1. Introduction
The upward trend in economic growth, industrialization, rising energy consumption and urbanization with weak regulatory instruments has caused unprecedented environmental pollution [
1]. This points to the need for more research to provide a basis on which policy formulation and enforcement are grounded to mitigate pollution [
2,
3,
4]. According to recent estimates by the World Health Organization (WHO), approximately 99% of the world’s human population breathe air with pollution parameters surpassing WHO permissible limits [
5]. From this standpoint, air pollution has attracted research interest because it has been incriminated in the etiology, progression and aggravation of airborne diseases (e.g., COVID-19, tuberculosis, ischemic heart disease), as well as neurodevelopmental and neuropsychiatric conditions, e.g., depression, bipolar and autism spectrum disorders [
6,
7,
8]. Moreover, the WHO has substantiated that the burden of diseases emanating from inhalation of polluted air is currently almost equivalent to those from major global health risks such as unhealthy diets and tobacco smoking [
9].
Air pollutants are known to contain a cocktail of hazardous materials, including aerosols of toxic elements (such as mercury, arsenic, chromium, lead, cadmium and nickel); pathogenic bacteria, viruses and fungi (yeast and molds); ozone; pollen; particulate matter; and acidic oxides [
10]. Among atmospheric contaminants, particles in sizes less than 2.5 µm or 10 µm have been a subject of intensive research [
11]. However, studies on the occurrence of particulates with larger grain sizes, e.g., dusts from roads, streets, highways, indoor environments and bridges, have not been explored in most countries [
12,
13,
14]. One of the contaminants of concern in particulate matter is potentially (eco)toxic elements (PTEs). Available literature shows that exposure to PTEs, especially heavy metals (HMs): copper (Cu), iron (Fe), chromium (Cr), mercury (Hg), nickel (Ni), cadmium (Cd), zinc (Zn), lead (Pb) and arsenic (As) in roadside, foliage and pedestrian bridge dusts can pose health threats [
15,
16]. In most reports, the background concentrations of the elements in roadside dusts surpassed their mean concentrations in the upper continental crust. Nazzal et al. [
17], for instance, found HMs in dusts from some Canadian highways to range from 0.51 to 40,052 mg kg
−1. Similar studies in the tourist city (Guilin) of China [
18], Central Scotland [
19], Dhanbad (India) [
20] and Luanda (Angola) [
21] have reported HMs in street and roadside dusts at levels above background concentrations.
PTEs can exert deleterious health effects. Zn, for instance, is an indispensable ion that is required in multiple metabolic processes, including protein synthesis and immunity construction. In addition, it constitutes over 1000 transcription factors and is a structural and regulatory component of up to 300 in vivo enzymes [
22]. However, excessive intake of Zn induces diarrhea, inappetence, abdominal cramps and headache. Chronic effects, such as low Cu status, altered Fe function and reduced immune function, have also been associated with excessive Zn intake. In supplements, continuous intake of Zn by men was cited to potentially increase the risk of and mortalities from prostatic adenocarcinoma [
23]. For the most part, HMs exert their toxicity through inducing reactive oxygen species generation, oxidative stress and DNA wreckage; enzyme inactivation; and disruption/weakening of the antioxidant defense system. On the other hand, some HMs mediate their toxicity through selective binding to specific macromolecule or cellular reductants, producing intermediates responsible for Fenton-type reactions that generate free radicals [
24]. Overall, the toxicity of PTEs depends on their bioavailability, ionic forms (valence states), chemical forms, dose, route and time of exposure, as well as the age, sex and nutritional status of an organism or individual. For instance, Fe can induce cell death by generating free radicals as it interconverts between ferrous (Fe
2+) and ferric (Fe
3+) forms [
25]. Similarly, Cr in the trivalent state (Cr
3+) is less mobile and toxic than the highly mobile hexavalent chromium (Cr
6+). Water-soluble hexavalent Cr is extremely irritating and toxic to human tissues as its solubility facilitates ionic active transport across biological membranes [
26]. Other examples are cited herein with (i) mercury (Hg), which is capable of exerting toxic effects in its vapor (elemental or metallic mercury; Hg
0) form. It is most toxic in its organic form as methyl or ethyl mercury (Me-Hg and Et-Hg) than the vapor and inorganic mercury forms (Hg
+ and Hg
2+) [
24]. Additionally, another example is (ii) arsenic, which exists as a metalloid (As
0), inorganic (As
3+ and As
5+), organic arsenic and arsine (AsH
3), with known toxicity arranged as organic arsenicals < As
0 < inorganic species (As
5+ < As
3+) < arsine [
24].
