Transport Dust in Poland: Tracking the Invisible Footprint of Transport on Ecosystem Health

Abstract

Urban road dust (URD) is a major source of particulate matter (PM) and pollutants, including trace elements and organic compounds, affecting human health and the environment. This study investigates the chemical composition, toxicity, and environmental transport mechanisms of URD from road and rail systems in two Polish cities. It compares trace element concentrations (e.g., Cu, Zn, Pb), chemical composition, toxicity of road vs. rail dust, and the impact of rainfall on contaminant dispersion. The oral pathway was identified as the main exposure route in both adults and children, followed by that of dermal contact. Railways pose additional challenges due to frequent maintenance and increased PM emissions. Results show that smaller cities like Rawicz may present higher health risks from URD due to local industry (e.g., metal processing) than larger cities like Wrocław. Rainfall mobilizes trace elements in urban dust, increasing pollutant runoff and exposure risks, highlighting the need for better runoff management. The highest road-related pollution was found in Rawicz (S5), with the highest railway-related pollution also found at the Rawicz station. Microtox showed no toxicity in Wrocław URD (except for short-term effect) but higher toxicity in Rawicz. Daphtoxkit showed the highest Daphnia magna mortality near roads (40.0%) in Rawicz. Ostracodtoxkit revealed strong growth inhibition in Wrocław (up to 94.29%). ECR confirmed a higher cancer risk in Rawicz, especially in children (Cr, As).

1. Introduction

Dynamic urbanization increases the emission of particulate matter (PM) and associated compounds, such as trace metals, organic compounds, and other pollutants [1,2]. Although road and railway networks are crucial for economic development, they are a significant source of air pollution and health risks. Emissions originate from both the operation and maintenance of road and railway infrastructure [2,3]. Road dust contains, among others, PM10, PM2.5, Pb, Mn, Cr, Fe, Cu, Cd, Zn, Ni, and PAHs [4,5] and is rich in trace metals (e.g., Pb, Cu, Zn), mainly due to tire, brake, and road surface wear [6]. Railway dust has different characteristics; it is dominated by mechanical wear particles (e.g., Fe, Mn, and Cr), although it contains fewer organic compounds [7]. URD has a wider range of impact, whereas railway dust has a local but long-term impact on the soil environment [7,8].

Studies in New York City have shown that subways are the main source of exposure to dust containing Fe, Mn, and Cr [9]. For adults and children, the main route of exposure to road dust is ingestion, followed by dermal contact [8]. Road and railway dust also negatively impacts the environment [10]. Railway infrastructure requires more frequent maintenance than road infrastructure—especially ballast replacement, which generates significant amounts of dust [3]. Dust samples from railway tunnels mainly contained Fe (ca. 45%), as well as Al, Cr, Ti, and Zr, which are used to increase hardness [11]. Other studies have shown that road traffic emits more trace metals than railways, and the highest concentrations of Cu, Ni, and Hg were recorded near highways [2].

According to the classifications of the Environmental Protection Agency (EPA), International Agency for Research on Cancer (IARC), and National Institute for Occupational Safety and Health (NIOSH) cancer guidelines, these dust emissions can be potential causes of carcinogenic impacts on humans [12,13]. Koh & Kim found a positive correlation between the organic carbon and metal content (Ni, Cr, and Zn) of road dust and skin and lung cytotoxicity [14]. Moreover, road dust causes respiratory tract inflammation (ex., asthma), cardiovascular system, central nervous system, and digestive tract [15,16,17].

Stormwater, flowing onto impermeable surfaces such as roads and pavements, transports pollutants to sewage systems and surface waters. This process, known as surface runoff, is a crucial mechanism for transporting substances in urban environments. Studies have shown that rainfall intensity and characteristics significantly impact the mobility and dispersion of pollutants [18]. For example, heavy rainfall can lead to the so-called “first flush” effect, in which the initial runoff contains the highest concentrations of pollutants. Furthermore, the presence of buildings and urban infrastructure affects the direction and speed of runoff, which, in turn, determines where pollutants accumulate [19]. Precipitation and strong winds tend to deposit dust particles into aquatic environments, leading to sedimentation and metal contamination, which subsequently adversely affect aquatic organisms [20,21]. Yang et al. found that seasonal variation is highly influenced by the spatial pattern of Cr and Hg, and the overall risk of potentially toxic elements is high in winter [22,23]. Ambient temperature, water solubility of the elements, and changes in anthropogenic activities also critically affect seasonal changes in road dust toxicity [23].

Therefore, there is an urgent need to investigate the chemical composition of urban road and rail dust, its transport mechanisms in the environment, and the impact of rainfall on the dispersion of pollutants. Such research is crucial for developing effective strategies to manage the quality of urban environments and protect public health. Moreover, systematic comparisons of the ecological impacts of road and railway dust, particularly their effects on soil and aquatic biodiversity, are limited.

The studies focused on two Polish cities: Wrocław, one of the largest cities in Poland, which is an example of a large urban agglomeration with complex transport infrastructure and diverse sources of pollutant emissions ([GUS], 2023) [24], and Rawicz, a smaller industrial city, representing a typical medium-sized urban center in which local emission sources, such as road transport and industrial plants, play a dominant role (Rawicz Municipal Office, 2021) [25]. The selection of these two locations allowed for the analysis of pollution problems under conditions of diverse urban development and can be related to the broader context of cities in Central and Eastern Europe (European Environment Agency [EEA], 2022) [26].

In light of the above, based on the literature and initial observations, we hypothesize that:

(1)

URD from smaller industrialized cities (e.g., Rawicz) contains a higher concentration of trace elements and poses greater toxicity than that from larger cities.

(2)

Rainfall and urban maintenance practices (e.g., street cleaning) significantly affect pollutant mobility, potentially increasing exposure through first flush effects and resuspension.

Accordingly, the aims of this study were as follows:

• Compare the concentrations of trace elements (in particular Cu, Zn, Pb) in dust from road and rail transport in urban conditions (we assessed two cities in Poland).
• Determine the differences in the chemical composition and toxicity of dust depending on the type of transport route (road vs. rail), although the direct comparison of dust from rail and road was not possible due to the use of different toxicity tests at each location and the different nature of the samples, which prevented a standardized, parallel comparison between the two types of transport.
• Investigate the effect of rainfall on the mobility and movement of pollutants in URD.
• Assess the “first flush” effect in the context of the intensity of pollutant transport during the initial phase of rainfall.
• Identify the possible health risks of road and rail dust in humans. Additionally, the possible impact of street cleaning was studied to understand the effect of URD distribution on pollution levels.

