Indoor PM₂.₅ as a Reactive Carrier of PAHs and Heavy Metals: Seasonal Chemistry and Women focused Exposure in Urban Households of Northern India

Abstract

Indoor PM₂.₅ is increasingly recognized as a chemically active aerosol capable of transporting toxic organic and inorganic constituents within household environments. The present study investigated seasonal indoor PM₂.₅, particle bound 16 USEPA recommended PAHs, selected heavy metals, and respiratory deposition characteristics across Industrial, Commercial, and Residential microenvironments in the capital of the most polluted state of Northern India. The study emphasis on women-focused domestic exposure and therefore, 24-hour indoor PM₂.₅ monitoring was conducted in 18 different households between November 2023 and October 2024, with sampling positioned between the kitchen and living area to represent the dominant household exposure zone for women. PM₂.₅ mass was determined gravimetrically, PAHs were analysed using GC-MS, selected metals were quantified by ICP-OES, and respiratory deposition was estimated using the Multiple Path Particle Dosimetry (MPPD) model. Indoor PM₂.₅ exhibited a clear seasonal trend of winter > summer > monsoon across all studied houses, with Industrial house showing the highest winter concentration (235 µg m⁻³). The compositional profile of indoor PM₂.₅ was strongly dominated by high molecular weight (HMW) PAHs, particularly InP, B(a)P, Chr, Pyr, and B(a)A, indicating substantial particle-phase persistence of combustion-derived compounds. Industrial households showed the strongest HMW PAHs contribution, accounting for 94.2% and 90.4% of total PAHs during summer and winter, respectively, while Commercial households exhibited a winter HMW contribution of 91.2%. Carcinogenic PAHs constituted a major fraction of total indoor PAHs, particularly in Industrial summer (74.1%), Industrial winter (61.2%), and Commercial winter (61.3%), confirming substantial predominance of particle associated toxic organic compounds within indoor aerosols. Heavy metal analysis further demonstrated the presence of a reactive inorganic fraction within indoor PM₂.₅, with Zn, Pb, Ni, Cu, Mn, Fe, and Cr showing distinct seasonal and microenvironmental variability. Respiratory deposition modelling indicated age-dependent differences in pulmonary deposition, with younger women showing comparatively greater deposited mass under elevated PM₂.₅ conditions. Winter Industrial households showed the highest deposited dose, reflecting enhanced accumulation and respiratory retention of the chemically elevated contribution of fine particles under stagnant atmospheric conditions. Collectively, the findings demonstrate that indoor PM₂.₅ in urban households functions not merely as suspended particulate matter, but as a chemically complex exposure medium carrying persistent PAHs and toxic metals. The study highlights the importance of integrating aerosol composition, seasonal behaviour, and respiratory deposition assessment for understanding indoor exposure dynamics among women living in rapidly urbanizing Northern India.