1. Background
Air pollution remains a major global public health challenge, particularly in rapidly developing and densely populated urban regions. According to the World Health Organization (WHO), exposure to ambient air pollution contributes substantially to morbidity and premature mortality, with notable effects on cardiovascular, respiratory, and hematological health outcomes (1, 2). Among air pollutant categories, volatile organic compounds (VOCs) are of particular concern because of their toxicity, photochemical reactivity, and key contribution to the formation of tropospheric ozone and secondary organic aerosols (SOA) (3-5). Within this group, benzene, toluene, ethylbenzene, and xylenes (BTEX) constitute a critical subgroup frequently detected in urban environments. These compounds are primarily emitted from anthropogenic activities, including vehicular exhaust, evaporative fuel emissions, petrochemical operations, industrial solvent use, and incomplete combustion processes (6-8). Their high volatility and atmospheric persistence enable them to remain suspended for extended periods, thereby contributing substantially to urban air pollution and the deterioration of urban air hygiene (9).
Among BTEX compounds, benzene is recognized as the most hazardous owing to its established carcinogenicity. The International Agency for Research on Cancer (IARC) classifies benzene as a Group 1 human carcinogen, and it is strongly associated with bone marrow suppression, disruption of hematopoiesis, leukemia, and other chronic hematological disorders (10, 11). Although toluene, ethylbenzene, and xylenes have lower direct carcinogenic potential, long-term exposure has been linked to neurological dysfunction, mucosal and respiratory irritation, and hepatic and renal impairment (12, 13). In addition to their toxicological effects, BTEX compounds play an essential role in atmospheric photochemistry; through chain reactions with hydroxyl (OH) radicals and nitrogen oxides (NOx), they accelerate the accumulation of ground-level ozone and SOA, thereby intensifying photochemical smog formation in urban basins (14, 15).
Recent evidence from 2020 to 2025 indicates that elevated BTEX concentrations persist in major global megacities despite the introduction of stringent regulatory protocols (16-18). Quantitative studies in Beijing, Abu Dhabi, New Delhi, and São Paulo have consistently reported substantial atmospheric BTEX burdens, often associated with dense traffic emissions, localized solvent applications, and limited regional dispersion during winter inversion episodes (19). These findings indicate that BTEX pollution remains a substantial and unresolved environmental and public health concern worldwide.
Tehran, the capital of Iran, is a densely populated megacity characterized by heavy traffic volumes, a large share of outdated or poorly maintained passenger vehicles, extensive fossil fuel consumption, and a complex bowl-like topographical structure surrounded by the Alborz mountain range. This geographic configuration frequently traps air toxics near the ground because of intense meteorological stagnation and surface-based temperature inversions (20, 21). Previous studies have reported that BTEX concentrations in several districts of Tehran exceed international guideline thresholds, particularly in high-traffic commercial corridors (22, 23). Moreover, chronic benzene exposure has been linked to elevated carcinogenic risks among residents living near traffic-dominated areas (24, 25). Despite these data, prior monitoring campaigns in Tehran often relied on short-term, single-season sampling, lacked multisite microenvironmental characterization, or omitted the simultaneous application of diagnostic ratios alongside modern USEPA deterministic inhalation risk frameworks (Inhalation Unit Risk [IUR]-based calculation updates). This limitation creates a critical knowledge gap regarding long-term seasonal exposure trends under changing urban fuel configurations and evolving vehicle fleet structures. Recent epidemiologic and environmental evidence from Iranian urban centers further highlights the multifaceted health burden of ambient air pollution, linking severe criteria pollutant exposures to systemic, cardiovascular, and neurological hospitalizations, such as pulmonary embolism in Mashhad and multiple sclerosis exacerbations in Kerman (26, 27). In addition, multidecade cohort syntheses, such as the Tehran Lipid and Glucose Study, underscore the chronic cardiometabolic risks associated with long-term exposure to urban atmospheric air toxics (28). Diagnostic ratios, particularly toluene-to-benzene (T/B) and xylene-to-benzene (X/B), are commonly used to evaluate dominant emission sources and characterize the photochemical aging of air masses. A T/B ratio of approximately 1.0 to 2.0 typically reflects fresh vehicular exhaust tailpipe emissions, whereas higher urban values may indicate industrial solvent use or substantial fuel evaporation and vapor leakage. Similarly, because xylenes have a much shorter atmospheric lifetime than benzene, the X/B ratio decreases over time because of faster hydroxyl radical reactions, making it a sensitive indicator of photochemical transformation and the relative aging of intercepted air masses (29-33). Incorporating these diagnostic fingerprints is essential for interpreting spatial VOC variability and identifying major sources across complex urban canopies.
