Impact of Atmospheric Polycyclic Aromatic Hydrocarbons (PAHs) of Falling Dust in Urban Area Settings: Status, Chemical Composition, Sources and Potential Human Health Risks

The present work is considered to investigate the sources, concentration, and composition of polycyclic aromatic hydrocarbons (PAHs) and associated health risk assessment of road dust in Riyadh City, Saudi Arabia. The study region included an urban area, strongly affected by traffic, a bare and an industrial area. A total of 50 locations were selected for sampling and 16 different PAHs were determined. The concentration of PAHs in road dust and their estimated lifetime average daily dose (LADD) for adults (human) ranged from 0.01 to 126 ng g−1 and 1950 to 16,010 mg kg−1 day−1, respectively. The ADDing was calculated separately for children (>6), teenagers (6–12), and adults (>12) for all PAHs with each collected sample. Moreover, the average daily exposure dose by ingestion (ADDing) and average daily exposure dose by dermal absorption (ADDder) were more in children (<6 years) as compared to teenagers (6–12 years) and adults (>12 years). Likewise, total equivalency factor based on BaP (TEQBaP) calculations pointed out that PAHs having more benzene rings or having high molecular weight showed high TEQBaP as compared to low molecular weight PAHs. The data revealed that the children population is at high risk for asthma, respiratory and cardiovascular diseases, and immunity suppression as compared to adults in the particular area of investigated region. These outcomes of this study can be used to deliver significant policy guidelines concerning habitants of the area for possible measures for controlling PAHs contamination in Riyadh City to protect human health and to ensure environmental sustainability.


Introduction
The PAHs are produced because of partial thermal decomposition and combustion of inorganic sources during industrial and power production activities, and desalination of water [1,2]. Natural sources are also present, such as volcanoes and forest fires [3,4]. Riyadh City, in Saudi Arabia, is a highly densely populated area, with several commercial and trade manufacturing regions having multiple anthropogenic sources of pollutants. During the past two decades, rapid growth because of urban and industrial expansion has occurred in Riyadh City. Nowadays, the urbanized, as well as industrialized regions, covers more than five thousand square kilometers. Population data predict an increase in the population of Riyadh up to 7.2 million by 2023. The main roads in Riyadh City consist of 3982 km, as well as 814 km of intercity ordinary roads [5,6]. The rapid expansion of any area results in dense traffic, diesel combustion, ultimately with outcomes of PAHs, heavy metals, and POPs release into the environment [7,8].
Previous studies indicated that the concentration of PAHs in air of Riyadh City was in the range of 1.8 to 13.5 µg/m 3 . The major PAH sources were characterized by petroleum product emissions caused by automobile combustion products and diesel emissions [9]. Local input is considered to be the main source with a minor influx from longer-range transport [10]. Therefore, it is anticipated that a variety of compounds are released during diesel

Dust Samples Preparation and Extraction by QuEChERS
The special extraction procedure was adopted by taking 2 g of dust sample and adding 1 mL of deionized water in a 50 mL centrifuge tube, vortexing shortly, and after vortexing, allowing it to homogenate for about 10 min total. Acetonitrile (6 mL) was added to each sample. Shaking (5 min) was carried out for the extraction of PAHs. The substances of citrate salts (ECQUEU750CT-MP) Mylar pouch were added to individual dust samples in the centrifuge tube. Instantaneously, samples were shaken for a minimum of 2 min and centrifuged for 5 min at ≥3500 rcf. Sample cleanup was carried out by transferring a 1.5 mL aliquot of supernatant to a 2 mL CUMPSC18CT (MgSO4, PSA, C18) dSPE tube. Samples were vortexed for 2 min and again centrifuged for 2 min at high rcf (i.e., ≥5000). The supernatant solution, using a 0.2 μm syringe filter, was transferred straight away into a GC vial (1.8 mL), while at the end, PAHs in the extracts solutions were determined by GC-MS/MSTQD.

PAHs Analysis by GCMSTQD
The EPA method was used to customize the GC MS/MS conditions (SVOC 8270). The auto SRM was used for the method development of MS/MS. To obtain good sensitivity, the modified method was divided into quantifier and qualifier ions. Scanning was performed in each segment (500 to 700 MS) while the maximum transitions were 51 per segment. Auto tuning of MS was performed before each batch of analysis, while helium (purity: 99.99%) gas was used as a carrier gas, nitrogen (purity: 99.9999%), argon (purity: 99.9999%) was used as collision gas obtained from a registered firm Linde gas (SiGas, Saudi Arabia). A total of sixteen (16) PAHs were analyzed, and their description and allotted symbols are shown in Table S1.