There has been increased publicity on the occurrence of PTEs in road and street dusts of megacities [
27]. In Africa, however, there are hardly any reports on the levels of PTEs in highway and pedestrian bridge dusts, except for some earlier reports from Angola [
21], Ghana [
28,
29] and Nigeria [
10]. Given the exponential rise in urbanization and industrialization activities, there is potential exposure to PTEs in dusts especially by pedestrians, cyclists and children. This study was therefore undertaken to quantify the levels, sources and the associated health risks of PTEs in dust from selected urban roads, highways and pedestrian bridges in Uganda.
2. Materials and Methods
2.1. Study Area
This contribution considered Eastern Uganda as a case study. It was of interest in this study for several reasons. First of all, it is made up of 32 districts, which represent 23.9% of the total number of districts in Uganda. Secondly, it is densely populated, with an estimated number of 10,836,500 people as of 2020 [
30]. Thirdly, this region has Malaba and Busia One-Stop Border Posts, which are by far the busiest border crossings on the Northern Corridor [
31,
32]. Thus, roads, highways and bridges in this region has one of the highest traffic in Uganda, especially with cargo trailers from Mombasa port (Kenya), which handles roughly 60% of the regional imports. Both imports and exports for Democratic Republic of Congo also pass through these border posts [
33]. Fourthly, the region has some distinct tourist attraction sites in Uganda, such as the source of the Nile River (the longest river in the world) and Itanda falls, with six whitewater rafting levels in Jinja, Nabugoye Synagogue and Mt. Elgon (with the largest volcanic base of 4000 km
2 in the world) in Mbale district. Other attractions include Nyero Rock Paintings (Uganda’s oldest rock-art site) in Kumi district, the Tororo rock (with various caves and ancestral paintings) in Tororo, Kagulu hill (a 3048 m high rocky prominence) in Buyende district, Sipi falls in Kapchorwa district, and Dolwe Island (Jewel of Lake Victoria) in Namayingo district [
34]. These tourist activities could potentially increase emission of PTEs due to increased vehicular activities. Fifth, the region has three prominent cement-producing factories in Tororo district (viz; Tororo Cement, Simba Cement and Hima Cement), which release industrial smoke, cement and quarrying dusts directly into the atmosphere [
35,
36]. According to Guinness World Records, Tororo is the most thundery place on earth as it thunders 251 days per annum [
37,
38]. This can increase weathering processes and hence the release of PTEs into the atmosphere as well as facilitate their long-range transport. Lastly, the region has been the epicenter of flash flood inundations and landslides, especially in the Manafwa watershed [
39,
40,
41].
The dusts were sampled from selected urban roads, highways and pedestrian bridges constructed over Manafwa River (0.9420° N 33.920° E), the longest river that flows through Mbale and Butaleja districts of Eastern Uganda (
Figure 1). The river is fed by various tributaries (such as Sala, Liisi, Wukha, Tsutsu, Pasa, Kufu, Nambale and Makhuba) and small streams from the transboundary Mt. Elgon [
42]. Manafwa River is about 14.63 km long, and its water quality has been degraded due to economic activities such as sand mining and frequent floods.