2. Objectives
Given Tehran’s growing urban population, the persistence of severe winter air quality crises, and the lack of recent, methodologically transparent, comprehensive health risk assessments, this study aims to evaluate ambient BTEX pollution using an integrated, updated approach. Specifically, this research:
1) Quantifies ambient concentrations of benzene, toluene, ethylbenzene, and xylenes across diverse land-use microenvironments over a full one-year campaign in 2025.
2) Applies empirical diagnostic ratios (T/B, X/B, and T/X), together with Pearson correlation matrices, to infer dominant mobile versus evaporative signatures.
3) Conducts a rigorous public health risk assessment modeled in strict alignment with the updated USEPA Human Health Risk Assessment guidelines to determine noncarcinogenic Hazard Indices (HI) and ILCR.
The findings are expected to move beyond generic environmental claims by providing quantitatively anchored evidence to support targeted, seasonal traffic control policies, improve urban fuel quality management, and contribute to the broader global understanding of volatile organic pollution in modern megacities.
3. Methods
3.1. Study Area
This study was conducted in Tehran, Iran, the country’s most populous metropolitan area, with a population exceeding 14 million in 2025. The city lies between longitudes 51.2°–51.6°E and latitudes 35.6°–35.8°N, at an elevation of 1100 - 1800 m above sea level. The mountainous northern boundary and flat southern plain create conditions conducive to atmospheric stagnation and inversion layers, allowing for pollutant accumulation.
Tehran’s main emission sources include vehicular traffic, industrial activities, and solvent use, all contributing to ambient BTEX levels. The climate is semi-arid, with cold winters, hot summers, and limited rainfall. Five representative locations were selected to capture extreme contrasts in land-use metrics and represent distinct urban macro-environments: A traffic-dominated site (Traffic), an industrial district (Industrial), a low-density urban baseline area (Urban Background), a high-density residential zone (Residential), and a peripheral low-density suburban site (Suburban). To ensure representative breathing-zone measurements and minimize immediate ground-level or high-altitude interference, all sampling inlets were positioned at a standardized height of 2.5 m above ground level.
3.2. Sampling Strategy
A total of 120 charcoal tubes (including 96 valid ambient air samples and 24 quality control blanks/replicates) were collected over a 12-month period in 2025, following NIOSH Method 1501 (34, 35). Sampling periods were categorized into cold (November through February) and warm (June through September) seasons. Active sampling was performed biweekly at each of the five representative land-use stations. To capture diurnal variations corresponding to peak commuting cycles, continuous 12-hour air collection blocks were implemented from 08:00 to 20:00. Each monitoring site yielded 12 valid environmental samples across the campaign (6 samples per site per season × 2 seasons), resulting in 96 urban environmental samples, which were further verified using 24 quality control frameworks. Sampling was conducted using low-flow personal pumps (SKC, USA) connected to activated charcoal adsorption tubes (100/50 mg). Flow rates were maintained at 0.2 L·min-1, ensuring adequate sample volumes for trace analysis (36).
3.3. Sample Collection and Analysis
After sampling, both ends of the tubes were sealed and kept refrigerated at < 4oC until analysis. In the laboratory, the front and back sections of each tube were separated and extracted with 1 mL of carbon disulfide (CS2) for 30 min (33, 37).