Dust Samples Preparation and Extraction by QuEChERS
The special extraction procedure was adopted by taking 2 g of dust sample and adding 1 mL of deionized water in a 50 mL centrifuge tube, vortexing shortly, and after vortexing, allowing it to homogenate for about 10 min total. Acetonitrile (6 mL) was added to each sample. Shaking (5 min) was carried out for the extraction of PAHs. The substances of citrate salts (ECQUEU750CT-MP) Mylar pouch were added to individual dust samples in the centrifuge tube. Instantaneously, samples were shaken for a minimum of 2 min and centrifuged for 5 min at ≥3500 rcf. Sample cleanup was carried out by transferring a 1.5 mL aliquot of supernatant to a 2 mL CUMPSC18CT (MgSO4, PSA, C18) dSPE tube. Samples were vortexed for 2 min and again centrifuged for 2 min at high rcf (i.e., ≥5000). The supernatant solution, using a 0.2 µm syringe filter, was transferred straight away into a GC vial (1.8 mL), while at the end, PAHs in the extracts solutions were determined by GC-MS/MSTQD.

PAHs Analysis by GCMSTQD
The EPA method was used to customize the GC MS/MS conditions (SVOC 8270). The auto SRM was used for the method development of MS/MS. To obtain good sensitivity, the modified method was divided into quantifier and qualifier ions. Scanning was performed in each segment (500 to 700 MS) while the maximum transitions were 51 per segment. Auto tuning of MS was performed before each batch of analysis, while helium (purity: 99.99%) gas was used as a carrier gas, nitrogen (purity: 99.9999%), argon (purity: 99.9999%) was used as collision gas obtained from a registered firm Linde gas (SiGas, Saudi Arabia). A Int. J. Environ. Res. Public Health 2023, 20, 1216 4 of 15 total of sixteen (16) PAHs were analyzed, and their description and allotted symbols are shown in Table S1.

Risk Assessment of Human Health
The soil absorption pathway was well thought out as the key pathway of PAHs for determining human health risks. The lifetime average daily dose (LADD) of PAHs through the soil is calculated in this study based on the concentration emitted from one box to the next. For the risk assessment analysis, exposure parameters established by the [31] have been used. Related studies in the literature have used these response parameters all over the world. The following formula was used to measure LADD: where Cs is the individual concentration of PAH in soil (µg kg −1 ), IR is the soil ingestion rate, F is a unit factor, EF is the exposure frequency (day year −1 ), ED is the lifetime exposure duration (year), BW is the body weight (70 kg for adult's calculation), and AT is the average time for carcinogens (day).

Health Risk Index
The health risk index is globally applied to measure surface dust particles' potential health risks [32,33]. Dermal absorption and hand-to-mouth ingestion have been recognized as the major entrance pathways for surface dust (toxins) to enter the human body. The health risk index includes calculating the following: where ADDing is the daily dose from the hand-to-mouth ingestion of substrate particles, and the ADDder is the daily dose via dermal absorption of PAHs in particles stuck to exposed skin. The characteristics of all remaining parameters used in Equations (2) and (3), and their adopted values are presented in Table 1.

Toxicity Equivalents of PAHs in Soils
The toxic equivalency factor (TEF) of BaP (TEQBaP) has been proposed as a capable approximation for determining the toxicology of PAHs in road dust toxic equivalent concentrations [35]. The TEF was designed to determine the danger posed by composite combinations, such as dioxins, which may help discern more precisely the carcinogenic components in composite combinations. The methodology defines a compound's separable matrix toxic potency, which is typically the deadliest complex in a mixture. In the case of PAHs, BaP is thought to be the most active and has a well-defined toxicological profile [35]. The concentration of PAHs at each sampling site was converted into TEQBaP using the corresponding TEF [35] according to the following equation: where C is the concentration of a PAH.

Statistical Analysis
To compare the mean values and outcomes from all parameters, a descriptive statistical analysis technique was performed using Microsoft Excel ® 2016.