2.2. Sample Collection
To minimize the effects of turbulence from rain and wind, the samples were taken following seven days of continuous sunshine [
18] in December 2019. Protective gears (face masks, gloves and an overall) were worn to eliminate potential exposure to the HMs in the dust samples. A total of 24 dust samples were taken in triplicate (
Figure 1 and
Table 1) from selected urban roads, highways and pedestrian bridges built over Manafwa River (viz; Manafwa historic bridge, Bushika bridge and Bubulo bridge). Briefly, the samples were collected from areas of 2 m
2 to 10 m
2 of roadside/pedestrian bridge areas. Each sample was approximately 100 g of the settled dust on cemented floor of the bridge pavement or highway side. Samples were collected using plastic brushes and scoops, and then wrapped in aluminum foil. They were transferred into polyethylene bags and taken to Kasese Cobalt Company analytical laboratory for spectroscopic analysis.
2.3. Spectroanalytical Procedure
Samples were initially dried at 100 ± 5 °C for 5 h in an oven and later cooled in a desiccator. Measured 1 g of the samples were digested with 20 mL of aqua-regia (1:3 v/v HNO3:HCl) until dryness. Thereafter, they were diluted to 10 mL with 1% nitric acid. The resultant mixture was cooled and filtered through Whatman filter papers into 50 mL sample vials. The resultant solutions were analyzed using atomic absorption spectrophotometer (AAS, Perkin Elmer Analyst 100) to determine the concentration of seven selected PTEs: Cu, Ni, manganese (Mn), Zn, Pb, Cd and Cr. The wavelengths used were 324.7, 232.0, 279.5, 213.9, 238.3, 357.9, 228.8 and 248.3 nm, respectively.
Working standards prepared from dilution of 1000 ppm stock solution of the nitrate and chloride salts of the PTEs were used to construct calibration curves. The concentration of the elements in the digests were determined from the calibration curves in mg/L, and thereafter converted into mg kg−1. The curves had acceptable linearity (R2 > 0.995). Quality control in the analysis was achieved by analyzing procedural blanks and spiked samples, and the recoveries obtained were in range (96 to 101%). The relative standard deviations from the experiments (as a measure of analytical precision) ranged from 3.1% to 4.8%. The method detection limits (in mg kg−1) were 0.01, 0.03, 0.05, 0.02, 0.05, 0.12, 0.002, and 0.08 for Cu, Ni, Mn, Zn, Pb, Cd and Cr, respectively.
2.4. Assessment of Dust Contamination and Toxicity Levels
To date, the undeniably most explored indices for evaluation of contamination risks and pollution extent of dusts are contamination factor, enrichment factor, geo-accumulation index, pollution load index and potential ecological risk index. They are, by and large, computed with respect to the crustal concentrations of the metals. In consequence, such assessments are invariably reliant on the values of the crustal concentrations chosen. Due to paucity of data on background values of PTEs in roadside dusts from Uganda, the concentrations used herein are global averages of the upper continental crust reported by previous authors [
43,
44], which respectively are 38.9, 29.0, 571.0, 64.0, 27.0, 0.40 and 59.5 mg kg
−1 for Cu, Ni, Mn, Zn, Pb, Cd and Cr.
Thus, the contamination factor (CF) was computed using Equation (1) advanced by Hakanson [
45]. Secondly, the index of geo-accumulation (I
geo), applied by Müller [
46] in the first instance, was also used (Equation (2)).
From which
Cd = element concentration in the sample,
CB = continental crustal average value of the same element and 1.5 is the background matrix correction factor [
47,
48]. The classification values and pollution extents based on CF and I
geo are detailed in
Supplementary Materials Table S1. For I
geo, the PTE pollution is categorized into seven distinguished enrichment classes (0 to 6), corresponding with normal background value to extreme metalliferous contamination.
To establish the cumulative pollution load in a particular dust sample, the pollution load index (PLI) was computed (Equation (3)).
From which CF1 to CF7 are the contamination factors for the seven PTEs.
Finally, the potential ecological risk index technique established by Hakanson [
45] was applied in this study. It was used to indicate the sensitivity of the biotic community to the PTEs and exemplifies the risk index caused by the overall contamination. The potential ecological risk coefficient (
) and potential ecological risk index (PERI) were computed (using Equations (4) and (5)) to obtain a clear evaluation of the ecological risks related to the PTE contamination by dusts per sampling site.
wherein
= biological toxic factor of PTEs: Cu = Ni = Pb = 5, Zn = Mn = 1, Cd = 30 and Cr = 2 [
20]. The risk characterization criterion is elaborated in
Supplementary Materials Table S1.
