INTRODUCTION
Urban areas in the main and even minor cities represent a base for all human activities, including transportation, construction, industry, and other activities1. Therefore, the pollution resulting from human sources in the urban areas are more than in the other regions2. Numerous studies have examined the types of pollutants, their distribution, their concentrations, and the dynamics of their spread from one site to another in urban areas. Multiple techniques were utilized such as satellite images, monitoring, and surveillance stations covering the most active areas of the cities. Furthermore, mathematical models were designed to calculate the daily and seasonal measures of the most affected areas by these pollutants3,4. Heavy metals are among the most important pollutants that have been studied, due to their severe toxicity and their clear impact on human health5-7. The levels of heavy metals in the urban environment are more than in other areas, which is due to different human activities that are concentrated there such as transport, mining, industry, use of pesticides, dyes and batteries, which are the essential sources of pollution of heavy metals8,9. Many previous studies were conducted to determine the spread of heavy metals in Baghdad and other Iraqi cities. Al Obaidy and Al Mashhadi10 investigated the levels of eight heavy metals (Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn) in the urban areas of Baghdad city for different land use (industrial, commercial and residential areas). Sultan et al.11 and Haleem et al.12 found high concentrations of heavy metals in dust samples carried in the dust storms that swept Baghdad city during repeated days of the year. While Essa and Al-Jibury13 detected four heavy metals (Cd, Pb, Zn and Ni) in the most crowded squares in Baghdad (Outer Karada, Al-Saadoon, Nidhal and Palestine streets); the findings indicated that the levels of the calculated pollution indices (geoaccumulation index, pollution index, carcinogenic and non-carcinogenic risks) for the heavy metals in the anthropogenic and side-road soils were affected by the emitted gasses from the vehicles’ exhausts and their fly ash13. The spread of heavy metals in this pattern and these concentrations may cause serious health problems for all exposed people especially pedestrians, workers, road sellers, and drivers who spend eight hours or more in the streets and are exposed to these pollutants by inhalation, ingestion, or direct skin contact14-16.
The two main objectives of this study were: 1) Evaluation of the concentrations of heavy metals on main side roads of the urban areas of Baghdad city and calculation of pollution indices such as geo-accumulation index (Igeo) and Integrated Pollution Index (IPI); 2) Assessment of the human health risk through chronic exposure to contaminated soil with heavy metals by calculating average daily dose (ADD), non-carcinogenic risk assessment, and carcinogenic risk assessment.
METHODS
Description of the study area
Baghdad city is the capital of Iraq and is located in the middle of the country. The Tigris River divides the city into two main areas, the west side (Al-Karkh) and the east side (Al-Rusafa). Baghdad is the largest and most populous city in Iraq, with about 8 million people in an area of 1000 km2 with a complex network of conventional and highway transport routes. Baghdad is characterized as a flat plain surrounded by low hills in its Northern and Northeastern parts. The land uses are varied, covering agricultural, industrial, commercial and residential use. It is expected that the soil nature will be different from one site to another depending on the type of activity.
Samples collection
A total of 110 dust samples were collected from the side roads of 22 areas within Baghdad city from July to October 2019, with five samples per site as shown in Supplementary file Figure 1. Samples were collected from the main roads of each area in which cars, trucks, motorcycles and buses are the main modes of transportation. We expected to find marked differences in the nature of pollutants and their concentrations in each selected site depending on the type of activity carried out in each area, such as commercial, industrial, agricultural, or residential.
All dust samples were collected from the main roads in urban areas with heavy traffic. Side-road dust samples trowel of up to 500 g were collected with a plastic brush and trowel from each location and were placed in self-sealed plastic bags and labeled with all the required information such as sample size, date of collection, location, and season. The samples were then taken to the laboratory (Soil and Water Laboratory, Environment Research Center, University of Technology)17. The collected dust samples were dried at ambient temperature and sieved through 2 mm mesh and stored in clean plastic bags for analysis.
