Ambient air pollution and daily mortality in ten cities of India: a causal modelling study

Background: The evidence for acute effects of air pollution on mortality in India is scarce, despite the extreme concentrations of air pollution observed. This is the first multi-city study in India that examines the association between short-term exposure to PM_2·5 and daily mortality using causal methods that highlight the importance of locally generated air pollution. Methods: We applied a time-series analysis to ten cities in India between 2008 and 2019. We assessed city-wide daily PM_2·5 concentrations using a novel hybrid nationwide spatiotemporal model and estimated city-specific effects of PM_2·5 using a generalised additive Poisson regression model. City-specific results were then meta-analysed. We applied an instrumental variable causal approach (including planetary boundary layer height, wind speed, and atmospheric pressure) to evaluate the causal effect of locally generated air pollution on mortality. We obtained an integrated exposure–response curve through a multivariate meta-regression of the city-specific exposure–response curve and calculated the fraction of deaths attributable to air pollution concentrations exceeding the current WHO 24 h ambient PM_2·5 guideline of 15 μg/m³. To explore the shape of the exposure–response curve at lower exposures, we further limited the analyses to days with concentrations lower than the current Indian standard (60 μg/m³). Findings: We observed that a 10 μg/m³ increase in 2-day moving average of PM_2·5 was associated with 1·4% (95% CI 0·7–2·2) higher daily mortality. In our causal instrumental variable analyses representing the effect of locally generated air pollution, we observed a stronger association with daily mortality (3·6% [2·1–5·0]) than our overall estimate. Our integrated exposure–response curve suggested steeper slopes at lower levels of exposure and an attenuation of the slope at high exposure levels. We observed two times higher risk of death per 10 μg/m³ increase when restricting our analyses to observations below the Indian air quality standard (2·7% [1·7–3·6]). Using the integrated exposure–response curve, we observed that 7·2% (4·2%–10·1%) of all daily deaths were attributed to PM_2·5 concentrations higher than the WHO guidelines. Interpretation: Short-term PM_2·5 exposure was associated with a high risk of death in India, even at concentrations well below the current Indian PM_2·5 standard. These associations were stronger for locally generated air pollutants quantified through causal modelling methods than conventional time-series analysis, further supporting a plausible causal link. Funding: Swedish Research Council for Sustainable Development.