Health risk assessment and source apportionment of potentially toxic metal(loid)s in windowsill dust of a rapidly growing urban settlement, Iran

PTM concentrations

In Table 1, the descriptive statistics of PTMs in 50 dust samples from Qom city are described. The background values were used based on the concentrations of metals in the Upper Continental Crust. The mean concentration of As, Cd, Cu, Mo, Pb, Sb, and, Zn exceeded the background value. Also, Cd, Cu, Mo, Pb, Sb, and, Zn had a coefficient of variation (C.V.) greater than 50%, indicating a severe variability in PTMs concentrations in the atmospheric dust of the studied area². Metals with C.V. < 50% (including Fe, Mn, Ni, Cr, Co, and As) might be from geogenic sources²².

Table 2 shows the comparison of the average concentration of PTMs in this study (determined in four land-use categories) with those reported in Iran cities and other countries. Industrial samples contained the highest amount of all PTMs—except for Mn and Cu. PTMs including As, Sb, Pb, and Zn were considerably higher than the background values in all the functional sectors.

Most of the studies have sampled dust from the street and roads. For this study, we used the windowsill dust at a height of 1.5 m. According to Table 2, dust contamination in Qom city is significant and as high as Tehran and Isfahan metropolises. Cobalt in Qom dust recorded the highest concentration among the other cities—same as the Isfahan—despite being lower than the background value. Manganese, Fe, and Zn in Qom dust were higher than in the other cities in Iran, except for Tehran (Iran). Lead concentration in Qom dust was almost similar to the value reported from Isfahan (Iran) and higher than the average values obtained in other cities of Iran (Table 2). Antimony had a significant amount in the industrial sector, and its average overall concentration is placed after Tehran and Isfahan. To sum up, Qom can be listed as the top three cities in Iran, along with Tehran and Isfahan in terms of dust pollution.

On the international scale, As and Pb concentrations in Qom were almost similar to Busan (Korea). Arsenic was significantly higher than the estimations from the other countries mentioned in Table 2, except Anyang (China). The concentrations of Pb, Cu, and Zn were found lower compared to Jiyuan (China) industrial city. The concentration of Zn was lower than Jiyuan (China), Madrid (Spain), and Busan (Korea), but higher than the other listed countries. Notably, Fe and Mn contents in this study were almost higher than in the other cities (except Tehran, Iran).

PTM pollution assessment

The EF and I_geo values of PTMs in Qom’s dust are given in Fig. 2 and the details are presented in Table S6. The mean EFs were found in the order of Sb > Pb > Zn > As > Cd > Cu > Mo > Cr > Mn > Ni = Co. Antimony (38.55) and Pb (35.13) had the highest average EF values, which means they were enriched very high in the windowsill dust. Also, they had a wide range of EF values in the 50 stations: from 4.0 to 227.0 for Sb, and from 8.3 to 140.8 for Pb which might reflect the existence of discrete multiple sources in the studied area. The degree of enrichment for Pb and Sb in the industrial sector was extreme and in the commercial sector was very high; also, the other sectors were significantly enriched. Zinc and As had a more homogenous enrichment in the area. In all the functional sectors, 95% and 84% of stations were significantly enriched by As and Zn, respectively. Copper, Cd, and Mo were moderately enriched in all functional sectors, but the greenspace sector had minimal enrichment by these elements. Some areas in the industrial sector had significant to very high enrichment of Cd. The EF value indicated Co, Cr, Mn, and Ni were minimally enriched in all the stations.

The highest average values of I_geo were obtained in the order of Pb > Sb > As > Zn. PTMs included Co, Cr, Ni, Fe, and Mn were categorized as unpolluted and Cd, Cu, and Mo were in the category of unpolluted to moderately polluted. In the industrial zone, the windowsill dust was extremely polluted with Sb and Pb. The sequence of contamination intensity with Pb, Zn and Sb according to land use was: industrial > commercial > residential > greenspace. The highest concentration of arsenic in the study area belongs to the industrial area.

To evaluate the pollution level based on land use, PLI and mC_d indices were utilized (Fig. 3). These cumulative indices showed that the dust in Qom city is considerably contaminated with PTMs. According to the PLI index, all the stations were categorized as polluted sites. The PLI_zone values were in the order of industrial (3.77) > commercial (2.05) > residential (1.67) > green space (1.38). This pattern was also repeated with the mC_d index. The mC_d for the industrial sector ranged from 6.98 (high contamination level) to 39.60 (ultra-high contamination level). In the commercial sector, fifty percent of dust samples were classified as having a high degree of contamination. All the greenspace stations were in the moderate pollution category. This shows the possible effect of tree density in diminishing the risk of dust pollution to the receptors.

