Spatial Distribution, Contamination Levels, and Health Risk Assessment of Potentially Toxic Elements in Household Dust in Cairo City, Egypt

Urban areas’ pollution, which is owing to rapid urbanization and industrialization, is one of the most critical issues in densely populated cities such as Cairo. The concentrations and the spatial distribution of fourteen potentially toxic elements (PTEs) in household dust were investigated in Cairo City, Egypt. PTE exposure and human health risk were assessed using the USEPA’s exposure model and guidelines. The levels of As, Cd, Cr, Cu, Hg, Mo, Ni, Pb, and Zn surpassed the background values. Contamination factor index revealed that contamination levels are in the sequence Cd > Hg > Zn > Pb > Cu > As > Mo > Ni > Cr > Co > V > Mn > Fe > Al. The degree of contamination ranges from considerably to very high pollution. Elevated PTE concentrations in Cairo’s household dust may be due to heavy traffic emissions and industrial activities. The calculated noncarcinogenic risk for adults falls within the safe limit, while those for children exceed that limit in some sites. Cairo residents are at cancer risk owing to prolonged exposure to the indoor dust in their homes. A quick and targeted plan must be implemented to mitigate these risks.


Introduction
Over the past few decades, a tremendous amount of hazardous waste materials has been released into various environmental media at increasing levels because of the rapid urbanization and globalization of economic and industrial activity [1][2][3][4][5]. Because the air in common is the primary carrier of fine particles, air pollution has produced a significant environmental impact (e.g., climate change and human health). The concentration of suspended particles in the air, which transports contaminants, especially potentially toxic elements (PTEs), has progressively increased, endangering humans. Because of their genotoxicity, carcinogenicity, chemical persistence, and non-degradability, PTEs attached to suspended particles would enrich in surface environments and have an acute or chronic impact on the health of vulnerable residents once they get into the human body [6][7][8][9][10]. PTEs can go through a human body via respiratory inhalation, ingestion of contaminated media, and dermal contact and accumulate over time [1,6,11,12].
Because indoor air can be significantly more polluted than outdoor air, it has captures remarkable attention from researchers. Imperfect air exchange and specific indoor emission sources combined with outdoor sources seems to be the leading causes of indoor air being a complex and contaminated environment [1,12]. People in megacities typically spend 80-90% of their own time indoors, in private homes, schools, and offices, potentially increasing their exposure to toxic substances being emitted from construction materials, household equipment, and electronic products, in conjunction with anthropogenic sources [13]. In Cairo City has a typical Mediterranean climate, with different temperatures through seasons: winter 14 • C, spring 21 • C, summer 36 • C, and fall 23 • C. Most of the year, wind speeds range from 3 to 8 m/s. The north and northeast were dominant wind directions [47,48]. It is surrounded by agricultural and industrial activities. It contains the main industrial zones that exist in the Northern and Southern regions which host cement manufacturing plants, steel, oil and gas, quarrying, rubber, petrochemicals, metallurgical, textile, and plastic products [26,48].

Sampling and Samples Preparation
A total of 38 composite household settled dust samples were collected from different regions and districts in Cairo City in 2021 ( Figure 1). The sample size was selected based on the major districts in Cairo City, in conjunction with budgetary constraints. To ensure a collection of representative samples at least 10 subsamples were collected from each main district representing a total of 473 private houses (1 sample per house) ( Table S1 in Supplementary Materials). The undisturbed surfaces, such as cupboards, fans, bookshelves, and refrigerators, were slowly brushed using precleaned polyethylene brushes and plastic dustpans to collect dust samples, which were then carefully blended and placed into transparent, zip-locked, and labeled plastic bags. The collected dust subsamples were carefully mixed and homogenized into 38 composite samples. The samples were then dried at 50 • C for 24 h in an oven followed by sieve analysis using a standard stainless-steel sieve (63 microns).