Heavy metal concentrations were determined by taking 5 g of each sieved soil sample and mixed with 25 mL of concentrated HNO3, then placed on a hot plate for one hour18. The remainder of the mixture was filtered through 0.45 Millipore filter paper to a volume of 10 mL using a volumetric flask with deionized water. The concentrations of the heavy metals were analyzed using a flame atomic absorption spectrophotometer (FAAS, Shimadzu AA- 6200, Japan).
Quality control
All dust samples were tested in duplicate including filter blanks and Standard reference substances.
Chemical materials used for digestion and analysis have been supplied from Sigma Aldrich, USA. All measuring devices and equipment were calibrated by the Central Organization for Standardization and Quality Control (COSQC), Iraq.
Pollution assessment
The pollution levels of the heavy metals in the side-road dust samples were determined by calculating the Igeo and IPI indices. To calculate the Igeo index, the reference values of the heavy metals Pb, Zn, Cr and Ni were taken from the baseline values of Baghdad soils13, which are 36.31 mg/kg, 56.23 mg/kg, 12.9 mg/kg, and 123.03 mg/kg, respectively. All background values were used from uncontaminated soil samples which were collected from 50 cm depth to avoid soil surface contamination19,20.
Geo-accumulation index (Igeo)
Geo-accumulation index which was mainly developed by Muller21 in the 1960s as the most valued tool to quantify heavy metal contamination levels in sediments (Supplementary file Table 1). Later, this method was used to determine the heavy metal contamination in soil and dust samples.
The (Igeo) is calculated from the equation:
where C represents the measured concentration of the heavy metal and B represents the background value for the same metal in the soil.
Integrated pollution index (IPI)
The second index, that was applied to assess the contamination of urban soil, was the integrated pollution index (IPI), which is determined at each polluted site22. The IPI for each heavy metal was calculated from the equation:
where Ci represents the element’s concentration, while Bi is the background value of each element at the same study site. The integrated pollution index (IPI) represents the mean, and was calculated from the equation:
The IPI was rated as IPI≤1 for the lowest level of contamination, 1<IPI≤2 for a moderate level of contamination, 2<IPI≤5 for a high level of contamination, and IPI>5 for an extreme level of contamination23.
Health risk assessment
According to the accredited methodology of the risk assessment by the Environmental Protection Agency of the United States (US EPA, 1989), we evaluated the health risks based on the concentration of heavy metals in the study area, depending on the average daily dose (ADD) (mg/kg/day) of only the inhalation exposure pathways, using the following formula24,25:
where ADD represents the average daily exposure value of metals through inhalation (mg/kg/day), and Supplementary file Table 2 shows the exposure factors with EPA guidelines. The standards of EPA were adopted in this study.
The American model (USEPA, 1989) was adopted to calculate the health risks assessment of road dust for both adults and children, because there is no local model for this type of exposure. The effect of road dust on adults is expected to be greater than on children due to exposure hours, and the respiratory effect of heavy airborne elements as a result of dust in the streets is expected to be greater than its ingestion effect. The non-carcinogenic effect was calculated for four heavy metals (Pb, Zn, Cd and Ni), while the carcinogenic effect was determined for two elements (Cr and Ni).
Non-carcinogenic risk assessment
The non-carcinogenic risk assessment can be calculated for the studied metals by dividing the average daily dose (ADD) by a particular reference dose (RFD), as in the following formula:
while the HI represents the sum of the HQ for the four studied metals:
The threshold values adopted to determine exposure severity26,27 are: H≤1 non-significant risk for non-carcinogenic; and H>1 significant risk for non-carcinogenic.
Carcinogenic risk assessment
The lifetime average daily dose (LADD) for inhalation exposure routes for Cr and Ni was applied to evaluate the carcinogenic risk depending on the IARC classification list. To calculate the carcinogenic risk for the two metals, we used the following formula:
where all the factors are annotated in Supplementary file Table 2. The lifetime cancer risk can be calculated using:
where SF value ranges from 10-6 to 10-4.