Spatial distribution of PTMs

The As, Cd, Cu, Sb, Pb, and Zn content in 100% of the dust samples exceeded the background value. Spatial distribution maps were generated for the hotspot PTMs (As, Sb, Pb, Cd, Cu, Mo and Zn) by applying the inverse distance weighted (IDW) interpolation method (ArcGIS 10.3). Figure 4 demonstrates that PTMs dispersions were slightly influenced by the prevailing wind direction (from the west), suggesting they came from the point- or area- sources. On the other hand, the K–S test showed that the overall distribution of PTMs was not normal in the studied region. This might signify the influence of industrial activities and the presence of multiple sources of dust.

The highest pollution load of PTMs belonged to the industrial section. The level of pollution gradually decreased from Shokouhieh to Mahmoudabad industrial zones. The reason is related to more active industries, a closed environment, and more construction existing in Shokouhieh industrial town than in Mahmoudabad industrial town.

There is a clear decreasing trend from the central part to southern (downtown area) and southwestern (suburb area) parts of the city. In fact, these parts are diffusely populated and the southwestern part is almost new with lots of barren lands. Copper, Mo, and Cd show high concentrations toward the central part of the city. Educational, cultural and commercial activities are mainly located in the central part of the city. Also, historical and religious districts in the city center are accompanied by a huge influx of tourists throughout the year. For this reason, the central part of the city has various public transportations such as bus stands and taxi stations, and is dominated by a high load of motorcycles.

In the eastern part of the city, some hotspots can be observed (Fig. 4). This part includes an important transportation system (like highways and a complex interchange) where exhaust traffic emissions might be a probable source of As, Sb, Pb, and Zn. Unlike Pb and Zn, several peaks of As are scattered in the western part, suggesting an area source might exist in the region. It is noteworthy that the western area is densely populated with lots of residential buildings. Bisht et al. (2022)³⁵ also observed hotspots of As in the residential area of Dehradun, India.

PTM potential sources

To evaluate the relationship between PTMs in dust samples, the Spearman correlation and PCA were developed (Fig. 5) and more details are given in Table S7. Statistical analysis can help to identify the potential source of contamination in urban dust. The Spearman correlation was significant at p < 0.05, and those correlations higher than 0.8 show a very strong correlation. The Kaiser–Meyer–Olkin (KMO) was 0.784 and Bartlett’s sphericity test was less than 0.05, which showed the reliability of PCA data. Three components with eigenvalues greater than 1.0, account for 77.9% of the total variances.

First component (PCA1) of PCA (55.40% loading) includes Cd (0.9), Mo (0.87), Sb (0.87), Zn (0.78), Pb (0.7), As (0.69), and Cu (0.6). The highest correlation was determined for Sb-Mo (0.87) and Mo-Zn (0.83) and Mo-Pb (0.81). In this study, more than 90% of these PTM values were higher than the background values in all the functional areas. The concentration of PTMs implies that the major part of this group originates from anthropogenic sources. However, the interpolated spatial distributions and Spearman analysis suggest that they might not stem from the same source. Traffic-related materials in the central and eastern parts of the city and industrial emissions in the northern region predominated over the other urban activities (like household activities and construction) have less contribution.

According to the pollution indices, the windowsill dust was enriched with Pb, Zn, Sb, and Cd. Metals such as As, Sb, Cd, Zn, and Pb are released from non-ferrous/ferrous alloy plants located in Mahmoudabad and Shokouhieh industrial zones. Arsenic also can stem from Cu-smelting slags around the city. Khorasanipour and Esmaeilzadeh⁴² stated that As in the topsoil came from Sarcheshmeh Cu-smelting slags via wind-blowing particles. Arsenic pollution in dust can also be attributed to household activities (like waste disposal and thermal system)⁴³ and construction work⁴⁴ (like paints, cement usage and demolition). Apart from smelting emissions, Sb can be derived from products like plastics and polyethylene terephthalate (PET), and textiles⁴⁵, all of which exist in the industrial section of the studied area. It is noteworthy that in today’s world, traffic pollution is an inevitable consequence of urbanization. Many previous studies also confirmed the presence of Pb, Zn, Cd, As, and Sb in the street dust of high traffic nodes^2,9,11. Lead, Zn, and As are correlated with motor vehicle exhausts and are widely used in automobile parts³³. Copper, Mo, Cd, and Sb are utilized in lubricating oils and automotive greases.