Contamination Levels 2.4.1. Contamination Factor (C f )
Anthropogenic activities' contribution to PTE contamination has been evaluated using the contamination factor (C f ). C f is calculated by the Equation (1) [50].
where C i s is the PTE concentration in analyzed samples, and C i b is the background value of the investigated PTE. In this investigation the Upper Continental Crust (UCC) element concentrations [51] were considered as the background values. The C f values are typically categorized in four distinct classes; class 1 (C f < 1.0 = low contamination); class 2 (1.0 ≤ C f < 3.0 = moderate contamination); class 3 (3.0 ≤ C f ≤ 6.0 = considerable contamination); and class 4 (C f > 6.0 = very high contamination) [50].

Contamination Degree (C deg )
To detect multielement contamination, C deg was used. It was calculated for each sampling site using Equation (2) [50].
where C f is contamination factor, and n is the number of the examined PTEs. The C deg values are typically categorized in four distinct classes; class 1 (C deg < 6.0 = low contamination); class 2 (6.0 ≤ C deg < 12.0 = moderate contamination); class 3 (12.0 ≤ C deg ≤ 24.0 = considerable contamination); and class 4 (C deg > 24.0 = very high contamination) [50].

Health Risk Assessment
PTEs measured in household dust in this investigation are typically known to possess noncarcinogenic effects on human health [52][53][54]. As, Cd, Cr, Ni, and Pb are believed to possess both noncarcinogenic and carcinogenic effects [52][53][54]. In the current study, health risks for children and adults in Cairo City were assessed using the noncarcinogenic Hazard Quotient (HQ) of a single element and Hazard Index (HI) of multiple elements via ingestion, inhalation, and dermal routes of exposure. Furthermore, the Cancer Risk (CR) was calculated using the concentrations of As, Cd, Cr, Ni, and Pb in the collected household dust samples. HQ, HI, and CR were calculated using the calculation model of exposure adopted by USEPA [52][53][54][55].
The average daily intakes (ADI) of PTEs in the household dust via nondietary inadvertent ingestion (noncarcinogenic) (ADI ing ), dust inhalation (noncarcinogenic) (ADI inh ), and dermal contact (noncarcinogenic) (ADI der ) routes are calculated using Equations (3)-(5) as follows: The noncarcinogenic risk HQ and HI of PTEs in the household dust is calculated using Equations (6)-(9) as follows: HQ der = ADI der R f D der (8) The lifetime average daily dose (carcinogenic) (LADD) and the carcinogenic risk (CR) of As, Cd, Cr, Ni, and Pb in household dust is calculated using Equations (10)- (13) as follows: where all the abbreviations, definitions, and reference values are given and explained in Table 1. If HI is less than one, there is no risk of noncarcinogenic effect; if HI is greater than one, there is a risk of noncarcinogenic effect. A value of CR less than 1 × 10 −6 is regarded as modest, a value of CR between 1 × 10 −4 and 1 × 10 −6 is regarded within the permissible level, and a value of CR greater than 1 × 10 −4 is likely to be harmful to humans [52][53][54][55].

Contamination Levels
The UCC element concentrations were used as the background values, and the C f and integrative C deg indices were applied to objectively analyze the contamination levels in the five administrative regions in Cairo City. The calculated C f values are presented in Table S2 and Figure 3. Altogether, the five regions were polluted to varying degrees by the measured PTEs. The lowest degrees of pollution were recorded for Al, Co, Fe, Mn, and V, while the highest degrees were recorded for Cd, Cu, Hg, Pb, and Zn, reaching considerably to very high pollution. Hg shows a wide range of C f values from low to very high pollution.
The calculated C f -based C deg values in the investigated five regions ( Figure 4) indicate that New Cairo recorded the slightest degree of contamination, ranging from considerably to very high pollution. On the other hand, eastern, northern, western, and southern regions' household dust were very highly polluted.

