Estimation of Pollution Levels and Assessment of Human Health Risks from Potentially Toxic Metals in Road Dust in Mymensingh City of Bangladesh

: The assessment of toxic metals pollution in road dust in Mymensingh city, Bangladesh and its impact on the health risk of human exposure to toxic metals, is inadequate. A comprehensive investigation was conducted in different land use areas, i


Introduction
Road dust, also known as street dust or roadside sediments, is an extremely fine, dry powder composed of microscopic particles of the Earth's crust and other toxic substances. It can settle on the hardened surfaces of the roads and be easily redistributed in the atmosphere. Dust from urban roads can be washed away by runoff that goes through sewage and into water bodies [1,2]. In addition, environmental research has indicated that road dust is a substantial contributor to urban pollution. Road dust may contain a wide variety of pollutants that originate from a diverse range of both natural and human-induced sources [3]. Since road dust may be redistributed into the atmosphere by wind, it is widely acknowledged as a valid indicator of poor air quality in cities [4].
The chemical constituents of road dust include toxic metals, also known as heavy metals, metallic ions, and organic pollutants, such as polycyclic aromatic hydrocarbons (PAHs), organophosphate esters, microplastics, etc. The chemical constituents of road dust change according to the features of the emission sources, the meteorology, and the geology of the sampling locations. Among them, toxic metals have sought a lot of attention in recent years due to the fact that they can persist in the environment for long periods of time, cause severe toxicity, and have the potential to bioaccumulate [5]. Road dust containing toxic metals is often described as "chemical time bombs". Road dust containing toxic metals has polluted several megacities around the world, i.e., Delhi (India), Beijing (China), Dhaka (Bangladesh), Tokyo (Japan), Sao Paulo (Brazil), Shanghai (China), and Kolkata (India) [6][7][8][9][10][11][12].
Toxic metals tend to accumulate in metropolitan areas due to a number of factors, one of which is the impact of land use type. Land use has a complex effect on the level of toxic metal pollution, primarily due to the contribution of land-use-dependent sources. The interplay between land use and metal concentrations can help pinpoint the metals' original point of entry into the environment. Numerous studies have been conducted to explore the impact of various land use types in cities on the level of urban road dust pollution [3,[13][14][15][16][17][18].
At the start of the twenty-first century, concerns regarding the pollution and health risks posed by dust metals have gained more attention [19]. Road dust contains harmful toxic metals that can cause a significant health risk to people with weak immune systems, such as children and ill patients. Recent investigations have demonstrated the presence of carcinogenic toxic metals, such as Cd, Pb, As, and Cr, in road dust [14,20,21]. Road dust containing toxic metals can enter the human body via multiple routes, including inhalation, ingestion, and dermal contact. A toxic metal can have both short-and long-term negative consequences. For instance, short-term contact with metals can cause skin irritations, breathing difficulties, and vomiting, and long-term exposure can lead to skin, lung, and kidney cancer [22,23]. Dust contaminants can cause eye ailment, tissue necrosis and disrupt gene expression in humans [24,25].
Bangladesh is a South Asian developing country with eight divisional cities. Urban road dust in Bangladesh is contaminated with toxic metals [8,26]. In several of Bangladesh's cities, studies have been carried out to investigate the levels of toxic metals present in the dust on the roads, as well as the health risks associated with it, its distribution, and the sources from which it comes [27][28][29]. Mymensingh is a newly founded divisional city, and the city has been redesigned. The number of hard surfaces, such as roads and highways, has increased, while the number of open spaces has decreased dramatically. Mymensingh City, similar to many other emerging cities throughout the world, is facing a major challenge when trying to implement pollution management methods that would allow for greater urban sustainability. The city has begun to experience air pollution as a result of the growth in its population, industrial sectors, vehicles, and other manmade activities. Due to overpopulation, rapid urbanization, and industry over the past few decades, Mymensingh is one of the most sensitive urban areas in Bangladesh for environmental pollution. Since there is no study on road dust pollution in Mymensingh, the city authorities are unable to predict the situation. In light of this, the authors consider the present study to be a timely investigation of the current state of road dust pollution and the following aims were pursued while conducting the research work; (i) to determine the concentrations of nine toxic metals (Mn, Cr, Co, Ni, Cu, Zn, As, Pb, and Cd) in the road dust in Mymensingh City, (ii) to investigate the spatial distribution of toxic metal characteristics in the studied area, (iii) to evaluate the pollution level and enrichment extent of the selected toxic metals using different pollution indices methods (I geo , CF, PLI, Er, EF, and C deg ), and (iv) to assess the human health risk posed by studied toxic metals in road dust of Mymensingh city.

