 × 10−7	8.336 × 10−7	9.656 × 10−7	1.718 × 10−6
HQdermal	1.710 × 10−7	1.306 × 10−7	1.130 × 10−7	1.310 × 10−7	2.331 × 10−7
As
HQoral	6.253 × 10−3	5.433 × 10−3	6.163 × 10−3	5.020 × 10−3	5.433 × 10−3
HQinh	1.839 × 10−5	1.598 × 10−5	1.812 × 10−5	1.476 × 10−5	1.598 × 10−5
HQdermal	7.486 × 10−4	6.503 × 10−4	7.380 × 10−4	6.013 × 10−4	6.340 × 10−4
Cd
HQoral	0.01479	0.01232	0.01273	0.01301	0.0150
HQinh	2.175 × 10−5	1.813 × 10−5	1.873 × 10−5	1.913 × 10−5	2.215 × 10−5
HQdermal	4.722 × 10−4	3.935 × 10−5	4.066 × 10−4	4.153 × 10−4	4.809 × 10−4
Co
HQoral	9.130 × 10−4	1.369 × 10−3	1.963 × 10−3	1.872 × 10−3	4.29 × 10−3
HQinh	6.715 × 10−6	1.007 × 10−5	1.443 × 10−5	1.376 × 10−5	3.155 × 10−5
HQdermal	3.643 × 10−6	5.463 × 10−6	7.833 × 10−6	4.466 × 10−6	1.712 × 10−5
Cr
HQoral	5.141 × 10−6	3.278 × 10−6	3.497 × 10−6	4.511 × 10−6	5.168 × 10−6
HQinh	3.965 × 10−5	2.528 × 10−5	2.697 × 10−5	3.479 × 10−5	3.986 × 10−5
HQdermal	1.577 × 10−6	1.00 × 10−6	1.073 × 10−6	1.384 × 10−6	1.586 × 10−6
Cu
HQoral	2.191 × 10−4	2.099 × 10−4	2.150 × 10−4	1.890 × 10−4	2.318 × 10−4
HQinh	3.22 × 10−8	3.085 × 10−8	3.162 × 10−8	2.780 × 10−8	3.407 × 10−8
HQdermal	8.745 × 10−7	8.375 × 10−7	8.58 × 10−7	7.542 × 10−7	9.250 × 10−7
Fe
HQoral	1.179 × 10−4	7.238 × 10−5	7.542 × 10−5	6.548 × 10−6	9.720 × 10−5
HQinh	ND	ND	ND	ND	ND
HQdermal	4.704 × 10−7	2.887 × 10−7	3.01 × 10−7	2.612 × 10−7	3.877 × 10−7
Ni
HQoral	2.431 × 10−4	1.691 × 10−4	2.020 × 10−4	1.931 × 10−4	2.767 × 10−4
HQinh	3.575 × 10−5	2.487 × 10−5	2.971 × 10−5	2.84 × 10−5	4.069 × 10−5
HQdermal	3.592 × 10−6	2.50 × 10−6	2.985 × 10−6	2.853 × 10−6	4.088 × 10−6
Zn
HQoral	5.61 × 10−5	6.546 × 10−5	3.956 × 10−5	2.557 × 10−5	5.766 × 10−5
HQinh	8.25 × 10−9	9.60 × 10−9	5.820 × 10−9	3.760 × 10−9	8.480 × 10−9
HQdermal	2.239 × 10−7	2.612 × 10−7	1.579 × 10−7	1.02 × 10−7	2.301 × 10−7
Pb
HQoral	4.275 × 10−3	2.549 × 10−3	2.545 × 10−3	1.141 × 10−3	1.221 × 10−3
HQinh	1.132 × 10−5	6.745 × 10−6	6.735 × 10−6	3.021 × 10−6	3.233 × 10−6
HQdermal	1.535 × 10−6	9.155 × 10−7	9.140 × 10−7	4.097 × 10−7	4.385 × 10−7

. Here, the HQ for each metal(loid)s was estimated using the ratio of computed mean daily intake (CDI, Table 6) of an element chemical ingested with contaminated dust to the reference oral dose (RfD) through oral ingestion, inhaled ingestion, and dermal absorption by the toll booth workers. Table 7 shows the results of HQ obtained at different collection points along the edge of the highway. All values of HQs of metals for adults in Table 7 are less than 1.

