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.