2. Materials and Methods

2.1. Description of Study Sites

In this study, we chose two different types of cities: Wrocław, one of the largest cities in Poland, and Rawicz, a smaller industrial town. Wrocław struggles with air pollution, especially during the heating season, with the main sources of emissions being the burning of solid fuels (about 70% of pollution comes from individual heat sources, such as coal and wood furnaces), and road transport, which is responsible for over 20% of suspended PM emissions. Despite the lack of heavy industry, Wrocław experiences industrial emissions from electrical power and heating plants, as well as smaller industrial facilities, particularly during the heating season [23,27]. Rawicz, although smaller, also experiences air quality issues, especially during calm weather and the heating season. The main sources of emissions are individual solid fuel boilers and local transport. Additionally, emissions may be elevated due to local industries such as metalworking, construction, clothing, agri-food processing, and car accessories manufacturing [28].

The sampling points in Wrocław were chosen to represent different urban environments and traffic intensities (Table S1 and Figure 1). Legnicka Street (site 1) is a major exit route with heavy traffic, reaching 2500 vehicles per hour during peak times [29]. Wrocław Mikołajów Railway Station (site 2) is a key transit hub, serving 2500 passengers daily. Popowicka Park (site 3) is an urban green area near a busy road with 1500 vehicles per hour. The Poznańska-Legnicka intersection (sites 4a and 4b) is a high-traffic zone with up to 3000 vehicles per hour. It was sampled under both dry conditions (with a long period of drought lasting one month) and after heavy rain to assess the impact of precipitation on the pollution levels. All these sites provide a broad assessment of pollution levels across different urban settings.

The sampling points in Rawicz were selected to represent different urban and transport-related environments with potential pollution sources influenced by local traffic and industrial activities (Table S1, Figure 1). Site 1 ab (Śląska Street, Railway Station), near the railway bridge and Rawicz railway station, experiences 390 vehicles per hour and frequent train traffic (25,000 trains annually). Site 2 abc (Tadeusza Kościuszki Street, Green Areas), near city landmarks, is affected by road transport (290 vehicles per hour) and heating emissions, while green areas contribute to natural dust accumulation. Site 3 ab (Sarnowska Street) is a busy intersection near a shopping center with 760 vehicles per hour, including heavy-duty vehicles. Site 4 a and 4b (Road S5, viaduct), located near the S5 expressway, with 1500 vehicles per hour. Additionally, the samples were collected after a fire at an industry, which may have further impacted pollution levels in the area.

2.2. Sample Preparation and Metal Concentration Assessment

Samples were collected using clean brushes and sterile swabs to avoid contamination. At each sampling location, three subsamples were collected from adjacent surfaces (approximately 1 m² each) and homogenized to form one representative sample per site. The sampling campaign was conducted in July 2022 under consistent environmental conditions, including low wind speeds (below 2 m/s), comparable ambient temperatures (21–25 °C), and at the same time of day (between 9:00 and 11:00 a.m.) to minimize temporal variability. For ecotoxicological testing, sample preparation was adapted to the requirements of each bioassay. To simulate urban runoff conditions for the Microtox and Daphtoxkit F tests, a water extract was prepared by mixing one part of urban road dust (URD) with two parts of distilled water (1:2 w/v), following the procedure described by Watanabe et al. [30]. The suspensions were placed in sterile containers and shaken using a DLAB SK-0330-PRO orbital shaker (DLAB Scientific Inc., Beijing, China) at a frequency of 150 rpm for 24 h. After shaking, the mixtures were allowed to settle for 2 h to ensure complete sedimentation of suspended solids. The resulting supernatants were filtered through 0.45 μm cellulose membrane filters. The filtered aqueous extracts were then used for further ecotoxicological analyses. In contrast, the Ostracodtoxkit F test requires direct contact between the test organisms and the solid dust matrix. For this purpose, the URD samples were air-dried, homogenized, and sieved through a 2 mm mesh to remove coarse particles. The prepared dust was placed into test containers and moistened with standard reconstituted water, following the instructions provided with the test kit.

The URD collected in the laboratory was subjected to a series of preparatory steps, including weighing, homogenization, drying, and sieving using a 2 mm mesh sieve to remove larger dust particles. A total of 20 g of sample from each of the 14 collection stations was weighed and divided into two equal portions of 10 g each, with one part used for mineralization to determine the concentration of selected trace elements. The trace elements in the collected dust samples were determined using atomic absorption spectrometry (AAS), an analytical technique based on the absorption of radiation by free metal atoms. Each metal absorbs radiation at a specific wavelength, allowing the quantification of its concentration by measuring the absorbed radiation. The analysis process began by drying the dust samples at room temperature. Then, using a Shimadzu AUW120D analytical balance (Shimadzu Corporation, Kyoto, Japan), an approximately 0.2 g portion of dust was precisely weighed from each sampling site for further analysis. The prepared weighed samples were placed in Teflon vessels and treated with 65% nitric acid. The vessels were then placed in a sealed microwave digestion system (Start D Milestone), where the temperature was increased to 220 °C, and the samples were exposed to 800W microwave power. This process facilitated the decomposition of organic matter present in the URD samples. After mineralization, the extracts were used to determine the total concentrations of copper (Cu), zinc (Zn), cadmium (Cd), chromium (Cr), lead (Pb), iron (Fe), arsenic (As), and nickel (Ni). The analysis was conducted using Flame Atomic Absorption Spectrometry (FAAS) with a Thermo Solaar iCE 3500 spectrometer (Thermo Fisher Scientific Inc., Waltham, MA, USA) employing an air-acetylene flame. The typical limits of detection (LOD) for the method were as follows: Cu—0.003 mg/L, Zn—0.005 mg/L, Cd—0.001 mg/L, Cr—0.005 mg/L, Pb—0.007 mg/L, Fe—0.01 mg/L, As—0.01 mg/L, and Ni—0.006 mg/L. The entire procedure was carried out in accordance with the PN-ISO 11047:2001 standard [31] for soil analysis, as there are no specific legal regulations for road dust. To ensure data reliability and exclude possible laboratory contamination, a blank sample was also prepared and analyzed. The blank consisted of ultrapure water (Milli-Q) subjected to the same acid digestion and analytical procedures as the URD samples. No detectable concentrations of the analyzed elements were found in the blank sample, confirming the absence of contamination during the analysis.

3. Toxicity Assessment

Differences in the tests conducted (i.e., Microtox was performed in both cities, Daphtoxkit only in Rawicz, and Ostracodtoxkit only in Wrocław) resulted from organizational limitations, availability of test material, and specific local environmental conditions. Instead of complete test symmetry, methods most appropriate to the characteristics of a given city were selected. Although this hinders a direct comparison of toxicity results between road and rail systems, it allows for the assessment of the differential impact of local pollution sources on various test organisms. Regardless of the type or location of the bioassay, both negative and positive controls were included in each test to ensure the reliability and validity of the results.