2.5. Assessment of Potential Human Health Risks
Health risk assessments establishes the relationship between the environment and human health. Herein, the assertions detailed by the US Environmental Protection Agency (US EPA) [
49] were assumed (
Supplementary Materials Table S2). The risks were categorized into carcinogenic and non-cancer risks. The average daily doses (mg kg
−1 day
−1) were estimated to discern human exposure through direct ingestion (ADD
ingestion), inhalation (ADD
inhalation) and dermal contact (ADD
dermal contact) with the dust samples (Equations (6)–(8)) [
50,
51].
The hazard quotient (HQ) was calculated to establish non-cancer risks from the PTEs (Equation (9)). Since contaminants like PTEs can elicit augmentative effects, the hazard index (HI) was calculated (Equation (10)) [
51].
where R
fD is the oral (direct ingestion), inhalation or dermal reference dose of the specific element (
Supplementary Materials Table S3).
Carcinogenic health risk (CR) estimated as the incremental lifetime cancer risk for the carcinogenic elements (Ni, Pb, Cd and Cr) (Equations (11)–(13)) [
21,
51,
52].
where IRF is the chronic inhalation unit risk factor = 1.5 × 10
−4, 1.2 × 10
−5, 4.9 × 10
−4 and 1.2 × 10
−2 per μgm
−3 for Ni, Pb, Cd and Cr, respectively [
52,
53,
54]; CSF is the ingestion cancer slope factor for respective metal under consideration = 3.0 × 10
−4, 8.5 ×10
−6, 5.0 ×10
−4 and 3.8 ×10
−4 mg kg
−1 day
−1 for Ni, Pb, Cd and Cr, respectively [
55].
2.6. Statistical Analysis
Quantitative data from experimental analyses performed in triplicate were captured in Microsoft Excel 2016 (version 2211, Microsoft Corporation, Washington, DC, USA) where they were averaged and expressed as means ± standard deviations of replicates. Significant differences in the mean concentration of the PTEs among the sites was established using one-way analysis of variance, followed by Tukey’s test. Pearson’s bivariate correlation analysis was performed to test for the association between the PTEs in the dust samples. Principal component analysis (PCA) was harnessed to group the PTEs and apportion their origins [
13]. Further, PCA components were transformed using a varimax rotation with Kaiser normalization post analysis. Cluster analysis was harnessed to establish the different geochemical groups by clustering samples with similar PTE content. The analyses were executed at 95% confidence interval in GraphPad Prism statistical software (version 9.3.1, GraphPad software, San Diego, CA, USA).
4. Conclusions
This study showed that there is severe PTEs pollution of dusts from roads, highways and pedestrian bridges in Eastern Uganda when compared with their crustal averages, with the exception for Cu, Ni and Cr. Though, there is no Ugandan standard for PTEs concentration in road dusts, the concentrations of the elements found in Eastern Uganda are relatively lower when compared to previous studies around the world. Source apportionment studies suggests that the higher PTEs concentrations are due to a combination of both anthropogenic (combustion processes and vehicular traffic) and geogenic contributions. With regards to CF, there is low to very high contamination of dust from the studied parts of urban roads, highways and bridges, particularly with regard to Pb, Mn and Cd. Assessment using PLI indicated that dust from Zikoye–Bushika road, the junction of Zikoye–Bushika and Bududa–Manafwa roads and Manafwa town were substantially polluted as the indices were greater than 1. Further assessment of pollution degree of the dust samples using Igeo revealed that the dusts showed practically uncontaminated to medium-to-strong contamination. Health risks assessment showed that there are discernable non-carcinogenic health risks that could arise from ingestion of PTEs in dusts from the studied areas of Eastern Uganda by children. This study, therefore, opens the lead for investigation of the occurrence and health risks from PTEs in road dusts of busy Ugandan cities, highways and industrial areas.