Correlations between PTEs
The multivariate statistical analysis including Pearson's Correlation Coefficient matrix (PCC), Hierarchical Cluster Analysis (HCA) in Q mode, and Principal Component Analysis (PCA) were utilized to reveal and emphasize the correlation intensity and linkage between the analyzed PTEs. Toxics 2022, 10, x FOR PEER REVIEW 11 of 23 The calculated Cf-based Cdeg values in the investigated five regions ( Figure 4) indicate that New Cairo recorded the slightest degree of contamination, ranging from considerably to very high pollution. On the other hand, eastern, northern, western, and southern regions' household dust were very highly polluted.

Correlations Between PTEs
The multivariate statistical analysis including Pearson's Correlation Coefficient matrix (PCC), Hierarchical Cluster Analysis (HCA) in Q mode, and Principal Component Analysis (PCA) were utilized to reveal and emphasize the correlation intensity and linkage between the analyzed PTEs.
Correlations values 0.00-0.19, 0.20-0.39, 0.40-0.59, 0.60-0.79, and 0.80-1.00 can be considered as very weak, weak, moderate, strong, and very strong correlations, respectively [73]. As shown in Table 4 The most significant finding that can be deduced from these positive linear relations is the role played by Al, Fe, and Mn as scavenging elements in the distribution of PTEs, especially As, Co, V, and Zn [48,74]. The strong to very strong positive correlation between the measured PTEs indicates their close distribution and association and may suggest a shared source. It appears to imply that household dust with more elevated levels of one toxic element additionally contain higher levels of other PTEs. Correlations values 0.00-0.19, 0.20-0.39, 0.40-0.59, 0.60-0.79, and 0.80-1.00 can be considered as very weak, weak, moderate, strong, and very strong correlations, respectively [73]. As shown in Table 4 The most significant finding that can be deduced from these positive linear relations is the role played by Al, Fe, and Mn as scavenging elements in the distribution of PTEs, especially As, Co, V, and Zn [48,74]. The strong to very strong positive correlation between the measured PTEs indicates their close distribution and association and may suggest a shared source. It appears to imply that household dust with more elevated levels of one toxic element additionally contain higher levels of other PTEs.
HCA ( Figure 5) reduced data into two main clusters. Cluster (1) includes: (a) Al, As, and V and (b) Co, Mn, and Fe. Cluster (2) was subdivided into (c) Cd, Ni, Zn, and Pb; (d) Cr, Cu, and Mo; and (E) Hg. Figure 6 presents the PCA component. Three components, PC1 (49.60%; eigenvalue 6.44), PC2 (24.87%; eigenvalue 3.48), and PC3 (410.40%; eigenvalue 1.46), were extracted from PCA. The 3D plotting of the extracted three components positively confirms the association between Al, As, Co, Fe, Mn, and V (Figure 6a). The 2D plotting of PC1 and PC2 combined with sampling sites (Figure 6b) indicates that Al, As, Co, Fe, Mn, and V are more associated together in the southern region samples. It can be concluded that these elements originated from natural sources; this is in agreement with [22,75]. As enriched from intensive industrial activity in the southern region and adsorbed on Fe-Mn oxides surface [76]. HCA ( Figure 5) reduced data into two main clusters. Cluster (1) includes: (a) Al, As, and V and (b) Co, Mn, and Fe. Cluster (2) was subdivided into (c) Cd, Ni, Zn, and Pb; (d) Cr, Cu, and Mo; and (E) Hg. Figure 6 presents the PCA component. Three components, PC1 (49.60%; eigenvalue 6.44), PC2 (24.87%; eigenvalue 3.48), and PC3 (410.40%; eigenvalue 1.46), were extracted from PCA. The 3D plotting of the extracted three components positively confirms the association between Al, As, Co, Fe, Mn, and V (Figure 6a). The 2D plotting of PC1 and PC2 combined with sampling sites (Figure 6b) indicates that Al, As, Co, Fe, Mn, and V are more associated together in the southern region samples. It can be concluded that these elements originated from natural sources; this is in agreement with [22,75]. As enriched from intensive industrial activity in the southern region and adsorbed on Fe-Mn oxides surface [76].  