Study Area
Mymensingh is the eighth administrative division in Bangladesh, established in 2015. Mymensingh is one of the oldest cities in Bangladesh, and it is growing fast. It is adjacent to the country's capital, Dhaka, and its industrial and agricultural sectors are expanding rapidly. Mymensingh metropolis is the fourth largest city in Bangladesh and a key financial and educational hub in the country's north-central region. The city is situated on the Brahmaputra River's bank ( Figure 1). The area depicted on the map is located between 24 • 45 14" north and 90 • 24 11" east. It rises above sea level by more than 19 m. The city of Mymensingh covers an area of 90.173 square kilometers and has a population density of 9017 people per square kilometer [30]. The demographic composition is quite uniform. The city of Mymensingh experiences an average maximum temperature of 34.6 • C, a minimum temperature of 11.9 • C, and an average rainfall of 1610 mm [31]. Mymensingh city is significant for economic development due to its well-established connections with the capital city and its excellent road and rail connections. Residential land use is the most prevalent in the city, followed by educational land use. Commercial and urban services are centered in the municipality's central hub. Within the city limits, rickshaws and "Autos" are the primary modes of transportation, and the expansion of automobiles is extremely advanced.

Study Area
Mymensingh is the eighth administrative division in Bangladesh, established in 2015. Mymensingh is one of the oldest cities in Bangladesh, and it is growing fast. It is adjacent to the country's capital, Dhaka, and its industrial and agricultural sectors are expanding rapidly. Mymensingh metropolis is the fourth largest city in Bangladesh and a key financial and educational hub in the country's north-central region. The city is situated on the Brahmaputra River's bank ( Figure 1). The area depicted on the map is located between 24°45′14′′ north and 90°24′11′′ east. It rises above sea level by more than 19 m. The city of Mymensingh covers an area of 90.173 square kilometers and has a population density of 9017 people per square kilometer [30]. The demographic composition is quite uniform. The city of Mymensingh experiences an average maximum temperature of 34.6 °C, a minimum temperature of 11.9 °C, and an average rainfall of 1610 mm [31]. Mymensingh city is significant for economic development due to its well-established connections with the capital city and its excellent road and rail connections. Residential land use is the most prevalent in the city, followed by educational land use. Commercial and urban services are centered in the municipality's central hub. Within the city limits, rickshaws and "Autos" are the primary modes of transportation, and the expansion of automobiles is extremely advanced.

Sample Collection
Road dust was sampled from sixteen spots in Mymensingh City ( Figure 1) in the winter (February) season of 2020, covering the commercial areas (CA = 4), the residential areas (RA = 4), the medically facilitated areas (MFA = 4), and the park areas (PA = 4). Dust samples were collected at designated places along major roadways. Dust was collected from the paved shoulders of the road by sweeping with a brush and pan. Sampling points were carefully chosen, and it was confirmed that they had been dried for at least a week. Approximately 300 g of samples were collected from each point, a composite of at least six subsamples collected one meter apart. All samples were bagged in properly labeled polythene zip bags and brought to the laboratory of the Department of Agronomy at Bangladesh Agricultural University, Mymensingh. The samples were spread on plastic sheets and dried under shade for several days until a consistent mass was obtained. The dust was then passed through a 2.0 mm nylon mesh sieve to remove debris and larger

Sample Collection
Road dust was sampled from sixteen spots in Mymensingh City ( Figure 1) in the winter (February) season of 2020, covering the commercial areas (CA = 4), the residential areas (RA = 4), the medically facilitated areas (MFA = 4), and the park areas (PA = 4). Dust samples were collected at designated places along major roadways. Dust was collected from the paved shoulders of the road by sweeping with a brush and pan. Sampling points were carefully chosen, and it was confirmed that they had been dried for at least a week. Approximately 300 g of samples were collected from each point, a composite of at least six subsamples collected one meter apart. All samples were bagged in properly labeled polythene zip bags and brought to the laboratory of the Department of Agronomy at Bangladesh Agricultural University, Mymensingh. The samples were spread on plastic sheets and dried under shade for several days until a consistent mass was obtained. The dust was then passed through a 2.0 mm nylon mesh sieve to remove debris and larger blocks. The dried and sieved dust samples were then repacked in labeled zip bag and transported to the Pollution Control Laboratory of Saitama University, Japan, where they passed through a vibrating sieve shaker (AS 200-digit Retsch AS200) set to an amplitude of 60 for 10 min to achieve working samples of <32 µm particle size. Finally, toxic metals (Cr, Mn, Co, Ni, Cu, Zn, As, Cd, and Pb) concentrations were determined for each sample.

Chemical Analysis
Fifty milligrams of the collected sample were placed in a 100-mL conical flask and mixed with 20 mL of aqua regia (5 mL of 63% of HNO 3 and 15 mL of 36% of HCl) [29] and stirred. The suspension was positioned on a hot plate for 1 h 30 min at 150 • C until the digest was reduced to a volume of approximately 1 mL or nearly dry. The sample was allowed to cool to room temperature. After diluting the samples with 20 mL of 2% nitric acid, they were filtered using a Whatman filter paper of 5C 110-mm diameter (pore size: 11 µm) and refrigerated until analysis. The concentrations of toxic metals were determined using an inductively coupled plasma mass spectrometer (ICP-MS, Agilent Technologies, 7700 series, Santa Clara, CA, USA) at the Center for Environmental Science in Saitama (CESS), Japan. Solutions containing multiple elements of XSTC-662 (Spex Certi Prep, Metuchen, NJ, USA) were utilized to generate calibration curves. For the concentration calculation, the calibration curves with an R 2 > 0.999 were approved.