The hazard index (HI) recorded for adults due to oral ingestion of metals, inhalation ingestion, and dermal contact with metals in highway dust along several collection sites (P1, P2, P3, P4, and P5) are shown in

Table 8. Hazard index (HI) for adults due to exposure to metals in highway dust along various collection points (Equation (6)).

	P1 (0 km)	P2 (8.5 km)	P3 (16.5 km)	P4 (25.5 km)	P5 (34.46 km)
HI = Σ HQoral	0.0269	0.0222	0.0239	0.0214	0.0266
HI = Σ HQinh	1.394 × 10−3	1.020 × 10−4	1.150 × 10−4	1.148 × 10−4	1.552 × 10−4
HI = Σ HQdermal	1.231 × 10−3	7.010 × 10−4	1.158 × 10−3	1.026 × 10−3	1.139 × 10−3

. That is, the non-carcinogenic risk due to potentially hazardous metals present in the same media is assumed to be additive. The hazard index recorded for adults in points was obtained as follows: HI = ΣHQ = HQAl + HQAs + HQCd + HQCo + HQCr + HQCu + HQFe + HQNi + HQZn + HQPb. According to the results, all values of the HQs were below 1.

As described in Section 2.6, since more than one pathway is present, we can consider that the hazard index (HI_pathway) is the sum of the hazard quotients of the three forms of pollutant intake for each element (oral, inalatory, and dermal), HI_pathways = HQ_oral + HQ_inh + HQ_dermal [35]. The HI_pathway sum results for each individual element from each exposure route are shown in

Table 9. Hazard index (HI_pathway) for adults due to exposure to metals in highway dust along various collection points.

Element	P1 (0 km)	P2 (8.5 km)	P3 (16.5 km)	P4 (25.5 km)	P5 (34.46 km)
Al	1.304 × 10−3	3.382 × 10−5	3.392 × 10−5	3.392 × 10−5	6.038 × 10−5
As	7.019 × 10−3	6.099 × 10−3	6.919 × 10−3	5.636 × 10−3	6.082 × 10−3
Cd	0.01528	0.01237	0.0131	0.0134	0.0155
Co	9.233 × 10−4	1.384 × 10−3	1.985 × 10−3	1.890 × 10−3	4.338 × 10−3
Cr	4.636 × 10−5	2.955 × 10−5	3.154 × 10−5	4.068 × 10−5	4.661 × 10−5
Cu	2.200 × 10−4	2.107 × 10−4	2.158 × 10−4	1.897 × 10−4	2.327 × 10−4
Fe	1.183 × 10−4	7.266 × 10−5	7.572 × 10−5	6.809 × 10−6	1.01 × 10−5
Ni	2.824 × 10−4	1.964 × 10−4	2.346 × 10−4	2.243 × 10−4	3.214 × 10−4
Zn	5.633 × 10−5	6.573 × 10−5	3.972 × 10−5	2.567 × 10−5	5.789 × 10−5
Pb	4.287 × 10−3	2.556 × 10−3	2.552 × 10−3	1.144 × 10−3	1.224 × 10−3

.

The results using Equations (7) and (8) on Incremental Lifetime Cancer Risk (ILCR_pathway and ILCR_total) are presented in

Table 10. Incremental Lifetime Cancer Risk (ILCR) derived from ingestion exposure to individual elements, for adults.

