Residential indoor and outdoor exposure to VOCs and NO₂: insights from a multi-city pilot study in India

As part of the APEAL project (Longitudinal Effects of Air Pollution Exposure on Adolescent Lungs), a large-scale cohort study involving ~5,000 children, this pilot study examined indoor and outdoor concentrations of volatile organic compounds (VOCs) and nitrogen dioxide (NO₂) in urban residential homes during the summer season (April-May 2022). One-week integrated Passam passive samplers were deployed to measure VOCs and NO₂ in Delhi, Bengaluru, and Mysuru, and NO₂ additionally in Mumbai. Average indoor (outdoor) VOC concentrations in Delhi were: 123.7±34.8 (99.3±33.0)µg/m³, which were 42.7%(34.4%) higher than in Bengaluru and 58.1%(50.4%) higher than Mysuru, respectively. Indoor (outdoor) NO₂ levels in Delhi were 101.4±12.4 (61.2±12.4)µg/m³, which were 33.5%(40.7%) higher than in Mumbai, 34.1%(41.2%) higher than Bengaluru, and 68.0%(80.6%) higher than Mysuru, respectively. Spatial heterogeneity was observed for pollutants across cities (Coefficient of Divergence, COD>0.20). Strong indoor-outdoor correlations for BTEX and weak correlations for NO₂ suggest variable source influences. Health risk assessment for NO₂ indicated an elevated risk of adverse effects, while Incremental Lifetime Cancer Risk (ILCR) analysis suggested a 'probable' to 'possible' carcinogenic risk from BTEX, predominantly driven by benzene. The indoor models explained ~65% of the variability in BTEX and 87% in NO₂ concentrations, with outdoor levels, indoor sources such as incense burning, household characteristics such as separate kitchen as the main explanatory factors. Outdoor BTEX and NO₂ showed exponential decay with distance from the nearest road. These findings underscore the need for city-specific interventions to reduce children's exposure to air pollution and its associated health risks.