Quality Assurance
An analytical process using reagent blanks and sample replication assessed the precision, bias, and pollution. The analysis showed that the bias and precision were less than 10%. The PAHs show a wide spectrum of volatility and all 16 PAHs behave the same in the chromatographic area in the standard and the sample. The set of samples was analyzed along with a blank for PAH background correction. The results were corrected for recoveries using an internal standard method, assuming that PAHs and d-PAHs behave in a similar manner during extraction and analysis. Recoveries of d-PAHs were utilized to estimate recoveries of the native PAHs. The average recoveries of d-PAHs varied from 79.11 to 91.56% with relative standard deviation (RSDs) ranging from 7.02% to 8.86%.

Lifetime Average Daily Intake of PAHs
For the human health hazard/risk calculation in this experimentation, we adopt that human adults of 70 years were exposed for all the days in a year. The estimated lifetime average daily dose (LADD) of 16 PAHs for human adults is shown in

Lifetime Average Daily Intake of PAHs
For the human health hazard/risk calculation in this experimentation, we adopt that human adults of 70 years were exposed for all the days in a year. The estimated lifetime average daily dose (LADD) of 16 PAHs for human adults is shown in Figure 4. The LADD with respect to individual PAHs in the road dust samples decreased in the order P3 > P15 > P1 > P8 > P6 > P7 > P5 > P9 > P2 > P12 > P11 > P4 > P10 > P13 > P16 > P14. The LADD with respect to naphthalene (P1) ranged from 1135 mg kg −1 day −1 to 5233 mg kg −1 day −1 with an average value of 2556 mg kg −1 day −1 among 50 collected samples of road dust in Riyadh City, Saudi Arabia. The highest LADD with respect to acenaphthylene (P2) was found with the value 4638 mg kg −1 day −1 and the lowest was 1231 mg kg

Toxicity Equivalent Factor
The BaP has enough toxicology evidence to use as the basis for a toxicity factor. As a result, BaP toxic equivalent factors were used to measure the volume of toxicity in Riyadh City road dust, as suggested for our specific PAHs compounds. The benzo[a]pyrene toxicity equivalency (BaPTEQ) represents the carcinogenic ability of PAHs that conform to BaP. The TEF data were adapted from the TEQBaP according to Nisbet and Lagoy [36]. Results are presented in Table 3 along with the minimum, maximum, and mean concentration of each PAH investigated in the study. The largest TEQBaP was detected with P15 dibenz(a.h)anthracene, whereas the lowest was found with the same