Element/ ILCR	P1 (0 km)	P2 (8.5 km)	P3 (16.5 km)	P4 (25.5 km)	P5 (34.46 km)
As
ILCRoral	2.814 × 10−6	2.445 × 10−6	2.773 × 10−6	2.259 × 10−6	2.445 × 10−6
ILCRinh	4.138 × 10−10	3.595 × 10−10	4.078 × 10−10	3.322 × 10−10	3.595 × 10−10
ILCRdermal	3.369 × 10−7	2.926 × 10−7	3.315 × 10−7	1.804 × 10−7	2.853 × 10−7
ILCRtotal	3.151 × 10−6	2.737 × 10−6	3.134 × 10−6	2.439 × 10−6	2.703 × 10−6
Cd
ILCRoral	9.317 × 10−6	7.761 × 10−6	8.019 × 10−6	8.196 × 10−6	9.450 × 10−6
ILCRinh	1.370 × 10−10	1.142 × 10−9	1.179 × 10−9	1.205 × 10−9	1.395 × 10−10
ILCRdermal	-	-	-	-	-
ILCRtotal	9.317 × 10−6	7.762 × 10−6	8.020 × 10−6	8.197 × 10−6	9.450 × 10−6
Co
ILCRoral	-	-	-	-	-
ILCRinh	3.948 × 10−11	5.922 × 10−11	4.488 × 10−10	8.093 × 10−10	1.855 × 10−9
ILCRdermal	-	-	-	-	-
ILCRtotal	3.948 × 10−11	5.922 × 10−11	4.488 × 10−10	8.093 × 10−10	1.855 × 10−9
Cr
ILCRoral	3.856 × 10−6	9.834 × 10−6	1.049 × 10−6	1.353 × 10−6	1.550 × 10−6
ILCRinh	4.649 × 10−9	2.962 × 10−9	3.163 × 10−9	4.079 × 10−9	4.671 × 10−9
ILCRdermal	-	-	-	-	-
ILCRtotal	3.860 × 10−6	9.836 × 10−6	4.212 × 10−6	1.357 × 10−6	1.554 × 10−6
Pb
ILCRoral	1.308 × 10−7	7.801 × 10−8	7.789 × 10−8	3.492 × 10−8	3.737 × 10−8
ILCRinh	9.056 × 10−9	5.396 × 10−9	5.388 × 10−11	2.417 × 10−10	2.586 × 10−11
ILCRdermal	-	-	-	-	-
ILCRtotal	1.398 × 10−7	8.340 × 10−8	7.794 × 10−8	3.516 × 10−8	3.739 × 10−8

. The absence of ILCR calculations in Table 10 is due to the lack of CSF values for some elements such as Al, Cu, Fe, Ni, and Zn. In addition, for elements such as Cd, Co, Cr, and Pb, oral or dermal CSF values are not yet available. Therefore, the carcinogenic risk assessment (Table 10) showed that in all studied sites, the As (ILCR_inh and ILCR_dermal), Cd (ILCR_inh), Co (ILCR_inh), Cr (ILCR_inh), and Pb (ILCR_oral and ILCR_inh) risk values belong to the very low (allowable) level, while As (ILCR_oral), Cd (ILCR_oral), and Cr (ILCR_oral) exceeded safety level (1.0 × 10⁻⁶) in all sites and showed low levels of carcinogenic risk. For multi-element carcinogens, all ILCR total values for As, Cd, Co, Cr, and Pb did not exceed the value of 10⁻⁴.

4. Discussion

The concentrations of Al, Fe, Co, Cu, and Zn at sites P1, P2, P3, P4, and P5, As concentrations at sites P3 and P5, as well as Ni concentrations at site P2, are below the values established by Conama [39]. On the other hand, the concentrations of Cd and Co in P1, P2, P3, P4, and P5 are above the values established. The concentrations of As, Cd, Cu, and Ni in P1, P2, P3, P4, and P5 are above the concentrations of metal in soil from China and the state of Pará, Brazil. The concentration of Zn, Cd, Cr, Co, and Zn are above the values obtained in the soils of Pará, Brazil. The highest mean concentrations of metal(loid)s in soils were in the samples obtained at the rotary (P1) followed by the point next to the toll booth (P5). These results are probably due to the fact that the volume of vehicles in the rotary was higher than in the other rotary areas and the highway. At the roundabout, the flow of vehicles is high, but due to the mechanical wear, as well as tire and brake abrasion, speed reduction occurs [1,42]. From these findings, it is clear that different vehicles contribute to the increase in the deposition of Fe, Cu, Ni, Cd, Cu, Pb, and Zn in the soil [6,7,8].