BTEX compounds were quantified using gas chromatography with a flame ionization detector (GC–FID) (Agilent 7890 series). Compounds were identified according to retention times: benzene (3.52 min), toluene (6.13 min), ethylbenzene (10.65 min), and xylenes (11.04 min). The Limit of Detection (LOD) was calculated as three times the baseline noise signal, yielding 0.12 μg/m3 for benzene, 0.15 μg/m3 for toluene, and 0.18 μg/m3 for ethylbenzene and xylenes. The Limit of Quantification (LOQ) values were established at 0.40, 0.50, and 0.60 μg/m3, respectively.
3.4. Calibration and Quantification
Calibration curves for BTEX were prepared in carbon disulfide at five concentration levels (10 - 100 µg·mL-1). Linear regression showed excellent correlation (R2 ≥ 0.99) (17).
The airborne concentration (C, mg·m-3) was calculated using Equation (1):
and converted to ppm via Equation (2):
where Wf and Wb are the analyte masses in the front and back sections, Bf and Bb are blank values, V is the sampled air volume (L), and MW is the molecular weight. Breakthrough was controlled by comparing the back-to-front mass ratio, and samples with > 10% breakthrough were excluded.
3.5. Diagnostic Ratios
Diagnostic ratios of toluene/benzene (T/B) and xylene/benzene (X/B) were calculated to infer dominant emission sources (i.e., traffic versus non-traffic). Previous studies have demonstrated their reliability as fingerprints of vehicular emissions.
3.6. Human Health Risk Assessment
Inhalation risks were determined using the standard USEPA deterministic framework. Owing to the lack of fully validated local epidemiological exposure cohorts for Tehran’s population, standard USEPA default parameters for an adult urban resident were used as a conservative baseline.
The Chronic Daily Intake (CDI, mg/m3) was computed via Equation (3):
where C represents the measured ambient concentration (mg/m3). Noncarcinogenic health effects were quantified using the Hazard Quotient (HQ = CDI/RfC), where RfC is the specific toxicological Reference Concentration. The overall cumulative risk was characterized using the Hazard Index (HI = ∑H Qi). The ILCR for benzene was determined using Equation (4):
Table 1 lists all default adult exposure factors, Toxicological Reference Concentrations (RfC), and IUR used for the risk calculations.
Table 1.
Exposure Parameters and Toxicological Values Used for Health Risk Assessment
| Parameter | Definition | Value Used |
|---|---|---|
| ED | Exposure Duration | 30 (y); (Adult) |
| EF | Exposure Frequency | 365 (d/y) |
| AT-NC | Averaging Time (Non-Cancer) | ED × 365 (d) |
| AT-C | Averaging Time (Cancer) | 70 (y) × 365 (d) |
| BW | Body Weight (Adult) | 70 kg |
| IR | Inhalation Rate | 16 m3/d |
| RfC (Benzene) | Reference Concentration | 0.038 mg/m3 |
| IUR (Benzene) | Inhalation Unit Risk | 7.8 × 10-6 (μg/m3)-1 |
3.7. Quality Assurance and Quality Control
A total of 12 field blanks (10% of the 120 environmental samples) were carried, treated, and analyzed identically to field samples. Instrument calibration was checked every 10 analyses. Repeat injections showed a coefficient of variation (CV) below 15%. Spike recovery tests for the carbon disulfide extraction process yielded highly acceptable mean recovery efficiencies ranging from 92.4% to 104.2% across all target species, validating the accuracy of the analytical process. All statistical analyses, including independent t-tests, Mann-Whitney U tests, and Pearson correlation matrices, were performed using IBM SPSS Statistics.