PTEs in household dust can be attributed to indoor activities such as cooking, smoking, carpet, paper, clothing, cosmetic and personal care products, electric instruments, and cleaning products [65,71,77,78]. A substantial portion of the PTEs emitted by various outdoor activities can travel considerable distances via atmospheric particulate matter and enter the indoor environment in a variety of ways [71]. Al is geochemically stable, while Fe and Mn are geochemically related elements that are abundant in the earth's crust and considered as major elements in soil minerals. The weathering of pre-existing rocks, sediments, and soils primarily releases these major elements [22,71,79] because the levels of PTEs in household dust can be attributed to indoor activities such as cooking, smoking, carpet, paper, clothing, cosmetic and personal care products, electric instruments, and cleaning products [65,71,77,78]. A substantial portion of the PTEs emitted by various outdoor activities can travel considerable distances via atmospheric particulate matter and enter the indoor environment in a variety of ways [71]. Al is geochemically stable, while Fe and Mn are geochemically related elements that are abundant in the earth's crust and considered as major elements in soil minerals. The weathering of pre-existing rocks, sediments, and soils primarily releases these major elements [22,71,79] because the levels of Al, Fe, and Mn in the investigated household dust samples are not polluted and relatively deficient. These elements are probably of predominantly geogenic origin and were not enriched in the dust samples by anthropogenic activities. Some exceptions for Mn were recognized in some sites moderately polluted with Mn. Mn can be enriched by many anthropogenic sources such as Mn fungicides [80], Mn-Ni batteries [81], and pigment and paints [82]. Similarly, Co and V concentrations in the majority of the studied samples are below background levels and show a low degree of pollution, indicating that they originated from natural sources before being transported and settling in household dust.
Anthropogenic sources of As, Cd, Cr, Cu, Ni, Pb, and Zn include traffic emissions, braking engine wear, corrosion of vehicle parts, lubricating oils, coal, and fossil fuel combustion, building and construction materials, rubbers, pesticides, and industrial emissions [19,22,63,71,75,77,81,83]. Cr and Zn can be sourced from wood preservative furniture [12,65]. Chemical and pharmaceutical industries, coal combustion, municipal solid waste incineration, and cement manufacture are all anthropogenic sources of Hg. Building materials (interior decorations, paints, and fluorescent lamps), household appliances and electronic devices, LCD displays, monitors, batteries, clothes dryers, irons, washing machines, fluorescent bulbs, neon lights, and thermometers are other potential indoor sources [84].

Health Risk Assessment
Results of human health risk assessment show that the calculated HQ ing , HQ der , and HQ inh values for individual element (Table S3) and combined PTEs (Table 5; Figure 7a,b) in the household dust were less than one for children and adults. In addition, HI values for adults of the combined PTEs in the household dust were less than one, suggesting no potential noncancer risks (Table 5; Figure 7b). On the other hand, HI values for children were greater than those for adults; one site (site 16; Eastern region) recorded HI values higher than one, suggesting potential noncancer risks for children (Figure 7a). In terms of carcinogenic risk, LADDing, LADDder, and LADDinh va elements were in the safe limit (Table S4). ⅀LADDing values for As, were higher than 1 × 10 -4 in the majority of the investigated sites, indi of cancer risk. On the other hand, ⅀LADDder values for As and Cr were 6, and ⅀LADDinh values for As, Cd, Cr, Ni, and Pd were lower than Table 5). Alarmingly, CR values through the three routs of exposure w 10 -4 in the majority of the investigated sites, indicating a possible cance in Cairo City. The CR risks via various exposure pathways were a mouth ingestion > dermal contact > respiratory inhalation. Individua tion (%) for carcinogenic risk ⅀LADDing, ⅀LADDder, and ⅀LADDinh is 8b. The elements' contributions are Ni (61.84%) > Cr (29.39%) > As (7. Cd (0.46%), As (81.93%) > Cr (18.07%), and As (52.14%) > Ni (21.73%) (5.40%) > Pb (3.59%) in ⅀LADDing, ⅀LADDder, and ⅀LADDinh, respecti HQ ing was most likely to pose a noncancer risk of more than In terms of carcinogenic