Our results in Table 2 corroborate with Adamiec et al. (2016) [1] and Duong and Lee (2011) [42]; that is, sites such as P1 and P5 have high concentrations of elements such as Cd, Cu, Pb, Zn, and Ni which are associated with dust from tire wear, brake wear, and velocity. Furthermore, with the exception of Cr (Table 2), the concentration results for elements such as Cd, Cu, Co, Ni, Zn, and Pb (Table 2) are higher than those obtained for areas of native vegetation or with minimal anthropic interference in soils of the state of Paraíba, Brazil, where the quality reference values (QRVs) were: Cd (0.08 mg/kg), Cu (20.82 mg/kg), Co (13.14 mg/kg), Cr (48.35 mg/kg), Ni (14.44 mg/kg), Zn (33.65 mg/kg), and Pb (14.62 mg/kg) [43]. As can be seen, clear differences were found in the concentrations of the metal in the states of Pará and Paraíba, both Brazilian states. In fact, Brazil has a great diversity of soil; however, information on the mineral composition of the various soils in urban, industrial, mining areas, and contaminated areas is scarce [41]. In addition, according to Brazilian Conama Resolution 420 of 29 December 2009 [39], given their continental size and environmental heterogeneity, each state in the country should establish background values for contaminants in the soil. However, several states of Brazil do not have background values for their soils, which makes comparisons between the various Brazilian states difficult.

According to the PCA, there is a greater tendency for the accumulation of elements such as Al, As, Cd, Co, Cr, Cu, and Zn at the P5 site. On the other hand, there is a tendency for Fe and Pb to accumulate at the P1 site. Through the PCA, we observed that the P5 sampling site close to the toll booths had higher proportions of metal(loid)s in the soil samples; that is, the PCA indicated that the exposures to potentially toxic contaminants are highest close to tollbooths.

Our results are consistent with studies published by Ogundele et al. (2015) [44], on heavy metal concentrations in plants and soil along heavy traffic roads in North Central Nigeria, where high concentrations of Zn, Cu, Cr, Ni, and Pb were also determined in roadside soil. Traffic emissions in the highway area are the main cause of heavy metal accumulation in soil [21,45]. Thus, the highest concentration of metals in P5 may occur because this point is 150 m from the tollbooth. Here, vehicles must slow down by applying the brakes, or remain stopped to wait for other vehicles to cross the tollbooth. In fact, during rapid braking, the most intense brake wear occurs at intersections, corners, traffic lights, and through forced braking [1]. Studies confirmed that significant concentrations of Zn, Cd, Co, Cr, Cu, Ni, and Pb were associated with dust from tire wear, while Zn is the most abundant element from tire wear due to its addition (composition in ZnO and ZnS) in the vulcanization process [46,47]. In addition, according to Ozaki et al. (2004) [48], asphalt and sandpaper-like effects are significant sources of Ni and As in road dust. Thus, the high metal(loid)s content in soil is mostly due to the density of traffic because fuels, lubricating oils, tires, and vehicle brake discs are the main source of some chemical elements such as Fe, Cu, Zn, Al, Cr, Ni, Pb, Co, As, and Cd [45,49,50].

Our results corroborate the observations of Oliva and Valdés (2004) [48,51], where the reduction in metals due to washing the leaves of some plant species ranged from 32 to 68% of the Al, 29–51% of the Cr, 27% of the Pb, and 21–51% of the Fe. Other studies have reported that the percentage reductions in the metal levels by washing the roadside Vernonia amygdalina leaf samples were 77, 54, 31, and 20% for Pb, Cd, Zn, and Cu, respectively [52]. A high percentage of toxic elements were removed from other species of plants; that is, percentages of 84% Al, 75% As, and 56% Pb were removed [53]. However, Caselles (1997) [54] did not observe differences between washed and unwashed leaves of Citrus limon.

Differences in the percentage of metal removal in washed or unwashed leaves arise because the effect of washing on tissue metal concentration depends on the chemical nature of contaminants, the tissue samples [51], the species of plant, and the methodology used for the removal of metals in leaves. In fact, Bora et al. (2022) [55] observed that fruits and vegetables washed with vinegar (5 and 10% acetic acid) resulted in 33 to 37% lower concentrations in As, Cd, and Pb as compared to washing with tap water.