4. Results
4.1. Descriptive Statistics of BTEX Concentrations
Table 2 summarizes the descriptive statistics of ambient BTEX concentrations (μg/m3) measured across the five monitored stations in Tehran. Among the target VOC species, toluene exhibited the highest mean concentration (47.3 ± 19.8 μg/m3), followed by xylenes (34.1 ± 15.2 μg/m3), ethylbenzene (17.4 ± 7.4 μg/m3), and benzene (11.6 ± 5.1 μg/m3).
Table 2.
Descriptive Statistics of BTEX Concentrations at the Monitored Stations
| Compound | Mean ± SD | Median | Min - Max | CV (%) |
|---|---|---|---|---|
| Benzene | 11.6 ± 5.1 | 10.9 | 3.6 - 27.4 | 43.2 |
| Toluene | 47.3 ± 19.8 | 44.0 | 19.8 - 103.7 | 42.7 |
| Ethylbenzene | 17.4 ± 7.4 | 15.9 | 6.1 - 33.8 | 43.9 |
| Xylenes | 34.1 ± 15.2 | 31.2 | 11.4 - 71.5 | 45.9 |
The elevated coefficients of variation (CV%) (42.7%–45.9%) confirm the strong influence of localized emission sources and heterogeneous land-use characteristics on ambient VOC distributions in the study area.
4.2. Seasonal Variation and Statistical Testing
To evaluate temporal trends, the sampling year was divided into a cold season (November through February) and a warm season (June through September). Table 3 presents the mean concentrations of BTEX compounds during the warm (summer) and cold (winter) seasons. Independent-samples t test results indicated that the mean concentrations of all investigated compounds were significantly higher in the cold season than in the warm season (all P < 0.001). This pronounced seasonal increase is attributable to cold-season meteorological conditions, including low planetary boundary layer (PBL) heights, frequent surface-based thermal inversions, reduced atmospheric mixing, and slower OH-radical-initiated photochemical degradation in winter.
Table 3.
Seasonal Comparison of BTEX Concentrations (μg/m3) in Summer and Winter a
| Compound | Summer (Warm) | Winter (Cold) | t-Statistic | P-Value |
|---|---|---|---|---|
| Benzene | 8.8 ± 2.8 | 14.8 ± 3.2 | -10.93 | < 0.001 |
| Toluene | 40.5 ± 8.5 | 52.1 ± 14.1 | -5.47 | < 0.001 |
| Ethylbenzene | 14.6 ± 2.9 | 19.3 ± 5.1 | -6.20 | < 0.001 |
| Xylenes | 28.8 ± 6.2 | 37.4 ± 10.8 | -5.34 | < 0.001 |
a Values are expressed as mean ± SD.
4.3. Spatial Distribution of BTEX
Table 4 presents the mean concentrations of BTEX compounds (benzene, toluene, ethylbenzene, and xylenes) across five land-use areas, including traffic, industrial, urban, residential, and suburban zones. One-way analysis of variance (ANOVA) indicated that the mean concentrations of all investigated compounds differed significantly among the studied areas (all P < 0.001). The highest concentrations of all BTEX compounds were consistently observed in the traffic zone, whereas the lowest values were recorded in the suburban area.
Table 4.
The BTEX Concentrations (μg/m3) in Five Land-Use Areas of Tehran a
| Compound | Traffic | Industrial | Urban Background | Residential | Suburban | P-Value |
|---|---|---|---|---|---|---|
| Benzene | 17.3 ± 1.2 | 14.4 ± 1.8 | 11.5 ± 2.2 | 9.2 ± 1.1 | 7.3 ± 2.3 | < 0.001 |
| Toluene | 70.9 ± 5.8 | 58.3 ± 3.9 | 46.5 ± 6.2 | 38.3 ± 6.2 | 30.2 ± 4.9 | < 0.001 |
| Ethylbenzene | 24.5 ± 2.6 | 21.6 ± 4.6 | 17.8 ± 4.3 | 14.4 ± 6.7 | 11.3 ± 5.3 | < 0.001 |
| Xylenes | 48.2 ± 5.4 | 42.1 ± 6.3 | 33.6 ± 5.9 | 27.2 ± 6.2 | 21.4 ± 5.7 | < 0.001 |
a Values are expressed as mean ± SD.