3.1. Aqueous Phase—Runoff Extracts

3.1.1. Microtox

Acute toxicity assessment was conducted using a Microtox Model 500 analyzer (Tigret Sp. z o.o.). (Warsaw, Poland). The device functions as both an incubator and a photometer. The test followed the ISO 11348-3 standard [32] and was based on the bioluminescence inhibition of Alvibrio fischeri bacteria. The reduction in bioluminescence of the bacteria was measured after 5 and 15 min at 15 °C and compared with that of a control sample. A 2% sodium chloride (NaCl) solution was used as the control following a standardized procedure. The test was performed on aqueous runoff extracts of road dust collected from sampling sites in Wrocław and Rawicz. Nine different concentrations of the prepared URD solutions were tested. The stock solution, representing a 100% concentration, contained 0.5 g of dust per milliliter of solution. The test was carried out using the 81.9% Screening Test Procedure, utilizing the Microtox Omni Software system. In the assessment of road dust toxicity, samples were classified based on their toxic effects after 5 and 15 min of exposure. If the reduction in bacterial bioluminescence was below 20%, the sample was considered non-toxic. Effects ranging between 20% and 50% were classified as intermediate, while samples exhibiting a toxic effect greater than 50% were considered toxic. Additionally, for intermediate samples, an 81.9% basic test was performed, and EC50 values were determined following the methodology described by Persoone et al. (2003) [33].

3.1.2. Daptoxkit F Magna

This test was conducted using the Daphtoxkit F magna protocol (MicroBioTests Inc., Gent, Belgium) and followed the OECD 202 guidelines [34]. Surface water samples were prepared in five serial dilutions using distilled water as a solvent, resulting in concentrations of 100%, 50%, 25%, 12.5%, and 6.25%, labeled C1–C5 (with C1 representing the highest concentration and C5 the lowest). For the control setup, 10 mL of standard freshwater was added to each well in the designated control row. Test wells were filled with 10 mL of the respective diluted solutions, starting with the lowest concentration (C5). A total of 20 Daphnia magna organisms were distributed into the wells, with 5 individuals placed in each of the 4 wells per concentration level. The plates were sealed with parafilm, covered with lids, and incubated at 20 °C. After 24 and 48 h of exposure, the mortality rate of D. magna was assessed to determine the toxic effects of the test substance. The test was performed using aqueous runoff extracts of road dust collected from sampling sites in Rawicz.

3.2. Solid Phase—Urban Road Dust (URD)

Ostracodtoxkit F

The Ostracodtoxkit F test was conducted following the official protocol (“Ostracodtoxkit F, MicroBioTests Inc., Gent, Belgium”) [35] and in compliance with ISO 14371. The test was performed on urban road dust (URD) samples collected from different locations in Wrocław. Initially, standard freshwater was prepared, and the Ostracod cysts (H. incongruens) were hatched. After measuring the length of the newly hatched organisms, an algal suspension was prepared. Each test plate was set up with 1000 μL of sediment, 2 mL of standard freshwater, 2 mL of algal suspension, and 10 Ostracods. The experiment was conducted in six replicates for both reference and test sediments. The plates were sealed with parafilm, covered with lids, and incubated in the dark for six days. Upon completion of the test period, mortality rates and growth inhibition percentages were determined using the appropriate formulas:

$$\mathrm{m}\mathrm{o}\mathrm{r}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{i}\mathrm{t}\mathrm{y} \mathrm{\%}={\displaystyle \frac{\mathrm{t}\mathrm{h}\mathrm{e} \mathrm{n}\mathrm{u}\mathrm{m}\mathrm{b}\mathrm{e}\mathrm{r} \mathrm{o}\mathrm{f} \mathrm{d}\mathrm{e}\mathrm{a}\mathrm{d} \mathrm{o}\mathrm{r}\mathrm{g}\mathrm{a}\mathrm{n}\mathrm{i}\mathrm{s}\mathrm{m}\mathrm{s}}{60}}$$

where 60 is the total number of studied organisms.

$$\mathrm{\%} \mathrm{g}\mathrm{r}\mathrm{o}\mathrm{w}\mathrm{t}\mathrm{h} \mathrm{i}\mathrm{n}\mathrm{h}\mathrm{i}\mathrm{b}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}=100-[{\displaystyle \frac{\mathrm{g}\mathrm{r}\mathrm{o}\mathrm{w}\mathrm{t}\mathrm{h} \mathrm{i}\mathrm{n} \mathrm{t}\mathrm{e}\mathrm{s}\mathrm{t} \mathrm{s}\mathrm{e}\mathrm{d}\mathrm{i}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t}}{\mathrm{g}\mathrm{r}\mathrm{o}\mathrm{w}\mathrm{t}\mathrm{h} \mathrm{i}\mathrm{n} \mathrm{r}\mathrm{e}\mathrm{f}\mathrm{e}\mathrm{r}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{e} \mathrm{s}\mathrm{e}\mathrm{d}\mathrm{i}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t}}}]$$

4. Health Risk Assessment

4.1. Exposure Dose Calculation

Health risk assessment was performed based on the evaluation of the concentrations of Cu, Zn, Cr, Cd, Pb, Fe, Ni, and As in URD, following the methodology established by the US EPA [36,37]. The calculations considered two groups: children and adults, and the exposure pathways included ingestion, inhalation, and dermal contact. The exposure dose represents the amount of URD absorbed by the body within 24 h, calculated as per kilogram of body weight. The following formulas were used to estimate the exposure dose:

$$ADDing=\mathrm{C}·{\displaystyle \frac{\mathrm{I}\mathrm{n}\mathrm{g}\mathrm{R}·\mathrm{E}\mathrm{F}·\mathrm{E}\mathrm{D}}{BW·AT}}$$

$$\mathrm{A}\mathrm{D}\mathrm{D}\mathrm{i}\mathrm{n}\mathrm{h}=\mathrm{C}·{\displaystyle \frac{\mathrm{I}\mathrm{n}\mathrm{h}\mathrm{R}·\mathrm{E}\mathrm{F}·\mathrm{E}\mathrm{D}}{PEF·BW·AT}}\times {10}^6$$

$$ADDderm=\mathrm{C}·{\displaystyle \frac{\mathrm{S}\mathrm{L}·\mathrm{S}\mathrm{A}·\mathrm{A}\mathrm{B}\mathrm{S}·\mathrm{E}\mathrm{F}·\mathrm{E}\mathrm{D}}{\mathrm{B}\mathrm{W}·\mathrm{A}\mathrm{T}}}$$

where:

C—average metal concentration in URD [mg/kg];

IngR—value of daily accidental URD intake [mg/d];

InhR—daily lung ventilation [m³/d];

EF—contact frequency [d/year];

ED—duration of contact [year];

BW—average body weight [kg];

AT—averaging period [d];

PEF—particle emission factor [m³/kg];

SL—coefficient of dust adherence to the skin [mg/cm²·d];

SA—skin surface exposed to dust [cm²];

ABS—percutaneous absorption coefficient, unnamed quantity.

The parameter values used for risk assessment in children and adults are presented in

Table 1. Parameter values used in risk assessment.