Quality Assurance and Quality Control
The standard quality assurance and control (QA/QC) procedures were followed during the estimation of toxic metals in the samples. Every piece of glassware that was utilized for the analyses was first cleaned with acid and then given a thorough rinse with deionized water. Reagent blanks and certified reference material (CRM) (Gobi kosa Dust, NIES CRM No. 30) were digested in triplicate and handled similarly and in parallel with the samples. The CRM was analyzed to determine the recovery percentage of the method used. All digested samples were examined in triplicate. The standard deviation of the replicated values did not exceed 5%. We also achieved satisfactory percent recoveries (85% for Cr, 93% for Mn, 107% for Cu, 90% for Cd, 100% for Zn, 86% for Pb, 95% for Co, and 85% for Ni) from CRM. After every ten determinations, the standards were recalibrated. Detection limits (LOD) were set at 0.018 ng/mL for Cr, 0.041 ng/mL for Mn, 0.041 ng/mL for Ni, 0.048 ng/mL for Cu, 0.048 ng/mL for Zn, 0.048 ng/mL for As, 0.048 ng/mL for Pb, 0.018 ng/mL for Cd, and 0.09 ng/mL for Co, respectively. Geoaccumulation index (I geo ) is performed to compare the concentration of a metal in samples to the reference values (UCC or local soil) [32]. Muller introduced I geo for the first time [33]. It is computed using the following formula.
where C n represents the determined concentration of the toxic metal in road dust and B n denotes its background concentration. It was decided to apply a multiplier of 1.5 because of the decrease in background variations that could be attributable to lithogenic differences in samples. Ali et al. [34] categorized the I geo values into seven classes, as described in our previous work [29].

Contamination Factor (CF)
The contamination factor is calculated by dividing the concentration of a toxic metal in road dust by the concentration of that toxic metals in the background. The formula for calculating CF is as follows: where C n is the concentration of toxic metals contained in road dust and B n is the background value of toxic metals. The CF category is stated elsewhere [14,26,29]. The contamination factors are calculated by adding up contamination degrees (C deg ). The degree of contamination of Mymensingh's road dust with nine hazardous metals was evaluated as follows [35]: where n is the number of toxic metals that have been evaluated and CF stands for contamination factor. Bisht et al. [15] have defined four different levels of degrees of contamination.

Pollution Load Index (PLI)
The pollution load index (PLI) provides a prediction of the total pollution load caused by the accumulating toxic metals at a given location [36].
where CF is the element-specific contamination factor. The scale of PLI is described as follows: if the PLI is less than 1, it indicates that there is no pollution; if it is equal to 1, it indicates that there are baseline levels of pollutants; and if it is greater than 1, it indicates that the quality of the site is getting worse [37].

Ecological Risk (Er) and Risk Index (RI)
To evaluate the potential ecological risk of a metal, the following formula was used: where T r represents the level of toxicity and CF represents the contamination factor. The T r represents values for Cr(2), Mn, Zn(1), Co, Ni, Cu, Pb(5), As(10), and Cd(30), respectively [21,29]. According to Kamani et al. [38], the grades are categorized as follows: Er less than 40 represents 'low potential ecological risk'; 40-80 defines 'medium potential ecological risk'; 80-160 defines 'considerable potential risk'; 160-320 defines 'high ecological potential risk'; and Er over 320 defines 'very high potential risk'.
The risk index (RI) was a frequently used tool for assessing the possible biotoxin effect of hazardous metals in road dust [35,39]. The following formula was used to determine the risk index (RI): where n represents the total number of metals being considered and i stands for the individual metal. Hakanson [35] categorizes the RI values into five distinct ranks, i.e., RI < 150 denotes 'low ecological risk'; 150 < RI < 300 denotes 'moderate ecological risk'; 300 < RI < 600 denotes 'considerable ecological risk'; RI > 600 denotes 'very high ecological risk'.

Enrichment Factor (EF)
The enrichment factor is frequently used to evaluate the potential influence of human activities on the element's concentrations in road dust [40]. In this investigation, it was utilized to quantify the abundance of harmful metals in the samples and to establish whether their source was natural or manmade. EF can be estimated using the following equation: where the concentration ratios of the ith metal in the road dust samples and the background are denoted by C i /C ref and B i /B ref , respectively. The composition of the average UCC was employed as the B ref value in this investigation. The selection of the background value for the EF calculation is crucial for providing an accurate estimation of dust elemental pollution. Fe was used as the reference element for this calculation. Sutherland [41] listed the following five classes of EFs: EF values less than 2 denote a deficiency of minimal enrichment, EF values between 2 and 5 indicate moderate enrichment, EF values between 5 and 20 indicate significant enrichment, EF values between 20 and 40 indicate very high enrichment, and EF values over 40 indicate extremely high enrichment. An EF value close to 1 means that the metal is of natural origin, whereas a value of > 10 implies that the metal content was enhanced by human activity.