Post hoc comparisons using Tukey's HSD test revealed the following spatial patterns:
The traffic zone had significantly higher concentrations of benzene, toluene, and xylenes than all other areas (P < 0.01), whereas for ethylbenzene, the difference from the industrial zone was not significant (P > 0.05). Industrial and urban zones showed intermediate concentrations, with no significant differences between them for any compound (all P > 0.05). The residential area exhibited significantly lower concentrations than the traffic and industrial zones (P < 0.05), but differences from the suburban area were not significant for ethylbenzene and xylenes (P > 0.05). The suburban area consistently showed the lowest BTEX concentrations among all studied zones.
This spatial pattern suggests that vehicular emissions and fuel handling are the primary drivers of BTEX in Tehran’s urban environment, with localized industrial solvent use and small-scale manufacturing contributing to the intermediate levels observed at the urban background and industrial monitoring sites.
4.4. Source Diagnostic Ratios
Diagnostic ratios of toluene-to-benzene (T/B), xylene-to-benzene (X/B), and toluene-to-xylene (T/X) were calculated using the mean concentrations measured at each monitoring site to explore likely emission sources and characterize the photochemical age of the intercepted air masses. As presented in Table 5, the T/B ratios were relatively consistent across the monitoring network, ranging from 4.12 at the traffic site to 4.29 at the suburban site.
Table 5.
Diagnostic Ratios (T/B, X/B, and T/X) Across Tehran's Monitoring Stations
| Station | T/B Ratio | X/B Ratio | T/X Ratio |
|---|---|---|---|
| Traffic | 4.12 | 2.82 | 1.46 |
| Industrial | 4.14 | 2.98 | 1.38 |
| Urban background | 4.18 | 2.98 | 1.39 |
| Residential | 4.22 | 3.04 | 1.41 |
| Suburban | 4.29 | 3.04 | 1.43 |
A T/B ratio of approximately 1.0 - 2.0 is generally associated with fresh vehicular exhaust, whereas the higher T/B ratios observed in this study (4.12 - 4.29) are consistent with mixed urban emission profiles in which traffic-related emissions, together with evaporative fuel emissions and gasoline vapor losses (e.g., from fuel stations and parked vehicles), are likely important contributors. The relatively uniform T/B ratios across the monitoring network further suggest that these emission sources are broadly distributed throughout Tehran. In addition, the X/B ratios remained close to 3.0 at most monitoring sites, indicating relatively fresh air masses with limited atmospheric aging. However, these diagnostic ratios should be interpreted as supportive indicators rather than definitive evidence of emission sources.
4.5. Correlation Patterns Among BTEX Compounds
Given that the normality assumption was satisfied (P > 0.05 for all variables), Pearson correlation coefficients (r) were computed using all observations (n = 120) to evaluate relationships among BTEX compounds and to assess whether they were likely influenced by common emission sources. As shown in Table 6, strong positive correlations were observed among all BTEX compounds (r = 0.90 - 0.95, P < 0.001). The strongest correlation was between benzene and toluene (r = 0.95), followed by ethylbenzene and xylenes (r = 0.94). These findings suggest that the BTEX compounds exhibited similar temporal and spatial variation patterns and were likely affected by common emission sources. In urban Tehran, this correlation pattern is consistent with traffic-related emissions, including vehicle exhaust and evaporative fuel losses, although source attribution cannot be established based on correlation analysis alone.
4.6. Health Risk Assessment
Noncarcinogenic and carcinogenic health risks associated with chronic inhalation exposure to ambient BTEX were modeled in strict alignment with the standard USEPA Human Health Risk Assessment framework (as parameterized in Section 3.6).