Discussion
The concentration of PAHs in dust varies with the nature of sampling areas such as rural and urban. Urban areas have dense road structures with heavy traffic, automobiles, hospitals, industrial sites, railways, waste dumping sites, etc. This study was purely focused on an urban city, i.e., Riyadh. Our results are similar to previous studies in terms of levels and occurrence of PAHs in road dust samples [37]. The total PAHs (ΣPAHs) in urban street dust from Tianjin ranged from 538 µg kg −1 to 34.3 mg kg −1 , with a mean value of 7.99 mg kg −1 . As compared to this study, overall, the concentration of ∑PAHs was from 0.01 to 126 ng g −1 ( Table 2). [18] depicted that acenaphthene (PAH) varied in summer and winter ranging from 3 ng g −1 to 1485 ng g −1 and 50 ng g −1 to 3162 ng g −1 , respectively. Organic contamination in road dust was rich in PAHs that have more than 4 benzene rings and a high molecular weight was present, confirming the 4 and above benzene rings. [38] determined various PAHs concentrations in different areas of Bayreuth, Germany. Several sites with heavy industries, including gas plants and three railroad areas, had a large concentration of different PAHs (the sum of 20 PAHs). The concentration of these PAHs was similar to our findings ranging from 2.40 to 48.90 ng g −1 soil. The levels in the center of the city ranged from 0.63 to 20.7 ng g −1 soil.
The World Health Organization (WHO) and their agency namely International Agency for Research on Cancer [39] defined 7 PAHs as possible carcinogens for humans, i.e., indeno(1,2,3,cd)pyrene, dibenzo(a,h)anthracene, benzo(k)fluoranthene, chrysene, benzo(a)anthracene, benzo(b)fluoranthene and BaP. The concentrations of 7PAH which are potential carcinogens observed in soils from Gwalior, India, ranged between 41-460 ng g −1 with a mean of 181 ± 51 ng g −1 and accounted for 38% of 16 PAHs. The average concentration of individual probable human carcinogens was 27 ± 5 ng g −1 {forbenzo(a)athracene}, 71 ± 28 ng g −1 (chrysene), 22 ± 6 ng g −1 {benzo(b)fluoranthene}, 14 ± 4 ng g −1 {benzo(k)fluoranthene}, 17 ± 3 ng g −1 (BaP), 25 ± 3 ng g −1 {dibenzo(a,h)anthracene}, and 22 ± 3 ng g −1 {indeno (1,2,3-cd)pyrene}, respectively. The combinations of the benzene ring in PAHs revealed different properties concerning carcinogen nature and exposure to humans. The release of PAHs into the environment may change due to pyrogenic and petrogenic sources as these are the most important factors [40]. It has been depicted that previously, PAHs, which possess two to three rings and are considered low molecular weight, arise from petrogenic sources, while PAHs having four to six rings and have high molecular weight arise from pyrogenic sources such as natural gas, fossil fuels, diesel, coal, and combustion of gasoline [41,42]. Typically, low-molecular-weight PAHs with 2 to 4 benzene rings are in the category of non-carcinogenic but studies showed their toxicity towards aquatic organisms, while the high-molecular-weight PAHs with 5 and 6 member-ring PAHs are classified in the category of carcinogenic [43]. The PAHs with five and six rings were predominant in suspended sediments. Our results are similar to previous studies, which stated that PAHs are the main contributor to settled dust [44,45]. Our results indicated that dust is an important source of 3-5 ring PAHs, but we need to be cautious since the collected samples and dataset in this experiment were minor and need more detailed studies to authenticate this point.
The LADD is the amount of intake per kg of body weight per day of a chemical (e.g., PAHs) suspected of having adverse health effects when absorbed into the body over a long period. In this study, we also determined that residents in the age-specific group are exposed for 350 days a year during their life span. The estimated LADD values of individual PAH for an age-specific group are shown in Figure 4. Our results are in agreement with [46,47]. The ADDing showed a real picture of PAHs contamination in dust concerning human health. The ADDing was higher in children as compared to adults. Our results are in agreement with [48]. Moreover, children can receive 2.5-fold more carcinogenic PAHs through dust ingestion as compared to inhalation [49,50]. As children have lower body weight, the intake of PAHs (mg per kg of body weight per day) is believed to be greater as compared to adults. Children are also undergoing early organ, immune system, and nervous development; hence, they are becoming more sensitive to carcinogens [51]. Owing to such factors, children are at more risk of PAHs as compared to adults. (Hu et al. (2007) [52] revealed that urban dust of Tianjing (China) created human cancer risks because of exposure to PAHs because of higher values of dermal exposure (SA) along with the increased value of exposure duration in Children. While results are similar to) [53] Yang et al. (2015), who researched the soil samples of Guizhou province, southwest China, and identified two major sources of PAH comprising vehicle emissions and coal combustion. High molecular weight and TEQBaP in Riyadh dust suggest that high molecular weight species have much higher TEF values than low molecular weight species. Also measured the TEQ of various PAHs in street dust of Bushehr City, Iran. They found a positive correlation between high molecular weight PAHs and TEQ. The PAHs released from fuel combustion, have higher TEF than low molecular weight PAHs and mostly affect TEQBaP same as in the current study. Alghamdi et al. (2022) [34] (reported that the rapid industrialization and urbanization and their concentrations in dust may cause health problems in the near future in the north side as well as other sides of Riyadh City.

Conclusions
This study presents PAH concentration in the road dust of Riyadh City, Saudi Arabia. The 16 PAHs were determined from 50 different locations. The findings revealed variable concentration levels for all tested PAHs. Many factors altogether impacted the PAH concentrations in road dust samples such as traffic density, vehicle emissions, industrial emissions, pollutant accumulation/road dust, and sampling site locations. The LADD showed high values. The ADDing and ADDderm depicted more values for children as compared to adults. The total equivalency factor based on BaP (TEQBaP) calculations revealed that PAHs with more benzene rings or having high molecular weight showed high TEQBaP as compared to low molecular weight PAHs. In the future, there is a great need for the identification of spatial-temporal control drivers in Saudi Arabia that determines the dust storms events. Furthermore, there is a need to emphasize significant correlations between human health problems and road dust, particularly in densely populated areas.