Health Risk Assessment
The noncarcinogenic and carcinogenic risks from toxic elements in media, including soil and road dust, have been evaluated using human health risk assessment models (HHRA) developed by the USEPA [21,42]. Since there were no regional regulations for the city of Mymensingh, the USEPA's model was employed to determine the extent of exposure and potential health consequences from road dust contaminated with toxic metals. The values of the hazard quotient (HQ), the hazard index (HI), and the cancer risk (CR) are much more critical than the concentrations of toxic metals when figuring out the human health risk [43]. Health concerns for adults and children were examined separately to confirm the veracity of the information on behavior and physiology [21,44,45]. This measurement's goal is to determine the extent, frequency, and duration of human exposure to environmental pollutants. In the context of this investigation, an exposure assessment was conducted by using the following formula to figure out the average daily intake dose (ADD) of toxic elements from ingestion, inhalation, and dermal contact.
ADD of toxic metals via ingestion ADD for toxic metals via inhalation ADD for toxic metals via dermal contact The noncarcinogenic risk evaluation has been conducted using hazard quotients (HQ) and hazard indices (HI). The ratio of the average daily intake dose to the accepted reference dosage (RfD) is known as HQ. Table 1 lists RfD values for all studied toxic metals. The HI is determined by summing the HQ values and is used to quantify the health risk caused by multiple exposure routes. HI ≤ 1 indicates that there are no harmful health impacts, but HI > 1 means there is a possibility of adverse health effects [46]. The following equations are used to determine HQ and HI: The combined risk of toxic metals was expressed as the cHI for the metals, such as:

Carcinogenic Risk Estimation
The lifetime exposure to a contaminant through the average daily dose (LADD) of ingestion and inhalation was taken into consideration for the calculation of cancer risk (CR). This study assessed the carcinogenic risk associated with Cr, Ni, Cd, As (as known carcinogens), and Pb (as a possible carcinogen). The cancer risk (CR) was calculated by multiplying the exposure dosages at each exposure pathway by the relevant carcinogenic slope factor (SF) ((kg-day)/mg). The slope factors (for ingestion and inhalation) were (0.5, 0.41) for Cr; (0.91, 0.84) for Ni; (1.5, 15.5) for As; (1.5, 6.3) for Cd; and (0.0085, 0.042) for Pb, respectively [26]. The cumulative cancer risk index (CRI) was calculated by adding the carcinogenic risks associated with ingestion and inhalation. The CRI range of 1 × 10 −6 to 1 × 10 −4 is acceptable; less than 1 × 10 −6 has no notable effects, while beyond 1 × 10 −4 is deemed unsuitable and may be harmful to human health [47]. All equations pertaining to the evaluation of carcinogenic risk are listed below:

Statistical Analysis
The open source statistical package 'R' [51] was used to carry out data analyses and visualization. The assumption of normality of the data was tested by the Shapiro-Wilk method. Since the dataset violated the assumption of a normal distribution, nonparametric Kruskal-Wallis tests were performed for mean comparison, followed by Dunn's post hoc test. The correlation coefficient among the toxic metal concentrations was estimated using Spearman's rank-order correlation. The variables were standardized, and the 'princomp' function of 'R' was deployed for principal component analysis (PCA).

Spatial Distribution of Toxic Metals in Road Dust
The results of toxic metal concentrations (Cr, Mn, Co, Ni, Cu, Zn, As, Cd, and Pb) in road dust samples of Mymensingh City are presented in Figure 2. The ranges of mean concentration (mg/kg) of toxic metals determined from different land use areas were: Cr According to the findings, all of the toxic metals (except As and Cd) had greater average concentrations in the CA, whereas As and Cd were found to have higher concentrations in the RA and MFA, respectively.
An upper continental crust (UCC) value, also known as a background value, was taken from [32] and compared to the concentrations of toxic metals found in this investigation due to a lack of local background concentrations of toxic metals. The average concentrations of toxic metals were greater in all land use areas, for example, Cr (1.16-2.44 folds), Ni (1.22-1.83 folds), Cu (2.06-5.04 folds), Zn (4.54-8.98 folds), As (2.46-3.14 folds), Cd (3.22-10.54 folds), and Pb (1.62-4.74 folds), respectively. This result was comparable to the level of harmful metals found in the road dust of Dhaka, a nearby city to Mymensingh [29]. The high concentration of toxic metals in road dust may be the result of human-caused factors, such as traffic, building demolition and construction, sewage systems, and the use of chemicals and paints [52,53]. The average concentrations of Mn and Co were either the same as or lower than their respective background values (527 mg/kg and 11.6 mg/kg, respectively). This suggests that they most likely came from natural sources through the weathering process. However, relatively high concentrations of Mn in commercial areas are unusual ( Figure 2). It might be added to municipal solid waste, which is considered a source of Mn [54].
The following is a list of the sequences of the average metal concentrations (mg kg −1 ) found in different land use areas during the sampling events: CA-Mn (589. Based on these sequences of toxic metal concentrations in chosen land use zones, it may be deduced that Mn, Zn, Pb, Cr, and Cu were prevalent, whereas Co, As, and Cd concentrations were lagging. Among the toxic metals examined, Cd was the least prevalent, which is consistent with other research [55]. The increased levels of Cu, Zn, Cr, and Pb in urban road dust are mostly attributable to the deposition of particles derived from vehicles, including exhaust particles, brake lining, and lubricating oil residue [34]. Zn is quite high in all locations when compared to other metals, and this can be attributed to tire wear resulting from heavy traffic [56]. These elevated levels could be caused by tire rubbing on a deteriorating pavement surface [57]. The result also found that Pb concentrations varied in different functional areas in Mymensingh City in the order of CA > RA > MFA > PA, which was higher than the UCC value. Despite the fact that Pb content in gasoline was banned in Bangladesh approximately two decades ago [58], the long half-life of lead in soils, fossil fuel emissions, and historical Pb pollution could all be contributing factors to the Pb concentration in road dust [59]. According to the findings of this investigation, the highest possible concentration of Cd was ten times greater than the value found in the background. The city of Mymensingh is home to a large number of battery-powered vehicles called "Auto", many of which use Ni-Cd batteries that can be recharged and might be sources of Cd in road dust. The highest concentrations of Ni were found in CA, as auto-mobile workshops in this vicinity caused great Ni emissions [60]. An upper continental crust (UCC) value, also known as a background value, was taken from [32] and compared to the concentrations of toxic metals found in this investigation due to a lack of local background concentrations of toxic metals. The average concentrations of toxic metals were greater in all land use areas, for example, Cr (1.16-2.44 folds), Ni (1.22-1.83 folds), Cu (2.06-5.04 folds), Zn (4.54-8.98 folds), As (2.46-3.14 folds), Cd (3.22-10.54 folds), and Pb (1.62-4.74 folds), respectively. This result was comparable to the level of harmful metals found in the road dust of Dhaka, a nearby city to Mymensingh [29]. The high concentration of toxic metals in road dust may be the result of human-caused factors, such as traffic, building demolition and construction, sewage systems, and the use of chemicals and paints [52,53]. The average concentrations of Mn and Co were either the same as or lower than their respective background values (527 mg/kg