4.7. Noncarcinogenic Risks
As presented in Table 7, for all target compounds (toluene, ethylbenzene, and xylenes), individual Hazard Quotients (HQ) remained well below the critical threshold value of 1.0 across all monitoring stations. This indicates that exposure to individual noncarcinogenic VOCs is unlikely to cause adverse chronic health outcomes at current levels. However, the cumulative Hazard Index (HI), which represents the additive exposure of all four compounds, reached a maximum of 0.84 at the high-traffic site. Although this cumulative value remains below the concern threshold (HI ≥ 1.0), it indicates elevated noncarcinogenic pressure near busy roadways that warrants close monitoring.
Table 7.
Noncarcinogenic Hazard Quotients (HQ), Cumulative Hazard Index (HI), and Incremental Lifetime Cancer Risk of BTEX Across Station Types
| Station | HQ-Benzene | HQ-Toluene | HQ-Ethylbenzene | HQ-Xylene | Hazard Index (HI) | ILCR (Benzene) |
|---|---|---|---|---|---|---|
| Traffic | 0.46 | 0.18 | 0.11 | 0.09 | 0.84 | 6.8 × 10 -5 |
| Industrial | 0.38 | 0.15 | 0.09 | 0.07 | 0.69 | 5.5 × 10 -5 |
| Urban Background | 0.31 | 0.12 | 0.07 | 0.05 | 0.55 | 4.3 × 10 -5 |
| Residential | 0.24 | 0.10 | 0.06 | 0.05 | 0.45 | 3.6 × 10 -5 |
| Suburban | 0.19 | 0.08 | 0.05 | 0.04 | 0.36 | 2.1 × 10 -5 |
4.8. Carcinogenic Risks
Public health risk evaluation identified benzene as the primary compound of concern. The estimated ILCR for benzene ranged from 2.1 × 10-5 at the suburban site to a maximum of 6.8 × 10-5 at the traffic site (Table 7).
These values exceed the strict de minimis benchmark of 1.0 × 10-6 (one in a million) and lie within the 10-6 to 10-4 range defined by the USEPA as a potential regulatory concern. It is important to note that these cancer risk values are deterministic, worst-case estimates based on a standardized 30-year exposure duration and linear extrapolation over a 70-year lifespan. They do not represent absolute clinical predictions of cancer incidence, but rather indicate an elevated probability of risk that justifies targeted emissions control policies.
5. Discussion
The findings of this study indicate that toluene and xylenes were the dominant BTEX species across all monitoring sites in Tehran, followed by ethylbenzene and benzene. This concentration pattern, combined with the relatively high diagnostic T/B ratios (4.12 - 4.29), suggests a pervasive influence of vehicular emissions, fuel combustion, and evaporative losses in shaping the city’s volatile organic compound (VOC) profile (3, 4, 29). The near-perfect Pearson correlation coefficients (r = 0.90 - 0.95) among the BTEX species further support the presence of common emission pathways, strongly linked to high traffic volumes, a substantial share of aged gasoline vehicles, and diffuse fuel vapor releases at service stations and urban intersections (3, 19).
The pronounced temporal pattern observed, with winter concentrations significantly exceeding summer levels (P < 0.001), underscores the strong role of seasonal meteorological conditions. Frequent thermal inversions, reduced planetary boundary layer heights, and minimal vertical mixing during the cold months trap pollutants close to the ground (4, 17, 30). Slower photochemical degradation rates, driven by lower winter solar radiation, further prolong the atmospheric lifetime of these air toxics. From a public health and respiratory medicine perspective, this prolonged winter exposure aligns closely with documented seasonal peaks in airway irritation, asthma exacerbations, reduced lung function, and increased respiratory-related emergency hospital admissions (1, 2).
The health risk assessment identified benzene as the primary carcinogenic component of concern within the monitored mixture. The estimated ILCR for benzene reached a maximum of 6.8 × 10-5 at traffic-dominated locations, exceeding the strict USEPA de minimis benchmark of 1.0 × 10-6. These metrics should be interpreted with appropriate scientific caution: these values represent deterministic probabilities based on conservative, worst-case linear extrapolations of constant inhalation patterns over a hypothetical 70-year lifespan. They are not clinical diagnoses of cancer incidence; rather, they are standardized indices indicating an elevated probability of risk that justifies targeted environmental interventions.
