Personal exposure to outdoor particulate matter and greenspace in Delhi, India

Abstract

BACKGROUND AND AIM: Research suggests urban greenspace promotes better health, with reduced air pollution being a potential mechanism; however, much of this work is based on static environments in lower air pollution settings. Our aim was to study the association between particulate matter of 2.5 µm (PM₂.₅) and greenspace during outdoor walking trips in a higher air pollution context. METHODS: Study subjects included adolescents (age 10-18 years) with asthma recruited within the Delhi Air Pollution and Health Effects (DAPHNE) study. Participants were provided with an AirSpeck personal sensor to monitor continuously their exposure to PM₂.₅ during 48-hour monitoring periods. We identified walking journeys based on personal GPS data and assigned three indicators of greenspace according to four buffer sizes (25m, 50m, 100m, 250m): the mean Normalised Difference Vegetation Index (NDVI), mean tree cover density (TCD), and proportion overlap with green land use (GLU). We assessed the relationship between greenspace and PM₂.₅ using multilevel modelling, controlling for individual (e.g., age, sex), spatial (e.g., roads, population density) and temporal (e.g., season, meteorology) covariates. RESULTS:There were 86 participants who provided 219 walking journeys, a cumulative total of 2029 minutes. The mean PM₂.₅ concentration was 129 µg/m³ (standard deviation=121). Results from preliminary analyses have not elucidated clear associations with greenspace. For example, in fully adjusted models (100 m buffer), the corresponding change in PM₂.₅ (µg/m³) for a 1-unit change in greenspace was 0.03 (95% confidence interval [CI]: -0.71 to 0.78) for NDVI, -1.4 (95% CI: -5.5 to 2.7) for TCD, and 0.42 (95% CI: -0.04 to 0.87) for GLU. CONCLUSIONS:Our initial findings do not support lower exposure to PM₂.₅ with greenspace. A possible explanation is that any reductions linked to vegetation may be too modest to detect in the presence of substantial spatiotemporal variation as that which exists in Delhi. KEYWORDS: Air pollution, Particulate matter, Green space, Short-term exposure, Built environment