RESULTS
Concentrations of Pb, Zn, Ni and Cr have been determined in the side-road dust samples from 22 areas within Baghdad city for different land-use areas (industrial, commercial, and residential). The maximum, minimum, and mean of total heavy metal concentrations are shown in Table 1. The distribution pattern of the four toxic metals (Cr, Ni, Pb and Zn) followed the same distribution in all sites whether industrial, commercial, or residential. It is possible that the high concentrations of toxic metals in soil samples on the side road in urban areas may be due to the leakage of heavyduty fuel, as well as the wear of the tyres, motors and brakes, battery wastes and engine emissions may add high amounts of heavy metals to the dust.
Table 1
Heavy metal concentrations (mg/kg) and geo-accumulation index of heavy metals in 22 side-road dust samples collected from selected areas of Baghdad city
Contamination indices
Geo-accumulation index
The maximum, minimum, and mean values of Igeo were calculated for each study site and given in Table 1. All Igeo values of the study samples were <1, which refer to uncontaminated soil. The Igeo values of the four metals were as follows: Zn ranged from 0.042 in Baya’a to -0.152 in Al-Ghadeer, Pb ranged from -0.0165 in Sheikh Omar to -0.416 in Baya’a, Ni values ranged from 0.082 Sheikh Omar to -0.219 Za'franiya, while Cr values ranged from 0.158 in Sheikh Omar to -0.235 in Hayy Al-Jami'a. The Igeo indicates a logarithmic relationship to measure the degree of soil contamination, and thus the minus sign for any value expresses the absence of influence of pollution sources in the soil. This reflects the non-participation of the geological source of contamination of those soils with heavy metals and the concentrations of these metals from anthropogenic activities. The range of the four heavy metals in Iraqi soils were Pb range 20–45 mg/kg, Ni range 40–100 mg/kg, Cr range 4–810 mg/kg with a mean value 180 mg/kg, while for Zn the range was 20–117 mg/kg with mean 56 mg/kg. In Equation (1), we adopted a background value from the soils, at a depth of 50 cm in urban areas of Baghdad city, to avoid the large variation in the index of geo-accumulation. It is important to note that the mean values of the four heavy metals in the Baghdad soils were: Pb, 36.31 mg/kg; Zn, 56.23 mg/kg; Cr, 12.9 mg/kg; and Ni, 123.03 mg/kg. Most of the metal concentrations were within the normal range of their distribution in Iraqi soils, except for Ni, its concentration was slightly higher than the background.
Pollution index (PI)
The values of the pollution index for each metal (Cr, Ni, Pb and Zn) in the side-road soils for different areas of Baghdad city are given in Table 2. The pollution index values for Cr ranged from 0.0 in the Al-Ghadeer to 1.44 in Sheikh Omar, while for Ni, the values ranged from 0.0 in Al-Taji to 1.82 in the Sheikh Omar area, whereas for the Pb, the values ranged from 0.0 to 1.9, and for the Zn, the values ranged 1.05–1.74 in the Karradah. It is worth noting that all the pollution factor values were within the second level of contamination (1≤PI<3). Contamination from low to moderate, and the highest pollution index was recorded in the Sheikh Omar and Baya’a sites due to the presence of industrial activity for the maintenance of vehicles and diesel engines, as well as heavy traffic during the hours of the day.
Table 2
Pollution index of heavy metals in side-road dust samples of all studied areas in Baghdad city
Health risk assessment
The present study showed high health risks for adults through the inhalation route than for children, which can be associated with high exposure duration and inhalation rate of adults than in children.
Non-carcinogenic risk
Tables 3 and 4 present the HQ and HI values for noncarcinogenic health risks for tested metals. For adults and children, the poising metals take the following order (Cr > Pb > Ni > Zn). The highest value for HI for the adults was 0.0028 while the least value was 0.0013. The highest value for HI for the children was 0.001 while the least value was 0.0007. From the above results, we notice that HQ< 1, so all data are <1, with the safe level USEPA indicating low health risks from road dust in the studied areas.