Beyond carcinogenicity, chronic exposure to the broader BTEX group is associated with noncarcinogenic pathologies, including mucosal irritation, persistent cough, neuro-respiratory symptoms, and localized airway inflammation (12, 13). Vulnerable subpopulations, including children, the elderly, individuals with pre-existing chronic obstructive pulmonary disease (COPD), and outdoor workers (such as traffic enforcement officers, street vendors, and transport drivers), are disproportionately affected by these exposures. In evaluating health risk outcomes from chemical exposures, accounting for potential misclassification bias and baseline assessment uncertainties remains essential for realistic public health projections (38). Parallel evaluations in occupational and clinical environments, ranging from health risk management practices during epidemic surges to indoor bioaerosol control through structural ventilation, further emphasize the critical role of integrated air quality management across both indoor and outdoor microenvironments (39, 40).
5.1. Study Limitations and Uncertainty Analysis
When evaluating these findings, several inherent study limitations and structural uncertainties must be explicitly considered:
1) Spatial Density: Although the five monitoring sites were carefully selected to represent major urban land-use archetypes, this configuration cannot fully capture microscale neighborhood variations and the complex urban morphology of a megacity such as Tehran.
2) Source Apportionment: While empirical diagnostic ratios (T/B and X/B) and strong Pearson correlations are highly suggestive of primary mobile signatures, they provide qualitative source indicators rather than quantitative mass contributions. Definitive source apportionment requires advanced multireceptor models, such as Positive Matrix Factorization (PMF), which were beyond the logistical scope of this project.
3) Exposure Parameters: Risk parameters were derived using standard USEPA default exposure constants for adult urban residents because fully validated local, population-specific epidemiological cohorts or micro-environmental time-activity data for Tehran were unavailable. Consequently, age-specific risks (particularly for children) and sensitivity distributions could not be explicitly modeled.
5.2. Conclusions
This study demonstrates that ambient BTEX pollution in Tehran is characterized by a clear dominance of toluene and xylenes. Concentrations were highest at traffic-influenced locations and lowest at residential sites. The observed spatial patterns, clear seasonal variations, diagnostic ratios, and strong inter-species correlations indicate that vehicular emissions and evaporative fuel losses are the primary drivers of ambient BTEX levels across the urban canopy. The significant accumulation of these compounds during the winter months confirms the major impact of unfavorable meteorological conditions, particularly shallow boundary layers and thermal inversions, on increasing ground-level exposure. In addition to traffic restrictions and fuel vapor controls, urban planning strategies should incorporate the strategic expansion of green infrastructure using air-pollution-resistant plant species to naturally sequester airborne volatile compounds and improve overall urban canopy air hygiene (41).
From a public health perspective, the deterministic risk assessment identifies benzene as the most critical compound because its elevated lifetime cancer risk probability exceeds the conservative de minimis benchmark at traffic-dense sites. Chronic exposure to the combined BTEX mixture also represents a significant risk for respiratory irritation, airway inflammation, and the exacerbation of pre-existing pulmonary diseases, particularly among vulnerable urban populations and outdoor workers.
These findings indicate that BTEX pollution in Tehran is a critical environmental health issue that requires targeted, evidence-based policy solutions. Because atmospheric accumulation peaks during cold-season meteorological stagnation, emissions control strategies must be dynamically prioritized during the winter months. Consistent with the empirical findings, effective interventions should include:
1) Implementing temporary, stricter low-emission traffic zones and heavy-vehicle restrictions during forecast winter inversion periods.
2) Accelerating the installation of Stage II vapor recovery systems at urban fuel stations to mitigate evaporative benzene and toluene emissions.
3) Expanding continuous volatile organic compound monitoring networks to better manage exposure risks in highly populated megacities.