Parameter	Adults	Children
IngR (mg/d)	200	100
EF (days/year)	180	180
ED (years)	70	6
AT (days)	70·365	6·365
BW (kg)	70	15
InhR (m3/d)	20	7.6
PEF (m3/kg)	1.39·109	1.39·109
ABS	0.001	0.001
SL (mg/cm2·d)	0.7	0.2
SA (cm2)	5700	2800
ET (h)	14	8

.

4.2. Non-Carcinogenic Risk Assessment

To evaluate potential non-carcinogenic health risks, two key indicators were calculated: the Hazard Quotient (HQ) and the Hazard Index (HI). These were determined using the following equations:

$$HQ={\displaystyle \frac{\mathrm{A}\mathrm{D}\mathrm{D}}{\mathrm{R}\mathrm{f}\mathrm{D}}}$$

$$HI=\sum \mathrm{H}\mathrm{Q}$$

where ADD represents the ingestion, inhalation, or dermal dose, and RfD refers to the reference dose provided by the Integrated Information Risk System (IRIS). If HQ > 1, potential adverse health effects may occur. Similarly, if HI > 1, exposure may pose a significant health risk. If both values remain below 1, no significant health hazards are expected [31,33]. The RfD values for each metal are listed in Supplementary Materials Table S2.

4.3. Carcinogenic Risk Assessment

The Excess Cancer Risk (ECR) was estimated using the following equation:

$$ECR={\displaystyle \frac{\mathrm{C}·\mathrm{E}\mathrm{T}·\mathrm{E}\mathrm{F}·\mathrm{E}\mathrm{D}·\mathrm{I}\mathrm{U}\mathrm{R}}{BW·AT}}$$

Not all trace elements contribute to carcinogenic risk; therefore, only Ni, As, Pb, Cr, Co, and Cd were included in the ECR calculation. The Inhalation Unit Risk (IUR) values for these elements, according to IRIS, are: Ni: 2.6·10⁻⁴ (g/m³)⁻¹, As: 4.3·10⁻³ (g/m³)⁻¹, Pb: 1.2·10⁻⁵ (g/m³)⁻¹, Cr: 0.012 (g/m³)⁻¹, Co: 9·10⁻³ (g/m³)⁻¹ and Cd: 1.8·10⁻³ (g/m³)⁻¹.

An ECR value within the range of 10⁻⁶ to 10⁻⁴ is considered to represent a low cancer risk [36,37].

5. Statistical Analysis

We decided to analyze the results of the mean Ostracodtoxkit F (length of specimens) depending on sites; therefore, an equality of means test was performed. Since verification of the homogeneity of the variance with the Levene test showed a lack of homogeneity of the variance, the Welch test for equality of means in different conditions was used. A Games-Howell post hoc test was performed to determine the significance of the differences between pairs.

Due to the small sample size, the correlation analysis of metal concentrations with results of Ostracodtoxkit F (length of specimens) was performed using Pearson’s correlation coefficient along with testing their statistical significance at $\alpha =0.05$ level.

The data from Rawicz’s sampling points were of a different nature, as there were two factors: location and weather conditions (dry or wet). Therefore, a two-factor ANOVA analysis with interaction was performed. Analyses and graphical presentation were prepared using the R environment version 4.2.1 (R Core Team 2021) and the graphical package ggplot2 version 3.4.0 [38].

The level of exposure related to the connections of the studied elements in Wrocław and Rawicz was also compared. Due to the large variation in the values of the indicators (by several orders of magnitude), a logarithmic transformation using the decimal logarithm was applied.

6. Results and Discussion

6.1. Trace Element Concentration

The elemental composition of the URD revealed distinct differences between the two cities, shaped by traffic volume, urban form, and industrial background (Supplementary Materials Tables S3 and S4). In Wrocław, the highest concentrations of Cu, Zn, Cr, Pb, and Fe were found along the main city roads (Legnicka Street and the Legnicka–Poznańska intersection; Sites 1 and 4 a, b), confirming that areas with high traffic volumes are the main sources of trace metals due to vehicle exhaust emissions, brake and tire wear, and road surface degradation [39]. Elevated Cr and Pb concentrations suggest additional inputs from brake components and exhaust gases of older vehicles. Moderate levels of Cu, Zn, and Pb at the Wrocław Mikołajów (site 2) station likely stem from rail corrosion, airborne URD deposition, and fuel combustion, while high Fe levels suggest track abrasion and possible industrial influence [40]. In contrast, “Polana Popowicka” Park (site 3) showed the lowest element levels, acting as a buffer zone with minimal traffic and industrial impact.

In contrast, Rawicz, a smaller but industrialized city, showed even higher Cr levels, particularly on the S5 motorway (site 4a) during the drought (Cr: 402.11 mg/kg), indicating that local industrial activity plays a significant role (Table S3). It is worth noting that Rawicz is home to the Odlewnia Żeliwa i Staliwa (Iron and Steel Foundry), a foundry operating since the 19th century, producing large cast iron and steel components (up to 4000 tons per year) for the machinery, energy, automotive, and railway industries. Furthermore, several metalworking companies and RAWAG, a manufacturer of rail transport equipment, operate in this area [24]. These sources likely enhance Cr and Ni deposition in the URD, consistent with studies linking traffic and local metallurgy to elevated trace metals in dust [41].

These results highlight that while pollution in Wrocław was concentrated in transport hubs, Rawicz was characterized by a broader and more persistent contamination pattern with the combined impact of transport and local industry. This highlights the need for differentiated mitigation strategies tailored to the urban context and dominant emission sources.

Rainfall at the measurement point at the Legnicka–Poznańska intersection in Wrocław led to partial leaching of elements such as Cr, Cu, and Zn, confirming the role of rain in the mobilization of pollutants in the urban environment. This phenomenon is consistent with the so-called “first flush” effect, the rapid flushing of pollutants (trace elements, oils, dust, and microplastics) accumulated on urban surfaces during the initial phase of rainfall. At this stage, the concentration of pollutants in stormwater runoff is typically highest, which can lead to their rapid penetration into sewer systems and surface waters, especially in cities with a high proportion of impermeable surfaces [19]. The literature indicates that the amount of rainfall strongly influences the mobility of pollutants in the URD. The highest concentrations of elements and other pollutants are typically observed during the first 2–10 mm of rain, while heavier rainfall leads to their dilution and a decrease in concentrations in surface runoff. This phenomenon also depends on rainfall intensity and the length of the preceding rainless period [19,42].

In Rawicz, metal concentrations varied significantly depending on environmental conditions (Table S2). At the railway station (site 1), a slight increase in Ni, Cu, Zn, and Cr content was observed after rainfall, indicating mobilization of previously deposited contaminants, while As was significantly diluted, likely due to leaching. In the park (site 2), accumulation of Zn and Cr was observed during drought (2a), and a decrease after rainfall (2b), suggesting partial leaching. After street cleaning (2c), Cu, Zn, and Pb concentrations increased, which may be the result of resuspension of fine particles and their spreading on the surface—similar phenomena were previously described in the study by Mahbub et al. (2010) [41].