Contamination Factor (CF) and Pollution Load Index (PLI)
In Figure 5, the CF values for Cr, Mn, Co, Ni, Cu, Zn, As, Cd, and Pb are depicted. As seen from Figure 5, all the land use areas were contaminated by all the studied toxic metals. The results indicated that CA was highly contaminated by Zn and Cd. In addition, cadmium (Cd) contamination levels in MFA were very high. In terms of relevant toxic metals, both Mn and Co demonstrated low levels of contamination in MFA and PA, while Co alone revealed low levels of contamination in CA and RA. Furthermore, the result showed that considerable contamination levels were indicated by Cu, Pb (CA), Cu, Zn (MFA), Cu, Zn, Cd, and Pb (RA), and Zn, Cd (PA). The pollution status of the area under examination can also be indicated by analyzing the concentration of all studied toxic metals in the road dusts. The PLI values for all studied sites were higher than one, which indicated deterioration of site quality and that immediate remediation is needed to prevent further pollution. The highest PLI values were for the area of CA, followed by RA and MFA, whereas the lowest PLI was for PA. Additionally, pollution emissions are also decreased by the efficient handling of municipal and medical solid waste.

Contamination Factor (CF) and Pollution Load Index (PLI)
In Figure 5, the CF values for Cr, Mn, Co, Ni, Cu, Zn, As, Cd, and Pb are depicted. As seen from Figure 5, all the land use areas were contaminated by all the studied toxic metals. The results indicated that CA was highly contaminated by Zn and Cd. In addition, cadmium (Cd) contamination levels in MFA were very high. In terms of relevant toxic metals, both Mn and Co demonstrated low levels of contamination in MFA and PA, while Co alone revealed low levels of contamination in CA and RA. Furthermore, the result showed that considerable contamination levels were indicated by Cu, Pb (CA), Cu, Zn (MFA), Cu, Zn, Cd, and Pb (RA), and Zn, Cd (PA). The pollution status of the area under examination can also be indicated by analyzing the concentration of all studied toxic metals in the road dusts. The PLI values for all studied sites were higher than one, which indicated deterioration of site quality and that immediate remediation is needed to prevent further pollution. The highest PLI values were for the area of CA, followed by RA and MFA, whereas the lowest PLI was for PA. Additionally, pollution emissions are also decreased by the efficient handling of municipal and medical solid waste. Processes 2022, 10, x FOR PEER REVIEW 13 of 22

Geoaccumulation Index (Igeo)
Based on the geoaccumulation (Igeo) results, Cr (except CA), Mn, Co, and Ni in all sampling areas were uncontaminated classes ( Figure 6). According to Igeo results, the road dust of Mymensingh City in different land use areas was contaminated (Igeo > 0) with toxic metals, such as Cu, Zn, As, Cd, and Pb. CA showed 'moderately to heavily contaminated' ranked with Zn and Cd, whereas, MFA showed 'moderately to heavily contaminated' ordered only with Cd. The Igeo classification for CA was "moderately contaminated" with Cu and Pb and in MFA with Cu and Zn. The same Igeo categorization was achieved for Cu, Zn, As, Cd, and Pb in RA, however, in PA, only Zn was classified as 'moderately contaminated'.

