Health Risk Assessment of Formaldehyde in Tehran’s Air Using Remote Sensing Data

Formaldehyde (HCHO) is a toxic volatile organic compound (VOC) and a Group 1 human carcinogen. In the Tehran metropolitan area, rapid urbanization, heavy traffic, and substantial emissions of photochemical precursors contribute to elevated levels of reactive carbon compounds. Nevertheless, long-term urban-scale assessments of ambient formaldehyde and its associated health impacts remain limited. This study investigated tropospheric formaldehyde over Tehran during 2020–2023 using TROPOMI observations from the Sentinel-5P satellite, with all data processing conducted on the Google Earth Engine platform. After applying quality assurance filters, daily observations were aggregated to generate monthly, seasonal, and annual averages. Tropospheric vertical column densities were converted to near-surface concentrations using the ERA5 planetary boundary layer height. Human health risk was assessed by estimating chronic daily intake (CDI) and excess lifetime cancer risk (ELCR) following the RAIS/U.S. EPA framework, using an inhalation unit risk (IUR) of 1.3 × 10⁻⁵ (µg/m³)⁻¹. In addition, spatial hotspots were identified using the *Getis-Ord Gi statistic. The screening-level estimates indicated that monthly near-surface HCHO concentrations ranged from 2.7 to 9.0 µg/m³, whereas annual mean concentrations varied between 5.6 and 6.15 µg/m³, with persistent pollution hotspots located in central and northeastern Tehran. Seasonal analysis revealed maximum concentrations during summer, coinciding with intensified photochemical oxidation of volatile organic compounds. Estimated CDI values ranged from 0.00077 to 0.00257 mg/kg/day, while annual ELCR values ranged from 8.00 × 10⁻⁵ to 7.28 × 10⁻⁵, remaining within the upper bound of the U.S. EPA acceptable risk range (10⁻⁶–10⁻⁴). The corresponding population-equivalent lifetime cancer burden was estimated at approximately 633–695 excess cancer cases within Tehran's 8.69 million inhabitants. These findings indicate a moderate yet meaningful screening-level cancer risk and demonstrate the utility of a satellite-based approach for assessing formaldehyde exposure and associated health risks in urban areas lacking continuous ground-based monitoring.