Washing the leaves of the plant results in lowering the metal contamination in the samples, which indicates that airborne particles are one possible source of contamination [13,52,56]. In fact, urban street areas are also indicators of heavy metal (Pb, Cd, Zn, Cr, Ni, As, Mn, and Cu) contamination from atmospheric deposition [57]. Metals such as Pb, Cd, Cu, and Zn are major pollutants in roadside environmental areas, being released from fuel burning, tire wear, oil leaks, and corrosion of batteries and metallic vehicle parts [58]. The concentrations of Cr (3.59–5.66 mg/kg), Cd (0.90–1.10), Cu (5.52–6.77), Fe (33.47–60.25), Pb (3.0–11.24), and Ni (2.47–4.04) (Table 4) are lower than those obtained in motorway dust (Cr 232.0 mg/kg, Cu 198 mg/kg, and Ni 62.3 mg/kg). However, the concentration values of Cd, Fe, and Pb are higher for elements such as Cd (0.45 mg/kg), Fe (42.03 mg/kg), and Pb (0.033 mg/kg) [1]. The content values of Cd, Cu, Cr, Ni, and Pb at site P1 (near the roundabout) and at site P5 (150 m from the tollbooth) are higher than the concentrations at sites P2, P3, and P4. In fact, our results corroborate the paper published by Duong and Lee (2011) [42], which states that there are higher concentrations of elements such as Cd, Cu, Pb, Zn, and Ni in the road dust from areas with different characteristics such as traffic rotaries, downtown areas, circulation roads, and asphalt and highways.

Therefore, street dust contaminated by heavy metal poses a higher health risk to adults, as it comes from the gases expelled by vehicle exhaust. The sample collection site P1 = 0 km is located near one of the largest roundabouts in the city of Campo Grande, with high vehicle traffic, while P5 is next to a toll booth. Therefore, the high amount of metal concentration deposited on the leaves is largely caused by vehicles. The results obtained in Table 4 and Figure 4 are consistent with those published by Zheng et al. (2010) [9]; that is, the concentrations of Al, As, Cd, Co, Cr, Cu, Fe, Ni, Zn, and Pb in the dust deposited on the leaves decreases with the distance from more polluted sites, such as P1 or P5.

According to the transfer factor (TF) of Al, As, Cd, Co, Cr, Cu, Fe, Ni, Zn, and Pb concentrations in soils for washed and unwashed B. decumbens leaves (Table 5), only Al is accumulated among the investigated metals. The transfer factor of Al from soil to plant has the value of 1.0151 for washed leaves, and 1.8681 for unwashed leaves. By contrast, the lowest values were established for Cd. In other cases, the TFs were below 1 (Figure 5), indicating that this plant species has no phytoremediation potential. In addition, sites P1 and P5 were the ones with the highest transfer factor values for Al, As, and Fe for washed and unwashed leaves. In fact, the TFs of Al, As, Cd, Co, Cr, Cu, Fe, Ni, Zn, and Pb in the soil collection sites near P1 (near the highway) and P5 (next to a toll booth) are higher than P2, P3, and P5 (Table 5).

The calculated exposure values for the three routes of exposition are shown in Table 6, which indicates that the highest values of chronic daily intake of dust were obtained through the oral route, followed by dermal and inhalation exposure. These results corroborate those found by Conama (2009) [39], who obtained values of oral daily intake for adults higher than dermal and inhalational exposure. Furthermore, our results are consistent with research by Zheng et al. (2010), Ferreira-Baptista and De Miguel (2005), and Gope et al. (2017) [9,25,32], in which the ingestion of dust particles seems to be the main route of exposure of metal(loid)s for people, followed by dermal contact and inhalation. The highest CDI values (oral, inhalation, and dermal) were obtained from elements such as Al, Fe, Zn, and Pb in dust, followed by As, Cd, Co, Cr, Cu, and Ni. Although the CDI calculations for elements Al, Fe, Zn, Pb, As, Cd, Co, Cr, Cu, and Ni pose no major differences in values at the five sites, they have slightly higher concentrations at sites P1 and P5 and lower levels at sites P2, P3, and P5. The highest CDI (Table 6) for Al (5.843 × 10⁻⁵ mg/kg/day), As (1.876 × 10⁻⁶ mg/kg/day), Cd (1.50 × 10⁻⁶ mg/kg/day), Co (1.287 × 10⁻⁶ mg/kg/day), Cr (7.753 × 10⁻⁶ mg/kg/day), Cu (9.273 × 10⁻⁶ mg/kg/day), and Zn (6.546 × 10⁻⁵ mg/kg/day), all through oral intake of dust are lower than the values established by the minimal risk levels (MRLs) for As (1 mg/kg/day), As (3 × 10⁻⁴ mg/kg/day), Cd (1 × 10⁻⁴ mg/kg/day), Co (1 × 10⁻² mg/kg/day), Cr (9 × 10⁻⁴ mg/kg/day), Cu (1 × 10⁻² mg/kg/day), and Zn (3 × 10⁻¹ mg/kg/day) [59]. There are no oral intake values for Fe, Ni, Zn, and Pb established by the Agency for Toxic Substances and Disease Registry. We did not find recommended risk level values for inhalation or dermal exposure.