Table 3
HQs and HIs from heavy metals given with their concentrations and reference dose in the studied sites’ soils (adults)
Table 4
HQs and HIs from heavy metals given with their concentrations and reference dose in the studied sites’ soils (children)
Carcinogenic risk
Carcinogenic risk can be defined as the probability of developing any type of cancer in a person from lifetime exposure to carcinogenic hazards16. In this study, the cancer risk for Ni and Cr was assessed by calculating the lifetime average daily dose (LADD) for the inhalation exposure route and the results are given in Table 5. The highest values for RCr (1.08) and RNi (540.02) were recorded in the Sheikh Omar site, while the lowest values for RCr (0.05) were recorded in the Hayy-Al-Jamia site, and for RNi (271.75) for the Abu Ghraib site.
Table 5
Carcinogenic risks for metal elements in soil collected from areas of Baghdad city
DISCUSSION
Our results reveal that the Sheikh Omar site recorded the highest mean value of heavy metal concentration and Hayy Al-Jami'a recorded the lowest mean value. This is due to the nature of the land use and the overpopulation of each area; Sheikh Omar is located in the center of Baghdad city, with high industrial activity, high populated area and heavy traffic, thus all these reasons may explain the higher concentrations of the heavy metals. In contrast, Hayy Al-Jami'a is a residential neighborhood and shopping area with weaker traffic.
Lead concentration in Iraqi soils varies depending on the parent rock and weathering erosion; lead (as Pb2+) forms carbonates, joined in clay metals like Fe and Mn oxides and in organic matter28. In Mesopotamia, the highest concentration of Pb is recorded in mid Euphrates and further south toward the marshland. Lead records high rates in Iraqi soils (urban and rural) because it is added to engine fuel to reduce the octane number; this is the main source of soil pollution of this toxic metal29. Zinc concentrations are also correlated to parent rock more than to pedogenic processes. Its concentrations increase significantly in arid regions and in saline-alkaline soils30,31. Zinc is replaceable with Mg2+ in silicate and has good mobility in acid oxidizing soils32. Urban soil contamination by Zn is closely associated with industries and the use of composted materials, fertilizers, and pesticides. Zn also results from mechanical abrasion of vehicles, fuel spills, and abrasion of brass alloy32,33. Other reasons associated with the addition of Zn to the urban soils is fertilizing, traffic, vehicle emissions, and tyre abrasion31-33. The results of the present study are in agreement with many previous studies that measured toxic metals in Baghdad soils34. Chromium highest level was recorded in the arid and semi-arid areas, reaching 2400 mg/kg. Chromium exists as Cr3+ in many soils, has slow mobility except in very acid environments, and its compounds have high stability in soil31. Usually, the parent rock determines its concentration in soil. In the North of Iraq (mountainous area), Cr concentrations are higher than the background value mainly due to the composition of the source rocks29.
The observed concentrations were higher than the world average value (180 mg/kg); this may be due to its slow mobility in soil, so it will stay in the upper surface, but, at depth, its concentration is low (12.9 mg/kg). The Cr average value (247.36 mg/kg) is greater than that of chromium in the rural soil (17.33 mg/kg), about 14.27 times.
Nickel (Ni) is a well mobilized metal and precipitated with Fe and Mn oxides, or organically attached in soil, this explains the presence of high concentrations of Ni in the deep soil. The Ni presence in soil is strongly determined by climate and parent rock components. Its levels are high in arid and semi-arid regions, especially in saline-alkaline soils; in urban areas Ni is added to the soil as a result of continuous human activities, because it is widely used in electroplating and battery manufacturing30,31.
CONCLUSIONS
Road dust is a source of many dangerous pollutants for residents of urban areas, due to its high toxicity and wide exposure by inhalation, ingestion, or by skin penetration. In this study, we determined the concentrations of four toxic metals (Cr, Zn, Ni and Pb) in road dust from 22 sites of Baghdad city with different land uses. Pollution rates with the four toxic metals were characterized as being moderate to low in all sites of the capital. Despite the low to moderate pollution rate, it gave high toxicity indicators for exposed people, this may be due to the synergy of more than one type of pollutant in the severe or high toxicity, and that heavy dust-borne toxic metals are not the only ones responsible for the toxicity.
