The United Nations proposed Sustainable Development Goals to promote healthy, safe and resilient lives. In recent years, indoor air pollution has been attracting increasing consideration and listed as one of the prime environmental hazards. Thus, it is vital to investigate the influence of outdoor pollution on indoor air quality. To fill this lacuna, this study conducted simultaneous indoor–outdoor measurements of particulate matter with a diameter ≤2.5 μm (PM2.5) and their metallic composition (Zn, Na, Cr, Cu, Ba, Ni, Ca, Al, Mg, Fe, K and Pb) in urban residential buildings of Agra. Results showed higher PM2.5 concentrations indoors (83.2±15.8 μg/m3) compared to outdoors (73.5±12.2 μg/m3) with an indoor/outdoor ratio of 1.13, indicating dominance of indoor emission sources (cooking and building materials). Infiltration analysis was applied to study transport of outdoor PM2.5 particles. Zn, Na, Cr, Cu, Ba and Ni were high indoors (I/O > 1) while Ca, Al, Mg, Fe, K and Pb were dominant outdoors (I/O < 1). This study identified four potential source factors both indoor and outdoor. Field emission scanning electron microscope revealed a difference in morphologies of indoor and outdoor PM (mostly spheroidal particles with dominance of C, S, Na, K and Cl indoors). These findings will help to develop strategies to control metals in PM2.5, reducing their adverse health effects.
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