At the intersection with traffic lights (site 3a), Cu concentrations were very high during the drought (600.40 mg/kg), indicating emissions from brake pad abrasion. After rainfall, levels decreased significantly, confirming their high mobility. Cr and Ni remained relatively stable, confirming their strong binding to particulate matter. The highest contamination was recorded near the S5 expressway (site 4a), where Cu and Cr reached maximum values during the drought (1617.6 mg/kg and 402.1 mg/kg, respectively). After rainfall, the concentrations of most metals decreased, except for Pb, whose levels increased slightly, possibly due to low solubility or particle remobilization. Drought conditions favored the accumulation of trace elements in high-traffic areas due to the continuous deposition of contaminants and lack of leaching. These results confirm the observations of Mahbub et al. (2010), who observed the accumulation of heavy metals on road surfaces in dry conditions [41]. In turn, precipitation effectively reduced the concentrations of more mobile elements (Cu, Zn, Pb) but had no significant effect on Cr and Ni, which—as shown in the studies of Pochodyła-Ducka (2023) and Stinshoff (2025)—exhibit strong binding to particles and low leaching potential [43,44].

In summary, the mobilization of pollutants by rain and the ineffectiveness of traditional street cleaning methods (e.g., sweeping) in Rawicz lead to secondary metal emissions and emphasize the need to implement more effective strategies for managing urban dust and stormwater runoff depending on local urban conditions.

Interestingly, the type of pavement affects the composition and toxicity of URD. Due to their impermeability, asphalt pavements promote the accumulation of heavy metals and particles from vehicle operation, which can lead to higher dust toxicity [41]. Conversely, paved pavements, although partially permeable, can store contaminants in crevices and promote their resuspension during heavy traffic or improper cleaning [40]. In Wrocław, most sampling points were located on asphalt, which promotes lower retention of fine particles. Conversely, some areas in Rawicz (e.g., parks and side streets) had cobblestone or porous surfaces that can retain fine particles and facilitate the accumulation of trace elements over time. Studies indicate that porous surfaces extend the retention time of URD and reduce its leaching, which may explain its higher toxicity under dry conditions.

6.2. Toxicity Assessment

6.2.1. Aqueous Phase—Runoff Extracts

Microtox

A basic toxicity test was conducted on water extracts from URD collected from Wrocław and Rawicz, and toxic effects were evaluated based on EC₅₀ values measured at 5 and 15 min (

Table 2. Results of the basic test for water extracts of URD collected at sites in Wroclaw and Rawicz—EC₅₀ [%] after 5 and 15 min of exposure. EC₅₀ was not determined because the toxicity level in the Microtox test did not exceed the threshold required for its calculation. “-“_signifies that it was not possible to calculate the EC₅₀ value due to the too low toxicity level of the samples.

Site	EC50 5 min	EC50 15 min
1 (Wrocław)	-	-
2 (Wrocław)	60.74%	-
3 (Wrocław)	-	-
4a (Wrocław)	-	-
4b (Wrocław)	-	-
1a (Rawicz)	-	-
1b (Rawicz)	9.75%	13.25%
2a (Rawicz)	-	-
2c (Rawicz)	-	-
3a (Rawicz)	-	-
3b (Rawicz)	-	-
4a (Rawicz)	21.72%	20.19%
4b (Rawicz)	-	-

). In Wrocław, all samples were classified as non-toxic, indicating that toxicity did not exceed the threshold necessary for extended testing. The only exception was samples collected at site 2 (Wrocław Mikołajów Railway Station), which showed a short-term toxic effect. These results suggest that water extracts from URD samples from Wrocław did not contain significant amounts of water-soluble toxicants, which showed a temporary effect that disappeared over time (Table 2). Interestingly, according to the Microtox test, water extracts (runoff) from samples collected in Rawicz at site 1b (train station after rain) showed low toxicity: 9.75% at 5 min and 13.25% at 15 min. A slight increase in toxicity over time suggests the presence of substances with a gradual toxic effect, although the overall values remained below the thresholds, indicating high toxicity. Water extract from sample 4a (highway S5, drought) showed moderate toxicity: 21.72% after 5 min and 20.19% after 15 min (Table 2). According to the literature, EC₅₀ values in Microtox assays below 20% (v/v) indicate high toxicity, while values between 20 and 50% are considered moderately toxic, and above 50% are considered non-toxic or of low toxicity [33]. A clear contrast emerges between the two cities, Rawicz and Wrocław. The water extracts from URD collected from Wrocław were largely non-toxic, with only one case (site 2 Wrocław Mikołajów Railway Station) showing a short-term toxic effect. Interestingly, in both cases, some toxicity was noted in areas related to rail transport, probably because the Microtox test detected other toxic compounds related to rail transport but of organic origin, which may cause toxicity in areas where the railway operates. At railway-related sites, the following toxic organic compounds may be present: PAHs, formed during fuel combustion (e.g., diesel) and brake friction, which are highly toxic, mutagenic, and carcinogenic; nitrated PAH derivatives (nPAHs), resulting from PAH reactions with nitrogen oxides (NOx) in exhaust fumes, exhibiting strong mutagenic and genotoxic properties; phenol derivatives from railway sleeper impregnation e.g., creosote, which are potentially carcinogenic and toxic; lubricants, transformer oils, and preservatives (e.g., mineral oils in lubricated elements) containing toxic aliphatic and aromatic compounds that can migrate into the environment; and polychlorinated biphenyls (PCBs) used in railway electrical equipment (e.g., transformers, capacitors), which are highly toxic, environmentally persistent, and exhibit endocrine-disrupting and carcinogenic properties [45].

These results indicate site-specific environmental hazards: in Wrocław, the potential risk is low, mainly due to the lack of soluble toxicants. Despite several hotspots, overall metal concentrations are lower, and toxicity is negligible, indicating limited environmental risk, especially the runoff risk. In Rawicz, significantly higher metal loads and toxicity were recorded, indicating serious local contamination problems, probably due to road traffic, industrial sources, and ineffective mitigation practices (e.g., street cleaning).

