Spatial variation of outdoor and indoor NO2 in metropolitan cities in India

Abstract

BACKGROUND AND AIM: Exposure to traffic related pollutants is known to affect the cardio-respiratory health in urban population. The residential urban neighborhoods in low- and middle-income countries such as India are in close proximity to major roads, densely packed, and likely have high heterogeneity in air pollution exposures. However, there is very limited data on long-term exposures to air pollutants and their intra-urban spatial variability in such countries. Thus, this study aims to examine the spatial variation of outdoor NO2, a proxy of traffic air pollution, in metropolitan cities and its relationship with indoor NO2 levels. METHODS: As part of a multi-centric adult women cohort, biweekly averaged outdoor NO2 was measured in the winter at ~60 residential locations each in three metropolitan cities in India – Delhi, Mumbai, and Bengaluru - using passive samplers. Indoor NO2 was also measured at half of these residences. Household characteristics including indoor activities, likely sources and ventilation conditions were also collected through structured questionnaire surveys. RESULTS:Mean (±SD) outdoor NO2 were 58.5±8.8, 57.9±15.8 29.5±8.1 μg/m3 for Delhi, Mumbai and Bengaluru, respectively. Corresponding indoor concentrations were 61.8±18.2, 69.6±20.1 and 53.2±21.1 μg/m3 for the three cities, respectively. Indoor/Outdoor (I/O) ratios of 1.03±0.3 (Delhi), 1.25±0.3 (Mumbai) and 1.82±0.6 (Bangalore) suggest the strong influence of the indoor sources coupled with poor ventilation, in particular in Mumbai and Bengaluru. Moderate to poor correlation was observed between indoor and outdoor NO2 (Pearson R=0.36, 0.49 and 0.52 respectively). Socio-economic factors, building airtightness, cooking activities as well as occupant’s behaviour (e.g. opening windows) are also identified as key factors influencing the I/O ratios. CONCLUSIONS:Our findings show substantial differences in outdoor NO2 between and within the cities, the influence of indoor pollution sources and activities on indoor levels. Further investigation delineating the influence of indoor and outdoor sources is underway. KEYWORDS: Nitrogen dioxide, Exposure assessment, Long-term exposures, Socio-economic factors.