The second component (PCA2) involves Cr, Co, Fe, and Ni accounting for 13.46% of the variance. PCA 2 loaded heavily on Cr (0.92) and Co (0.86), with moderate loading on Ni (0.66) and Fe (0.70). Cobalt showed a remarkable positive correlation with Cr (0.82) and Fe (0.92), which reflects they stemmed from the same source. It is noteworthy that all the elements in this component were lower than the background value and had an EF < 1.5 which suggests the natural originality of the elements¹¹. Generally, a substantial proportion of Ni, Fe, Cr and Co comes from lithogenic materials deposited as dust in the atmosphere. However, 36% of Fe and 28% of Cr exceeded the background value. The correlation of Cr and Fe with Pb were respectively 0.87 and 0.86, indicating smelting activities as the potential source of Fe and Cr⁷. An iron/steel smelting plant is located in the Mahmoudabad industrial zone (Fig. 1) which could be the reason for increasing Fe and Cr concentrations in the windowsill dust of the industrial section. Consequently, Fe and Cr arise from a mixture of natural and anthropogenic sources (especially in the industrial section). Long et al.⁷ also reported Fe and Cr in the same PCA component with higher concentrations in the steel plant area compared to other functional areas in Panzhihua city, China.

The third component (PCA3) with a total variance of 10.06% included Mn (0.76). Mn had a weak to moderate correlation with the other studied PTMs. The manganese content in 95% of the samples exceeded the UCC value. Manganese had the lowest CV which shows this element was normally distributed in the studied area. Particularly, one station in a park had an extreme value that might be due to an accidental event; thus it was considered as an abnormal value. Fard et al.⁴⁶ also reported an approximately stable seasonal variation of Mn in the urban region of Qom. Venarj Mn mine, the largest Mn mine in Iran (1962–present), distances about 30 km from Qom city. The mining activities and related transportation disperse high amounts of Mn minerals into the atmosphere. Thus, PCA 3 might refer to the mine-associated sources in windowsill dust.

To sum up, finding the origination of PTMs in the dust is a laborious task, especially for contaminants with low concentrations. Descriptive statistics revealed three main routes of contamination in the windowsill dust: (1) a mixture of anthropogenic sources including industrial activities (alloys, chemical, textile and plastic productions) and urban activities (like traffic emissions—vehicle exhaust, bake and tire wear); (2) lithogenic/geogenic sources; and (3) mining activities (processing, transportation and waste).

Risk assessment

Ecological risk assessment

The average PERI values indicated that the samples from the industrial zone can be categorized as very high ecological risks (Fig. 6). PTMs in samples of all residential sectors and most greenspaces and commercial sectors had moderate ecological risk (about 62% of all samples). However, two points in the greenspace sector and the commercial sector and two points in commercial sectors demonstrated low ecological risk and considerable ecological risk, respectively; all of the four stations were located in the east of Qom.

In residential and greenspace sections, As had the highest contribution followed by Cd and Pb. While in commercial and industrial sections the highest contributions in RI belonged to Pb. Among the elements considered in RI, Cd had a low concentration with a remarkable contribution. Cd has the maximum T_r (30) which is related to its high half-life time of about 25–30 years⁴⁷. RI index was first introduced by Hakanson in 1980, and the contribution of modern contaminants like Sb has not been considered in the index. However, studies in the last two decades affirm the bioaccumulation and toxicological effects of Sb on humans⁴⁸ and the ecosystem⁴⁹.

Health risk assessment

PTMs in dust particles could cause health issues for local residents, especially sensitive individuals and children⁵⁰. The non-carcinogenic health risk of PTMs (HIs) through three possible pathways (oral ingestion, dermal, and inhalation) was measured for children and adults based on functional areas (Fig. 7, Table S8).

The average HI values of all PTMs regarding land-use type decreased in the order of industrial > commercial > residential ≈ greenspace. The five PTMs with the highest overall HI are ranked as follows: Pb > As > Cr > Mn > Sb (Fig. 7). The HI values in all the sections were lower than the permissible level (1.00), except for Pb. In the industrial section, Pb recorded the highest HI value for children (HI = 1.73) which exceeded the acceptable value. The HI values were 10 times higher for children than adults indicating they are more susceptible to PTMs in the dust.

The dominant pathway for noncancerous risk was ingestion followed by dermal contact and inhalation. The trend is in line with previous research^25,51,52. However, for Co and Mn, the descending order was different as follows ingestion > inhalation > dermal contact. The highest contribution of HQ_inh and HQ_derm to HI was measured for Co (34.0%) and Cd (29.0%), respectively.