risk, LADDing, LADDder, and LADDinh values for individual elements were in the safe limit (Table S4). ⅀LADDing values for As, Cd, Cr, Ni, and Pd were higher than 1 × 10 -4 in the majority of the investigated sites, indicating a probability of cancer risk. On the other hand, ⅀LADDder values for As and Cr were higher than 1 × 10 -6, and ⅀LADDinh values for As, Cd, Cr, Ni, and Pd were lower than 1 × 10 -6 ( Figure 8a; Table 5). Alarmingly, CR values through the three routs of exposure were higher than 1 × 10 -4 in the majority of the investigated sites, indicating a possible cancer risk to inhabitants in Cairo City. The CR risks via various exposure pathways were as follows: hand-tomouth ingestion > dermal contact > respiratory inhalation. Individual element contribution (%) for carcinogenic risk ⅀LADDing, ⅀LADDder, and ⅀LADDinh is presented in Figure  8b. The elements' contributions are Ni (61.84%) > Cr (29.39%) > As (7.28%) > Pb (1.02%) > Cd (0.46%), As (81.93%) > Cr (18.07%), and As (52.14%) > Ni (21.73%) > Cr (17.14%) > Cd (5.40%) > Pb (3.59%) in ⅀LADDing, ⅀LADDder, and ⅀LADDinh, respectively. In terms of carcinogenic risk, LADDing, LADDder, and LADDinh values for individ elements were in the safe limit (Table S4). ⅀LADDing values for As, Cd, Cr, Ni, and were higher than 1 × 10 -4 in the majority of the investigated sites, indicating a probabi of cancer risk. On the other hand, ⅀LADDder values for As and Cr were higher than 1 × 6,
LADD inh values for As, Cd, Cr, Ni, and Pd were lower than 1 × 10 −6 ( Figure 8a; Table 5). Alarmingly, CR values through the three routs of exposure were higher than 1 × 10 −4 in the majority of the investigated sites, indicating a possible cancer risk to inhabitants in Cairo City. The CR risks via various exposure pathways were as follows: hand-to-mouth ingestion > dermal contact > respiratory inhalation. Individual element contribution (%) for carcinogenic risk In terms of carcinogenic risk, LADDing, LADDder, and LADDinh values for individual elements were in the safe limit (Table S4). ⅀LADDing values for As, Cd, Cr, Ni, and Pd were higher than 1 × 10 -4 in the majority of the investigated sites, indicating a probability of cancer risk. On the other hand, ⅀LADDder values for As and Cr were higher than 1 × 10 -6, and ⅀LADDinh values for As, Cd, Cr, Ni, and Pd were lower than 1 × 10 -6 ( Figure 8a; Table 5). Alarmingly, CR values through the three routs of exposure were higher than 1 × 10 -4 in the majority of the investigated sites, indicating a possible cancer risk to inhabitants in Cairo City. The CR risks via various exposure pathways were as follows: hand-tomouth ingestion > dermal contact > respiratory inhalation. Individual element contribution (%) for carcinogenic risk ⅀LADDing, ⅀LADDder, and ⅀LADDinh is presented in Figure   LADD  In terms of carcinogenic risk, LADDing, LADDder, and LADDinh values for individual elements were in the safe limit (Table S4). ⅀LADDing values for As, Cd, Cr, Ni, and Pd were higher than 1 × 10 -4 in the majority of the investigated sites, indicating a probability of cancer risk. On the other hand, ⅀LADDder values for As and Cr were higher than 1 × 10 -6, and ⅀LADDinh values for As, Cd, Cr, Ni, and Pd were lower than 1 × 10 -6 ( Figure 8a; Table 5). Alarmingly, CR values through the three routs of exposure were higher than 1 × 10 -4 in the majority of the investigated sites, indicating a possible cancer risk to inhabitants in Cairo City. The CR risks via various exposure pathways were as follows: hand-tomouth ingestion > dermal contact > respiratory inhalation. Individual element contribution (%) for carcinogenic risk ⅀LADDing, ⅀LADDder, and ⅀LADDinh is presented in Figure   LADD  In terms of carcinogenic risk, LADDing, LADDder, and LADDinh values f elements were in the safe limit (Table S4). ⅀LADDing values for As, Cd, C were higher than 1 × 10 -4 in the majority of the investigated sites, indicating of cancer risk. On the other hand, ⅀LADDder values for As and Cr were high 6, and ⅀LADDinh values for As, Cd, Cr, Ni, and Pd were lower than 1 × 10 Table 5). Alarmingly, CR values through the three routs of exposure were h 10 -4 in the majority of the investigated sites, indicating a possible cancer risk in Cairo City. The CR risks via various exposure pathways were as follo mouth ingestion > dermal contact > respiratory inhalation. Individual elem