In addition, all metal hazard quotient (HQ) and hazard index (HI) values for adults calculated are less than 1 (Table 7, Table 8 and Table 9). Therefore, exposures are unlikely to result in non-cancerous adverse health effects during the lifetime of exposure. It is important to note that an HQ < 1 does not necessarily mean that adverse effects will not occur. However, the effects of exposure to any hazardous substance depend on the dose, duration, how you are exposed, personal traits and habits, and whether other chemicals are present [21,22].

For ILCR, which is calculated only for As, Cd, Co, Cr, and Pb, the results differ slightly between collection sites P1, P2, P3, P4, and P5 (Table 10). However, the ILCR_pathway values exceed 1 × 10⁻⁶ for As and Cd. Therefore, the probability of an adult developing cancer from the consumption of metal in dust is greater than the USEPA threshold risk limit (>10⁻⁶) for As and Cd. In addition, the total excess lifetime cancer risk for an individual calculated from the average contribution of the individual heavy metals for all the pathways using Equation (8) is below 10⁻⁴.

Various elements are essential nutrients for humans. However, when they are ingested or inhaled in large amounts or over a long term, they can cause health damage [60,61]. In fact, Cr, Cd, Pb, Ni, Pb, and As can induce human poisoning and cause cancer [62]. On the other hand, the carcinogenic effect of Al has not been proven to date. According to the Agency for Toxic Substances and Disease Registry, workers who inhale large amounts of Al dust may experience lung problems or have reduced performance on some tests that measure nervous system function [63]. In addition, high Fe concentration was positively associated with liver cancer and inversely associated with brain cancer [64].

Street dust may have become one of the main air quality problems in the atmospheric environment [65,66]. Dust can be deposited on the walls of school buildings and school playgrounds and may contain metallic pollutants, which can induce soil contamination. Dry and wet depositions near roads, together with transport by runoff water, are the main causes by which pollutants are transported to soils. Different studies have proved that air pollution through soil and dust does not only affect human beings who are regularly exposed to it, but also affects climate, agriculture, and natural environments [32,67].

The calculations used for dermal and inhalation absorption are estimates derived from user input parameters and default parameters based on industry data collected by the USEPA [68]. Therefore, direct methods of chemical substances can be analyzed in the blood or feces of people who work at toll booths and are exposed to heavy metals. Thus, research involving various types of workers must be carried out to obtain real information on the degree of exposure to heavy metals from dust and their oral, dermal, and particularly inhalation ingestion. In fact, topsoil and road dust can be used to monitor heavy metal contaminations [69,70], as they are not only non-biodegradable but can also be stored for long periods of time [71].

5. Conclusions

This study revealed that there are significant concentrations of Al, As, Cd, Co, Cr, Cu, Fe, Ni, Zn, and Pb in soil collected along the BR-163 highway in Brazil. The concentration of metal(loid)s in some sites exceeds the values of metals set by Brazilian legislation for soil and obtained in countries such as China and Brazilian states. Furthermore, the PCA indicated that Al, As, Cd, Co, Cr, Cu, Fe, Ni, Zn, and Pb showed greater accumulation at site P5 near the toll booth, which may be due to braking, smoke, and vehicle stops. On the other hand, the lowest concentrations of these metals are at site P1, located near a roundabout. Therefore, we observed that the P5 sampling site close to the toll booths had higher proportions of metal(loid)s in the soil samples, indicating that tollbooth operators are more exposed to a higher concentration of potentially toxic contaminants from the soil.

The concentrations of the metal(loid)s were high in the unwashed leaf samples compared to the washed. It could be concluded that vehicular emission, industrial emission, and burning of all forms of waste along the highway could have significantly contributed to the elevated metal(loid)s loads in atmospheric deposits, and consequently in the leaves of the roadside B. decumbens plants grown in the area.

The Incremental Lifetime Cancer Risk (exposure routes: inhalation, ingestion, or dermal) values exceed the limit of 1 × 10⁻⁶ for As, Cd, and Cr established by the USEPA. Therefore, an adult has a non-negligible risk of developing cancer from exposure to the metal(loid)s in highway dust.