Integrated Assessment of BTEX in Tehran’s Ambient Air: Concentration Levels, Diagnostic Ratios (T/B, X/B), and Human Health Risk Evaluation
Abstract
Background: Benzene, toluene, ethylbenzene, and xylenes (BTEX) are key volatile organic compounds (VOCs) that pose substantial environmental and public health risks in urban environments. Objectives: This study presents a methodologically transparent assessment of BTEX concentrations, spatial and seasonal variations, emission sources, and associated human health risks across distinct land-use microenvironments in Tehran, Iran. Methods: Ambient BTEX concentrations were monitored biweekly over 12-hour sampling periods (08:00 - 20:00) from January to December 2025 at five strategic stations (traffic, industrial, residential, urban background, and suburban). Active sampling was conducted using charcoal tubes (NIOSH 1501) and analysis was performed by GC-FID. Seasonal variations were evaluated using an independent-samples t-test, whereas spatial differences among the five monitoring sites were assessed using one-way analysis of variance (ANOVA). Source attribution was evaluated using empirical diagnostic ratios (T/B, X/B) and Pearson correlation (n = 120). Carcinogenic and noncarcinogenic inhalation risks were modeled using the updated United States Environmental Protection Agency (USEPA) Human Health Risk Assessment framework. Results: Toluene was the most abundant species (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). All target species exhibited statistically significant winter accumulation (P < 0.001). BTEX concentrations were significantly higher during the cold season than during the warm season for all target compounds (P < 0.001), with benzene and toluene showing the greatest seasonal increases. Uniformly high diagnostic ratios (T/B = 4.12 - 4.29, X/B ≈ 3.0, and T/X = 1.38 - 1.46), together with strong positive correlations among BTEX compounds (r = 0.90 - 0.95), were consistent with the influence of common traffic-related emission sources, including vehicle exhaust and evaporative fuel losses. Although individual noncarcinogenic hazard quotients were below 1.0, the cumulative Hazard Index (HI) reached 0.84 at the traffic site. The deterministic Incremental Lifetime Cancer Risk (ILCR) for benzene ranged from 2.1 × 10-5 to 6.8 × 10-5, exceeding the strict USEPA de minimis benchmark (1.0 × 10-6). Conclusions: BTEX pollution in Tehran is a critical environmental health concern driven by high mobile fleet intensity and winter meteorological stagnation. Given the pronounced cold-season accumulation, emergency seasonal traffic zoning restrictions and stage II vapor recovery systems at fuel stations should be dynamically prioritized during winter months to mitigate inhalation exposure and protect public respiratory health.