Daphtoxkit F Magna

The test was conducted only on water extracts from URD collected in Rawicz. The results of the Daphttoxkit F magna test indicate varying mortality rates among the tested samples, reflecting differences in environmental conditions and potential toxic effects (Table S5). The highest mortality rate was recorded in water extract collected at site 4a (Highway S5, drought- 40.0%). At the other sites, the mortality rate of D. magna ranged from 13.3 to 33.3%. The impact of railways on the composition of URD was not as high as that of other types of transport. i.e., vehicles. (mortality rate 20–26% versus 13–40%). According to Al-Shidi and Sulaiman (2024), Daphnia magna showed no significant mortality in road dust extracts despite elevated metal levels, suggesting LC₅₀ values were higher than tested concentrations [46]. This supports the interpretation that mortality rates of 13.3–40.0% in this study reflect moderate, not acute, toxicity—even at site 4a. The mortality of D. magna individuals can be influenced by two factors changing simultaneously: site type and weather conditions (drought or rain) (Figure 2). Therefore, a two-factor ANOVA was performed. The results of the ANOVA analysis indicate that both in terms of location and weather conditions (dry, rain), mortality of the organisms differed significantly ($F=5.85, p-\mathrm{v}\mathrm{a}\mathrm{l}\mathrm{u}\mathrm{e}=0.0003$; $F=4.89, p-\mathrm{v}\mathrm{a}\mathrm{l}\mathrm{u}\mathrm{e}=0.010$ respectively).

A detailed analysis of the pairwise differences was performed using Tukey’s post hoc test. Significant differences were identified only between the control group and sites 1, 2, and 4. Moreover, significant differences in weather conditions were detected between sites 2b (park-rain) and 2c (park after street cleaning). The intergroup effect, i.e., the effect of rainfall, which varies between sites, was also found to be significant ($F=4.32, p-\mathrm{v}\mathrm{a}\mathrm{l}\mathrm{u}\mathrm{e}=0.008$). At site 2, the environment was more toxic after rainfall than under dry conditions (Figure 3), suggesting site-specific pollutant mobilization during storm events. These findings highlight the complex interactions between pollution sources and hydrological dynamics in urban water systems. In particular, rainfall appears to act as both a flushing and dispersion mechanism that can either dilute pollutants or mobilize previously deposited pollutants into the water column, depending on site characteristics. Gasperi et al. [42] showed that rainfall can lead to the rapid release of accumulated contaminants from urban surfaces into aquatic systems. This supports the observed increased toxicity at site 2 following rainfall, likely due to remobilization of surface-deposited contaminants. Moreover, the presence of other contaminants that were not studied in URD samples from this site cannot be ruled out. Kayhanian et al. [47] reported that stormwater runoff can significantly alter the concentration and toxicity of pollutants in receiving water bodies, and these changes may vary with location, land use, topography, and previous drought periods. The increase in toxicity after rainfall may be due to first flush phenomena, especially in highly urbanized or industrialized areas where the accumulation of pollutants is significant between rainfall events [48].

6.3. Solid Phase—Urban Road Dust (URD)

Ostracodtoxkit F

The test was conducted only on water extracts from URD collected in Wrocław. The growth inhibition analysis of Ostracods in water runoff revealed variability in individual organism growth over the six days of the experiment, with inhibition values ranging from 0% to 94.29% (Figure 4). Water extracts from URD collected at site 4a (intersection of Legnicka and Poznańska streets, during drought) exhibited the highest growth inhibition at 94.29%, and samples derived from site 2 were close behind with 91.43% growth inhibition (Wrocław Mikołajów Railway Station). Therefore, the inhibition of Ostracoda growth was also high at site 2 near the railway. This result suggests the influence of this form of transport on the growth of organisms, although the trace element content was not as high in the URD samples as in the case of sites 1 or 4; therefore, other factors may also have an impact on toxicity, i.e., organic compounds that were not studied in this research. Water extract from URD collected at site 4b (intersection Legnicka and Poznańska street, rain) showed 82.86% growth inhibition, and at site 1 (Legnica st.) A 74.29% growth inhibition was noted. In contrast, water extracts from URD collected at site 3 exceptionally increased growth up to 320 μm (Polana Popowicka). This suggests the presence of very favorable environmental conditions and a lack of toxicity. These results are consistent with previous findings, as the highest concentrations of trace elements were recorded on Legnicka Street (site 1) and at the busy intersection of Legnicka and Poznańska streets (sites 4a, b). Simultaneously, “Polana Popowicka” Park (site 3) showed the lowest element concentration, confirming its function as a buffer zone with limited exposure to traffic and industrial sources.

Based on the observed differences (Figure 4), the statistical significance of these differences was assessed. Due to the lack of homogeneity of variance in all groups confirmed by the Levene test (F = 15.45, $p-\mathrm{v}\mathrm{a}\mathrm{l}\mathrm{u}\mathrm{e}=9.4\bullet {10}^{-14}$). Based on the observed differences (Figure 4), the statistical significance of these differences was determined. As expected, the Welch test indicated the existence of statistically significant differences between the mean lengths of Ostracods in the groups (F = 1480, $p-\mathrm{v}\mathrm{a}\mathrm{l}\mathrm{u}\mathrm{e}<2.2·{10}^{-16}$). A Games-Howell post hoc test was performed to determine the significance of the differences between pairs. Statistically significant differences were observed in the means between all pairs, with the p-value for sites 2 and 4a being closest to the cut-off. Verification of equality of variance for this pair of locations only (p-value = 0.74) entitles us to use the Tukey test to compare the two averages, which indicates that the mean Ostracod lengths in sites 2 and 4a are not significantly different (p-value = 0.812). The findings indicate that water extracts from URD collected at sites 2, 4a, and 4b had the most significant inhibitory effects on the growth of Ostracods, suggesting the presence of contamination. These results highlight the potential impact of environmental factors on growth rates, emphasizing the need for continued monitoring to assess possible sources of contamination. It can be concluded that most of the trace elements were in a soluble form and passed into the water extracts, suggesting an unfavorable effect of these elements on the water biocenosis. Interestingly, the toxicity of these extracts was high when there was no rainfall (URD from site 4a) and decreased after rainfall (site 4b), suggesting that toxic components of urban runoff sediments (URD) are rapidly leached into surface waters during rain. This observation is consistent with the results of studies showing that stormwater runoff is the main vector for transporting accumulated pollutants from urban surfaces to aquatic ecosystems [49,50,51]. Rainfall has been documented to mobilize trace elements and organic pollutants from sediments and URD, increasing their bioavailability in the initial runoff phases [46].

In general, rainfall plays a dual role in the dispersion of particulate matter (PM): on the one hand, it partially removes pollutants from urban surfaces, but on the other, it mobilizes fine particles along with associated toxic elements (e.g., Cu, Pb), increasing their transport to surface waters. This phenomenon increases the risk of exposure, especially during the initial phase of rainfall (the so-called first flush effect) in areas with heavy traffic and poor drainage [39]. Interestingly, water extracts had a negligible effect on bacteria (Microtox test) at all sites in Wrocław. This suggests a differential sensitivity of organisms to URD components, which may reflect differences in physiological or metabolic pathways between bacteria and crustaceans. Furthermore, microbial communities can show adaptive resistance in urban environments due to chronic exposure to low levels of pollutants, which may explain the limited impact observed in this study [52].