Ecological Risk (Er) and Ecological Risk Index (RI)
The ecological risks of Mymensingh's road dust, based on Hakanson's approach, are presented in Figure 7 for a better understanding of the pollution and associated risks of Mymensingh's road dust. The order of the mean Er values was Cd > As > Cu > Pb > Zn > Ni > Co > Cr > Mn for MFA, RA, and PA, respectively. While the order of mean Er value

Geoaccumulation Index (I geo )
Based on the geoaccumulation (I geo ) results, Cr (except CA), Mn, Co, and Ni in all sampling areas were uncontaminated classes ( Figure 6). According to I geo results, the road dust of Mymensingh City in different land use areas was contaminated (I geo > 0) with toxic metals, such as Cu, Zn, As, Cd, and Pb. CA showed 'moderately to heavily contaminated' ranked with Zn and Cd, whereas, MFA showed 'moderately to heavily contaminated' ordered only with Cd. The I geo classification for CA was "moderately contaminated" with Cu and Pb and in MFA with Cu and Zn. The same I geo categorization was achieved for Cu, Zn, As, Cd, and Pb in RA, however, in PA, only Zn was classified as 'moderately contaminated'.

Geoaccumulation Index (Igeo)
Based on the geoaccumulation (Igeo) results, Cr (except CA), Mn, Co, and Ni in all sampling areas were uncontaminated classes ( Figure 6). According to Igeo results, the road dust of Mymensingh City in different land use areas was contaminated (Igeo > 0) with toxic metals, such as Cu, Zn, As, Cd, and Pb. CA showed 'moderately to heavily contaminated' ranked with Zn and Cd, whereas, MFA showed 'moderately to heavily contaminated' ordered only with Cd. The Igeo classification for CA was "moderately contaminated" with Cu and Pb and in MFA with Cu and Zn. The same Igeo categorization was achieved for Cu, Zn, As, Cd, and Pb in RA, however, in PA, only Zn was classified as 'moderately contaminated'.

Ecological Risk (Er) and Ecological Risk Index (RI)
The ecological risks of Mymensingh's road dust, based on Hakanson's approach, are presented in Figure 7 for a better understanding of the pollution and associated risks of Mymensingh's road dust. The order of the mean Er values was Cd > As > Cu > Pb > Zn > Ni > Co > Cr > Mn for MFA, RA, and PA, respectively. While the order of mean Er value

Ecological Risk (Er) and Ecological Risk Index (RI)
The ecological risks of Mymensingh's road dust, based on Hakanson's approach, are presented in Figure 7 for a better understanding of the pollution and associated risks of Mymensingh's road dust. The order of the mean Er values was Cd > As > Cu > Pb > Zn > Ni > Co > Cr > Mn for MFA, RA, and PA, respectively. While the order of mean Er value in CA was Cd > As > Pb > Cu> Zn> Ni > Cr > Co > Mn. According to Figure 7, Cd has the highest ecological risk value, which is the 'high ecological potential risk' class for MFA (Er = 271) and CA (Er = 207), the 'considerable potential risk' class for RA (Er = 127), and the 'medium potential ecological risk' class for PA (Er = 64), respectively. Thus, Cd poses an ecological risk to the surrounding ecology, necessitating precautionary measures in vulnerable locations. The other studied toxic metals have shown a 'low potential ecological risk' level (Er < 40) in different land use areas of Mymensingh's road dust. The result suggests that these potentially toxic metals may not be having any influence on the environment. The Er values of Cr, Mn, Co, Ni, Cu, Zn, As, Cd, and Pb were in the ranges of 1.9-6.68, 0.53-1. 33 an ecological risk to the surrounding ecology, necessitating precautionary measures in vulnerable locations. The other studied toxic metals have shown a 'low potential ecological risk' level (Er < 40) in different land use areas of Mymensingh's road dust. The result suggests that these potentially toxic metals may not be having any influence on the environment. The Er values of Cr, Mn, Co, Ni, Cu, Zn, As, Cd, and Pb were in the ranges of 1.9-6.68, 0.53-1. 33 (Table S1). The results also show that 53.3% of RI values are in the range of 150-300, indicating that the road dust samples in Mymensingh present a "moderate ecological risk". In contrast, 33.3% of RI values are in the range of 300-600, indicating a "considerable ecological risk" of Mymensingh road dust. Cd have the highest contributions (64%, 77%, 55%, and 58% in CA, MFA, RA, and PA, respectively) to the RI. The second most contributing element was As (8.7%, 6.6%, 13.5%, and 17.3% in CA, MFA, RA, and PA, respectively) to RI of Mymensingh road dust samples.  (Table S1). The results also show that 53.3% of RI values are in the range of 150-300, indicating that the road dust samples in Mymensingh present a "moderate ecological risk". In contrast, 33.3% of RI values are in the range of 300-600, indicating a "considerable ecological risk" of Mymensingh road dust. Cd have the highest contributions (64%, 77%, 55%, and 58% in CA, MFA, RA, and PA, respectively) to the RI. The second most contributing element was As (8.7%, 6.6%, 13.5%, and 17.3% in CA, MFA, RA, and PA, respectively) to RI of Mymensingh road dust samples.