tion (%) for carcinogenic risk ⅀LADDing, ⅀LADDder, and ⅀LADDinh is presen LADD inh is presented in Figure 8b. The elements' contributions are Ni (61.84%) > Cr (29.39%) > As (7.28%) > Pb (1.02%) > Cd (0.46%), As (81.93%) > Cr (18.07%), and As (52.14%) > Ni (21.73%) > Cr (17.14%) > Cd (5.40%) > Pb (3.59%) in In terms of carcinogenic risk, LADDing, LADDder, and LADDinh values for individual elements were in the safe limit (Table S4). ⅀LADDing values for As, Cd, Cr, Ni, and Pd were higher than 1 × 10 -4 in the majority of the investigated sites, indicating a probability of cancer risk. On the other hand, ⅀LADDder values for As and Cr were higher than 1 × 10 -6, and ⅀LADDinh values for As, Cd, Cr, Ni, and Pd were lower than 1 × 10 -6 ( Figure 8a; Table 5). Alarmingly, CR values through the three routs of exposure were higher than 1 × 10 -4 in the majority of the investigated sites, indicating a possible cancer risk to inhabitants In terms of carcinogenic risk, LADDing, LADDder, and LADDinh values for individual elements were in the safe limit (Table S4). ⅀LADDing values for As, Cd, Cr, Ni, and Pd were higher than 1 × 10 -4 in the majority of the investigated sites, indicating a probability of cancer risk. On the other hand, ⅀LADDder values for As and Cr were higher than 1 × 10 -6, and ⅀LADDinh values for As, Cd, Cr, Ni, and Pd were lower than 1 × 10 -6 ( Figure 8a; Table 5). Alarmingly, CR values through the three routs of exposure were higher than 1 × 10 -4 in the majority of the investigated sites, indicating a possible cancer risk to inhabitants LADD der , and In terms of carcinogenic risk, LADDing, LADDder, and LADDinh values f elements were in the safe limit (Table S4). ⅀LADDing values for As, Cd, C were higher than 1 × 10 -4 in the majority of the investigated sites, indicating of cancer risk. On the other hand, ⅀LADDder values for As and Cr were highe 6, and ⅀LADDinh values for As, Cd, Cr, Ni, and Pd were lower than 1 × 10 Table 5). Alarmingly, CR values through the three routs of exposure were hi 10 -4 in the majority of the investigated sites, indicating a possible cancer risk t The spatial distribution maps of the calculated HI (children), HI (adults), an risks are presented (Figure 9) to inform decision makers about the riskiest districts s mitigation measures could be implemented. The presented maps show the same dis tion for noncancer and cancer risk, with hot spots concentrated in the eastern, nor and western regions due to condensed road networks in these regions with perm traffic congestion (Figure 1). In addition, the southern region showed considerabl distribution due to the intensive industrial activity in this region. One of the most s cant limitations of this investigation is the analysis of few composed samples and th determined indoor microenvironments. Additional investigation in highly pollut gions should include specific indoor microenvironments such as entrances, kitchen ing rooms, children's rooms, and bedrooms to provide a more comprehensive analy household dust geochemistry in various microenvironments and to differentiate be PTE outdoor and indoor sources. The spatial distribution maps of the calculated HI (children), HI (adults), and CR risks are presented (Figure 9) to inform decision makers about the riskiest districts so that mitigation measures could be implemented. The presented maps show the same distribution for noncancer and cancer risk, with hot spots concentrated in the eastern, northern, and western regions due to condensed road networks in these regions with permanent traffic congestion (Figure 1). In addition, the southern region showed considerable risk distribution due to the intensive industrial activity in this region. One of the most significant limitations of this investigation is the analysis of few composed samples and the undetermined indoor microenvironments. Additional investigation in highly polluted regions should include specific indoor microenvironments such as entrances, kitchens, living rooms, children's rooms, and bedrooms to provide a more comprehensive analysis of household dust geochemistry in various microenvironments and to differentiate between PTE outdoor and indoor sources. Toxics 2022, 10, x FOR PEER REVIEW 18 of 23 Figure 9. Spatial distribution of HI for children and adults and CR risks for the household dust exposure in Cairo City.