6.4. Health Risk Assessment

6.4.1. Exposure Dose

The analysis of Average Daily Dose (ADD) (Supplementary Materials—Tables S6 and S7) values for trace elements in URD samples from Wrocław and Rawicz indicates differences in exposure levels. In general, URD samples collected in Rawicz exhibited higher metal concentrations than those in Wrocław, suggesting a greater contamination burden in this city. Children consistently showed higher ADD values than adults due to their lower body weight and increased susceptibility to ingestion, dermal, and inhalation exposure (Figure 5). Among the studied elements, Fe and Zn were the most abundant, with Fe reaching 1.15·10⁵ mg/kg/day in Rawicz, significantly exceeding other elements. Pb and Cr posed potential health risks, particularly in Rawicz, where Pb ingestion exposure reached 4.26·10² mg/kg/day in children. Inhalation exposure (ADDinh) was relatively low for all trace elements, whereas dermal exposure (ADDderm) was more pronounced, particularly for Zn and Fe. In general, the results demonstrated higher exposure levels in Rawicz than in Wrocław, with children being the most vulnerable group (Tables S6 and S7).

6.4.2. Non-Carcinogenic Risk Assessment

The Hazard Index (HI) values from Wrocław and Rawicz indicate significant differences in potential non-carcinogenic health risks associated with metal exposure (Supplementary Tables S8 and S9). In Wrocław, all HI values remained below 1, suggesting that cumulative exposure to trace elements from URD does not pose a significant health risk. The highest values for adults were recorded for Cr (HI = 1.39·10⁻¹, site 1, Legnicka st.) and Fe (HI = 7.17·10⁻² site 1, Legnicka st.), while children showed slightly higher values, with Cr reaching 2.08·10⁻¹, and Fe 1.65·10⁻¹ for URD at site 1, indicating exposure within safe limits. In Rawicz, collected URD samples showed higher HI values, with some exceeding 1, particularly for children (Supplementary Table S9). The highest values were observed for Cr (HI = 5.67·10⁻¹, site 4a, highway: drought, children) and Pb (HI = 1.26·10⁻¹, site 2c, park after street cleaning), indicating a potential, non-carcinogenic health risk. Although HI values for adults in Rawicz were below 1, the elevated levels for children suggest greater cumulative exposure, likely due to their increased sensitivity and lower body weights. Therefore, the levels of exposure related to the presence of trace elements in the URD samples from Wrocław and Rawicz were compared. Due to the large variation in the values of the indicators (by several orders of magnitude), a logarithmic transformation using the decimal logarithm was applied (Figure 6 and Figure 7).

The largest difference was noted for ADD_inh_Zn in Rawicz. However, in Wrocław, the exposure HQ_ing_Zn was higher than that in Rawicz. In order to determine whether health exposure (for children) differed significantly between the two locations, a paired t-test was performed. Its assumptions were previously verified: the distribution of transformed values for Wrocław and Rawicz is consistent with the normal distribution (p-value for Shapiro-Wilk normality test equal to 0.793 and 0.787, respectively), and the variance is homogeneous (p-value for Levene’s test is equal to 0.054). A comparison of the logarithmized values of exposure indicators in Wrocław and Rawicz showed statistically significant differences (p-value = 0.008), indicating higher levels of individual exposure indicators in Rawicz (Wrocław—Rawicz mean difference = −0.322) (Figure 6).

The same procedure was used to calculate the exposure of adults.

Only for ADD_ing and ADD_derm related to Zn content and HQ_inh related to Pb, the risk in Wrocław is higher. All other indicators suggest a higher health risk in Rawicz, in particular related to the adverse effects of Cu on the bodies of adults. A statistical test of the significance of differences in logarithms of indicators between Wrocław and Rawicz was performed. The pair t-test assumptions are also met: compliance with the normal distribution confirmed by the Shapiro-Wilk test (p-value equal to 0.869 and 0.525 for Wrocław and Rawicz, respectively), homogeneity of variance was confirmed by the Levene’s test (p-value = 0.505). The differences in the logarithm of exposure indicators were also statistically significant (p-value = 1.073 10⁻⁶) with a mean value of −0.297, which confirms a statistically significantly higher exposure in Rawicz (Figure 7).

These findings demonstrate a clear contrast between the two studied locations: Wrocław showed no immediate risk, whereas Rawicz URD samples, particularly for children, posed a potential health concern due to high Cr and Pb exposure. This suggests that environmental pollution in Rawicz city may be more severe, requiring further investigation into contamination sources and possible mitigation measures to reduce exposure risks.

6.4.3. Carcinogenic Risk Assessment (ECR)

ECR values for Wrocław and Rawicz are presented in

Table 3. Excess Cancer Risk (ECR) for adults and children in URD samples from Wrocław.

Elements	Group/Site	1	2	3	4a	4b
	Adults	3.79·10−3	1.08·10−3	9.77·10−4	3.12·10−3	2.43·10−3
Cr	Children	1.01·10−2	2.88·10−3	2.61·10−3	8.33·10−3	6.47·10−3
	Adults	4.38·10−5	2.07·10−5	7.81·10−6	3.91·10−5	3.95·10−5
Pb	Children	1.17·10−4	5.52·10−5	2.08·10−5	1.01·10−4	1.05·10−4

and

Table 4. Excess Cancer Risk (ECR) for adults and children in URD samples from Rawicz.

Elements	Group/Site	1a	1b	2a	2b	2c	3a	3b	4a	4b
	Adults	2.82·10−3	1.65·10−3	8.35·10−4	1.52·10−3	3.30·10−3	1.14·10−3	1.39·10−3	1.62·10−3	4.41·10−4
As	Children	5.53·10−2	6.31·10−2	9.03·10−3	1.18·10−2	3.65·10−2	4.31·10−2	5.23·10−2	2.34·10−1	6.76·10−2
	Adults	1.25·10−3	1.43·10−3	2.05·10−3	2.66·10−4	8.29·10−4	9.76·10−4	1.19·10−3	5.30·10−3	1.53·10−3
Ni	Children	3.34·10−3	3.82·10−3	5.46·10−4	7.11·10−4	2.21·10−3	2.60·10−3	3.16·10−3	1.41·10−2	4.09·10−3
	Adults	4.27·10−5	4.60·10−5	2.64·10−5	2.13·10−5	1.53·10−4	8.96·10−5	9.95·10−5	3.23·10−5	3.65·10−5
Pb	Children	1.14·10−4	1.23·10−3	7.04·10−5	5.68·10−5	4.09·10−4	2.39·10−4	2.65·10−4	8.61·10−5	9.73·10−5
	Adults	4.26·10−5	4.44·10−5	2.13·10−5	1.60·10−5	4.79·10−5	2.66·10−5	2.13·10−5	2.13·10−5	3.20·10−5
Cd	Children	1.14·10−4	1.18·10−4	5.68·10−5	4.26·10−5	1.28·10−4	7.10·10−5	5.68·10−5	5.68·10−5	8.52·10−5
	Adults	6.68·10−2	7.76·10−2	8.89·10−2	1.80·10−2	5.81·10−2	8.77·10−2	8.90·10−2	4.76·10−1	1.16·10−1
Cr	Children	1.78·10−1	2.07·10−1	2.37·10−1	4.81·10−2	1.55·10−1	2.34·10−1	2.37·10−1	1.27·100	3.08·10−1