In this study, the carcinogenic risk from windowsill dust was estimated for the carcinogens including Cd, Co, Cr and Ni, Pb, and As through the possible routes (Fig. 8, Table S9). The contribution of PTMs to CR decreased in the order of Cr (3.24E−05) > As (2.05E−05) > Pb (2.52E−06) > Co (6.91E−09) > Ni (1.72E−09) > Cd (2.58E−10). The average CR values for target PTMs through inhalation ranged from 7.9E−10 to 1.7E−07, which remained in the safety zone (CR < 1 × 10⁻⁶). The highest amount of CR belonged to Cr (4.9E−05) in the industrial zone, while pollution assessments identify the level of Cr pollution as uncontaminated in the windowsill dust. The high CR value of Cr mainly is related to the high slope factor (42 kg day/mg). Therefore, this is difficult to ascertain Cr cancerous effect considering its low speciation in the studied dust. Several studies on dust pollution have reported Cr as the highest contributor to carcinogenic effects^4,7,53. Rehman et al.⁵⁰ indicated that despite a moderate level of pollution, Cr in the indoor dust had a high CR close to the unacceptable range. Taiwo et al.⁴ also identified Cr as the highest contributor of CR in Nigeria road dust, while its concentration was lower than the background value.

The contribution of Pb and As to CR values in the windowsill dust regarding carcinogen routes was ingestion > inhalation > dermal (Fig. 8). While the contribution of Cr to carcinogenic risk was higher through inhalation than ingestion. The reports concluded that the primary exposure route of Cr is inhalation⁵⁴. Considering the predominant forms of Cr in the environment, Cr^VI is more toxic than Cr^III. Exposure to Cr^VI can cause immunological diseases, dental effects and carcinogenic effects (lung cancer, nose and nasal sinus cancer, suspected laryngeal and stomach cancers)^54,55.

The result of health risk from target PTMs in windowsills of Qom indicates significant chronic exposure to Pb can take place for children in the industrial zone. The ingestion route is the most probable pathway for children due to their hand–to–mouth behavior⁵⁶. Lead can bio-accumulate in the body without any obvious symptoms of toxicity⁵⁶. The total CR values for Pb, Cr and As in different land-use types were in the range of tolerable carcinogenic risk (1 × 10⁻⁴ < CR < 1 × 10⁻⁶), except for Pb in the greenspace and residential sections (CR < 1 × 10⁻⁶). Lung cancer and skin cancer show a clear correlation with As exposure through oral and inhalation routes⁵⁷. Moreover, Long-time exposure to As increases the risk of skin lesions and Blackfoot disease⁵⁷. Since urbanization and industrialization continue in Qom city, the carcinogenic risk of As, Pb, and Cr in the dust may exceed the safe level in the future. Furthermore, the present study only identifies possible exposure to PTMs via street dust, while exposure from other sources (like food, indoor dust, and soil) would aggravate the exposure and carcinogenic risks. United States Environmental Protection Agency (USEPA) has not yet included Sb in the category of carcinogen elements. However, reports confirmed the possible carcinogenic effects of Sb on animals⁵⁸ and humans due to long-term exposure⁴⁸.

limitations and uncertainties

Several limitations and uncertainties still exist in the present study. The concentration of HMs in dust may be influenced by meteorological conditions (such as rainwater and dust storm). Nevertheless, seasonal variations were not taken into account. Sampling was done from a height of 1–2 m above the ground. In order to investigate the effect of height on the concentration of HMs in dust, it is suggested to take samples from low-, medium-, and high-rise buildings. The possible concentration difference in various particle sizes is not considered. This study did not determine the bioavailability of metals in dust, an easy transport indicator in the food chain. The average upper continental crust (UCC) values were used as the background reference contents in this study due to the lack of background surface dust values. Other routes of exposure to HMs (contaminated soil, food, and water) and the presence of organic pollutants (e.g., microplastics and polycyclic aromatic hydrocarbons) have not been evaluated in the studied dust, although they would elevate the aggregation risk. Uncertainty is inherent in human health risk assessment even when the most accurate data and the most sophisticated models are used. Given the significant uncertainties associated with the estimates of toxicity values, exposure factors, and the absence of site-specific biometric factors, these results should be regarded as screening data. Further research should be undertaken before conclusions regarding potential health effects are drawn.

Source: Ecology - nature.com