Conclusions
This study is the first one to comprehensively measure the chemical composition of household dust in Cairo City, Egypt. In general, the following important conclusions can be gained: (1) The levels of As, Cd, Cr, Cu, Hg, Mo, Ni, Pb, and Zn surpassed the background values of UCC, indicating anthropogenic influences. The lowest degrees of pollution were recorded for Al, Co, Fe, Mn, and V, while the highest degrees were recorded for Cd, Cu, Hg, Pb, and Zn, reaching considerably to very high pollution. (2) New Cairo recorded the slightest degree of contamination, ranging from considerably to very high pollution, while in other Cairo regions household dust is very high polluted. Elevated PTE concentrations in Cairo's household dust may be due to industrial activities and heavy traffic emissions. (3) The health risk assessment model revealed that the vital route of potential PTE exposure that leads to both noncarcinogenic and carcinogenic risks is ingestion, followed by dermal and inhalation pathways. The noncarcinogenic risk was generally in the safe range for adults' exposure. Children are at risk in some sites, where HI values for the measured PTEs in household dust are higher than the recommended safe limit. Prolonged exposure to household dust in Cairo City would produce cancer risk to inhabitants.

Conclusions
This study is the first one to comprehensively measure the chemical composition of household dust in Cairo City, Egypt. In general, the following important conclusions can be gained: (1) The levels of As, Cd, Cr, Cu, Hg, Mo, Ni, Pb, and Zn surpassed the background values of UCC, indicating anthropogenic influences. The lowest degrees of pollution were recorded for Al, Co, Fe, Mn, and V, while the highest degrees were recorded for Cd, Cu, Hg, Pb, and Zn, reaching considerably to very high pollution. (2) New Cairo recorded the slightest degree of contamination, ranging from considerably to very high pollution, while in other Cairo regions household dust is very high polluted. Elevated PTE concentrations in Cairo's household dust may be due to industrial activities and heavy traffic emissions. (3) The health risk assessment model revealed that the vital route of potential PTE exposure that leads to both noncarcinogenic and carcinogenic risks is ingestion, followed by dermal and inhalation pathways. The noncarcinogenic risk was generally in the safe range for adults' exposure. Children are at risk in some sites, where HI values for the measured PTEs in household dust are higher than the recommended safe limit. Prolonged exposure to household dust in Cairo City would produce cancer risk to inhabitants.
(4) The critical contributors to noncancer risk are Pb, As, Cr, Mn, V, and Al. The main causes of cancer risk are Ni, As, and Cr. (5) The study's findings call for regular detection and assessment of the PTE concentrations and health risk in indoor dust in Cairo City, as well as initiation and facilitation of public health policy development, prevention of anthropogenic source pollutants, and implementation of specific control measures.
Supplementary Materials: The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/toxics10080466/s1, Table S1: Samples distribution in administrative regions and districts in Cairo City.; Table S2: Calculated C f and C deg values; Table S3: Calculated noncancer HQ ing , HQ der , and HQ inh values. Table S4: Calculated cancer LADD ing , LADD der , and LADD inh values.