Degree of Contamination (C deg )
The degree of contamination reflects the cumulative impact of all toxic metals at a given place. The degree of contamination by nine toxic metals in the road dust from the CA, MFA, RA, and PA is represented in Figure 8. The range of C deg was as follows: 28.5-41.4 in CA, 20.6-44.6 in MFA, 21.6-35.8 in RA, and 12.26-21.7 in PA, respectively. These findings indicate that CA, MFA, and RA all have a very high degree of contamination (C deg ≥ 24). In contrast, the degree of contamination in PA was at a considerable level (12 ≤ C deg < 24). Eleven of the sixteen sampled locations exhibited extremely high levels of pollution.

Degree of Contamination (Cdeg)
The degree of contamination reflects the cumulative impact of all toxic metals at a given place. The degree of contamination by nine toxic metals in the road dust from the CA, MFA, RA, and PA is represented in Figure 8. The range of Cdeg was as follows: 28.5-41.4 in CA, 20.6-44.6 in MFA, 21.6-35.8 in RA, and 12.26-21.7 in PA, respectively. These findings indicate that CA, MFA, and RA all have a very high degree of contamination (Cdeg ≥ 24). In contrast, the degree of contamination in PA was at a considerable level (12 ≤ Cdeg < 24). Eleven of the sixteen sampled locations exhibited extremely high levels of pollution.

Enrichment Factor (EF)
The enrichment factor of different toxic metals was calculated with respect to the background values of the corresponding metals, and the results are presented in Table 2. Significant anthropogenic influence was suggested by the high EF values for Zn in CA and RA, and for Cd in MFA. Minimal enrichment was reported for Cr, Mn, Co, and Ni in all studied land use areas in Mymensingh City (Table 2). While, in the CA, there was a moderate enrichment of As, Cu, and Pb, whereas in the MFA, there was a moderate enrichment of As and Pb. In RA, PA, Cu, As, Cd, and Pb reported moderate enrichment classes. In Mymensingh City, variations in EF values were seen for the toxic elements Cr  49-9.1). The highest EF values for Zn were obtained at sampling points in CA and PA, respectively. The highest EF value for Cd was achieved at sampling points in MFA, whereas the highest EF value for Pb was found at sampling points in RA. The average EF value of Cd in MFA above 10 was regarded as the result of human activities [61,62]. For each toxic metal, there was a large difference between the lowest and highest EF values. Large differences may be attributable to a substantial change in the material component, natural processes, or anthropogenic involvement at multiple layers. If the EF value is below 1.5 or 2, the metal is exclusively derived from crustal sources or natural processes. However, EF values > 1.5 or 2 imply that anthropogenic causes exert a growing influence [63]. Therefore, it appears that EFs can be used as an effective tool for distinguishing between natural and anthropogenic sources in this research. Cr, Mn, Co, and Ni all had EF values below 2, indicating that they were

Enrichment Factor (EF)
The enrichment factor of different toxic metals was calculated with respect to the background values of the corresponding metals, and the results are presented in Table 2. Significant anthropogenic influence was suggested by the high EF values for Zn in CA and RA, and for Cd in MFA. Minimal enrichment was reported for Cr, Mn, Co, and Ni in all studied land use areas in Mymensingh City (Table 2). While, in the CA, there was a moderate enrichment of As, Cu, and Pb, whereas in the MFA, there was a moderate enrichment of As and Pb. In RA, PA, Cu, As, Cd, and Pb reported moderate enrichment classes. In Mymensingh City, variations in EF values were seen for the toxic elements Cr  49-9.1). The highest EF values for Zn were obtained at sampling points in CA and PA, respectively. The highest EF value for Cd was achieved at sampling points in MFA, whereas the highest EF value for Pb was found at sampling points in RA. The average EF value of Cd in MFA above 10 was regarded as the result of human activities [61,62]. For each toxic metal, there was a large difference between the lowest and highest EF values. Large differences may be attributable to a substantial change in the material component, natural processes, or anthropogenic involvement at multiple layers. If the EF value is below 1.5 or 2, the metal is exclusively derived from crustal sources or natural processes. However, EF values > 1.5 or 2 imply that anthropogenic causes exert a growing influence [63]. Therefore, it appears that EFs can be used as an effective tool for distinguishing between natural and anthropogenic sources in this research. Cr, Mn, Co, and Ni all had EF values below 2, indicating that they were predominantly influenced by crustal materials or natural sources, as opposed to the other toxic elements (Cu, Zn, As, Cd, and Pb) studied (EF > 2), which were primarily influenced by human activity [64]. Sampling locations with the highest total EF value were in the following order: medical facility areas, commercial areas, residential areas, and park areas.  Table 3 presents an assessment of the noncancerous human health risks from toxic metals based on daily metal intake via inhalation, ingestion, and skin contact from road dust collected from different land use areas in Mymensingh city. The noncarcinogenic indicator, hazard quotient (HQ), and hazard index (HI) values for children and adults were calculated independently. The results of our research show that ingestion was the prominent exposure pathway for Ni, Cu, Zn, As, and Pb. In contrast, dermal contact is the predominant route of exposure for Cr (except for PA for both children and adults), Mn, Co (except for PA for adults), and Cd (except for MFA for adults and PA for both groups). All land use areas had high Mn intakes (mg kg −1 day −1 ) for children and adults, whereas total intakes of Zn were lowest in CA, MFA, and RA. As far as PA is concerned, only Cd intakes are particularly low ( Table 3). The intake of dust-bound toxic metals among children was higher than the intake in adults for all land use categories. According to the findings of a great number of research, children have a significantly higher risk of being exposed to potentially harmful metals [20,37,[65][66][67]. Therefore, children are in greater danger than adults, because of their propensity to play in the dirt and their habit of putting their hands in their mouths frequently. Health problem of children due to respiratory diseases was 12.50% in Mymensingh district where 69.44% children were exposed to air pollution [68].
The HQ values for the three ways of being exposed showed that all toxic metals studied in all land use categories are less than one for both adults and children (except Mn in dermal contact).
The  Table 4). The HI values of the majority of toxic metals (Cr, Mn, Ni, Cu, Zn, Cd, and Pb) were lower in PA compared to other land use zones, whereas they were highest in CA. Therefore, CA was more dangerous for children than other regions.  The HI values were obtained for adults for the following elements: Cr (0.01-0.11), Mn (0.006-1.32), Co (0.0003-0.04), Ni (0.0007-0.001), Cu (0.0005-0.001), Zn (0.0006-0.001), As (0.014-0.018), Cd (0.0002-0.002), and Pb (0.006-0.01), respectively, in the road dust of Mymensingh City. The HI values were lowest in PA and highest in CA, similar to children. The HI value indicated that there are no adverse effects on humans (HI less than 1). Similar findings were found in the road dust of Madrid City in Spain [37], Jalalabad and Kabul, Afghanistan [69], and Gazipur City, Bangladesh [28]. However, only with the exception of Mn whose HI up to the threshold level indicates a possibility of noncancer risk occurring which was similar with the road dust of Dhaka City [26]. Manganese affects the lungs and brain. Hallucinations, forgetfulness, and nerve damage define manganese poisoning. Manganese can cause Parkinson's, and pneumonia [26]. In addition, the combined hazard index cHI of toxic metals in children and adults varies according to the type of land use: CA > RA > MFA > PA, and the obtained values exceeded the permissible limit (Table 4) [42]. The results also aligned with road dust collected in e-waste recycling areas in China [70].