. In Wrocław, the concentration of Cr posed a cancer risk, with ECR values exceeding the 1·10⁻⁴ threshold, particularly in children. The highest ECR for Cr, adults was recorded at site 1 (Legnica St. 3.79·10⁻³), while for children, even higher exposure (1.01·10⁻², site 1) was noted. These findings suggest that exposure to Cr in Wrocław’s URD samples may pose a potential long-term health risk. In contrast, the ECR values for Pb were lower, with the highest risk observed in children at site 1 (1.17·10⁻⁴, the upper limit of the acceptable range). Samples collected in Rawicz were characterized by considerably higher ECR values, particularly for Cr and As, suggesting a greater long-term cancer risk in this city than in Wrocław. Cr exhibited the highest carcinogenic potential, reaching an ECR of 1.27, far exceeding the acceptable limit for children at site 4a (highway, S5, drought). Similarly, URD collected at sites 3a (traffic lights, drought) and 3b (traffic lights, after rain) also showed elevated Cr-related risk, with ECR values of 2.34·10⁻¹ and 2.37·10⁻¹, respectively. For As, the highest risk was again observed in children, particularly at site 4a (highway S5, drought, 2.34·10⁻¹), whereas adults exhibited much lower values, with the highest value recorded at 1.62·10⁻³ (site 4a, highway S5, drought). Lower cancer risk values were recorded for Pb, Ni, and Cd than for Cr and As, although some Pb-related ECR values approached the moderate-risk range. The highest Pb ECR for children was recorded in URD collected at site 1b (train station after rain, 1.23·10⁻³), while for adults, the values remained significantly lower. The ECR values for Ni and Cd did not exceed the moderate risk, but still contributed to overall exposure.

These results clearly show a difference in the potential cancer risk between URD samples collected in Wrocław and Rawicz, indicating that the latter poses a significantly higher risk of cancer, especially in children, due to elevated Cr and As levels. Extremely high ECR values were recorded for URD samples collected at site 4a (highway, S5, drought, Rawicz), suggesting that this area is a contamination hotspot requiring further environmental monitoring and potential mitigation measures to reduce the risk of future exposure. Considering the high toxicity and carcinogenic potential associated with Cr and As, this finding highlights the urgent need for targeted management strategies in Rawicz. Such measures may include implementing effective street cleaning, dust control practices, and green infrastructure solutions to minimize the spread of contaminants. While the goal of street cleaning is to reduce pollution, our studies indicate that improper sweeping can lead to increased road dust emissions (sites 2a, 2b, and 2c in Rawicz, park: drought, rain, and after cleaning). Similar observations were made in Toronto, where scientists found that the use of regenerative air sweepers could effectively remove road dust particles, but in some cases, increased the concentration of trace elements in the remaining dust, which can negatively affect air quality [53]. Other studies have emphasized that traditional mechanical sweepers can be less effective at removing fine dust particles and can even contribute to their resuspension in the air, especially in dry conditions [54]. Therefore, for street cleaning to be effective in reducing pollution, it is necessary to use appropriate technologies and adapt the frequency and cleaning methods to local environmental conditions.

Additionally, local authorities should prioritize public health risk communication and community engagement, especially in areas near major highways and high-traffic corridors, to increase awareness and promote safe practices for vulnerable populations, such as children. High values of ECR (particularly for Cr and As in Rawicz) also indicate the need to implement preventive measures.

Table 5. Summary of key contamination indicators and human health risks (Zn, Pb, and Cr concentrations; ECR and HI values) at the most polluted URD sampling sites in Wrocław and Rawicz.

Sample ID	Cr (mg/kg)	Pb (mg/kg)	Zn (mg/kg)	ECR Cr (Children)	HI (Adults)
1 WROCŁAW	147.7	37.0	268.1	1.01·10−2	1.39·10−1
3a RAWICZ	74.10	75.70	189.5	2.34·10−1	3.45·10−2
2c RAWICZ	49.10	129.60	324.4	1.55·10−1	5.91·10−2
4a RAWICZ	402.10	27.30	202.7	1.27·100	3.79·10−1

. Summary of selected contamination and health risk indicators (Cr, Pb, and Zn concentrations in URD, ECR, and HI values) at the most contaminated sites in Wrocław and Rawicz. The table highlights locations with the highest recorded values of Cr, Pb, and Zn, as well as the corresponding elevated non-carcinogenic (HI) and carcinogenic (ECR) risks, especially in Rawicz. Notably, site 4a (Highway S5, drought) stood out as a hotspot with the highest ECR values for both Cr and As and elevated HI in children, highlighting the urgent need for targeted mitigation.

These include enforcing stricter emission limits for local industries, modernizing street cleaning equipment by introducing vacuum or regenerative systems, and implementing permeable surfaces or buffer zones with vegetation to capture stormwater runoff (URD) before it runs off. Public awareness campaigns, especially those aimed at parents of young children, should promote exposure minimization during periods of drought.

Comprehensive risk assessments and longitudinal studies are essential to understand the cumulative impact of URD exposure over time and to assess the effectiveness of mitigation interventions.

7. Conclusions

This study revealed significant differences in the composition, toxicity, and environmental behavior of road dust (URD) in two contrasting Polish cities. Higher levels of pollution and health risks were recorded in the smaller, industrialized city of Rawicz—particularly in areas with heavy road and rail traffic—than in the larger city of Wrocław. In Rawicz, URD exhibited greater toxicity (Daphnia magna mortality up to 40%) and a higher cancer risk (ECR for Cr up to 1.27), particularly among children. In Wrocław, toxicity was lower, although a strong inhibition of Ostracods’ growth (up to 94%) was noted in transport areas. Precipitation mobilized pollutants but did not always remove them; in some cases (e.g., Cu and Pb), it increased their displacement. Street cleaning in Rawicz proved ineffective and may have promoted the re-entrainment of toxic particles, unlike in Wrocław, highlighting the importance of urban practices in shaping URD.

Although differences in test methods limit the direct comparison of URD in road and rail traffic, the findings highlight the need to better understand the role of organic pollutants, especially in railway areas.

Overall, the results confirm that even small- and medium-sized cities can experience disproportionately high environmental and health risks from transport-related pollution. This emphasizes the need for tailored mitigation strategies not only in large urban centers but also in smaller industrialized towns.

Future research should focus on long-term monitoring, multitrophic bioassays, and a better understanding of pollutant runoff dynamics to inform urban particulate matter management strategies and public health protection.