Carcinogenic Risk (CR) Assessment
The CR of Cr, Ni, As, Cd, and Pb via ingestion and inhalation exposure pathways are presented in Table 5. In this study, the CR value via the ingestion pathway was higher than the inhalation exposure pathway. In ingestion exposure pathways, the calculated carcinogenic risks of Cr, Ni, and As for adults and children in all land use areas were within the acceptable range; whereas Cd and Pb values were less than 1 × 10 −6 . Therefore, a serious long-term health risks to the adults and children is not associated with Cr, Ni, As, Cd, and Pb from the road dust. On the other hand, considering the inhalation pathway, all carcinogenic toxic metals obtained values less than 1 × 10 −6 for all land use areas. Therefore, inhalation exposure pathway also would not create any cancer risk to the resident of the city. The findings were consistent not only with those of previous research conducted in Bangladesh [8,28,71], but also with worldwide cases [72]. The CA has the highest cumulative cancer risk (CRI) values compared to all other zones due to commercial and human activities. The cumulative cancer risk index (CRI) through ingestion and inhalation exposure ranged from 1.86 × 10 −7 to 3.4 × 10 −5 , suggesting that the cumulative carcinogenic risk of toxic metals is below tolerable levels and will pose no risks, in accordance with Rabin et al. [26].

Conclusions
The pollution levels, along with their spatial distributions, and potential risk of toxic metals in road dust in the Mymensingh city of Bangladesh were investigated. The concentrations of Cr, Ni, Cu, Zn, As, Cd, and Pb were higher than their corresponding UCC values. Spatial distributions across different land use areas revealed that CA was enriched by Cr, Mn, Co, Ni, Pb, and Zn, while Pb, As, and Cd was abundant in RA and MFA, respectively. The I geo findings indicated that Zn and Cd represented "moderately to heavily contaminated" in CA, while only Cd was in the "moderately to heavily contaminated" category in MFA. Similarly, the CF and PLI demonstrated that road dust in CA was strongly polluted by Zn and Cd. The ecological risk (Er) and risk index (RI) demonstrated low to high potential ecological risk across the study area, whereas, the CA and MFA were more vulnerable than other zones in Mymensingh City. The enrichment factor (EF) revealed that all of the land use areas were highly enriched by Zn and Cd. Furthermore, EF results suggested that As, Cd, Pb, Cu, and Zn were accumulated in road dust from anthropogenic sources. Health risk assessment indicates that children are more vulnerable to toxic metal contamination than adults. However, it was determined that all tested toxic metals (except Mn for children) were at safe levels when assessed by HQ and HI indicators. There was no carcinogenic risk caused by toxic metals in the road dust of Mymensingh city. Since this was an initial investigation of road dust pollution, therefore, more research should be conducted taking into account seasonal variations, source identification, and fine particle size (PM 2.5 and PM 10 ). Finally, the bioavailability of toxic metals should be researched to achieve more realistic and precise results for health risk assessment in Mymensingh City. The administration and environmental organizations should devote more attention to the comprehensive assessment of toxic metals in the road dust of Mymensingh city in order to safeguard public health and environmental integrity.