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Air pollution and extreme heat increase mortality in India

A new study from the Institute of Environmental Medicine, Karolinska Institutet reveals that days with both high air pollution and extreme heat substantially raise the risk of death in Indian cities more than either factor alone. The findings are in the journal Environment International.

Previous research on the interaction between air pollution and heat has been limited, particularly in regions like India, where both factors frequently reach extreme levels. This study included daily counts of death from 10 major Indian cities between 2008 and 2019.

Researchers applied two advanced spatiotemporal models to estimate daily exposure levels of ambient air pollution and temperature. By analyzing approximately 3.6 million deaths, they found that the association between PM_2.5 and mortality was particularly strong at high temperatures.

A 10 μg/m³ increase in fine particulate matter (PM_2.5) was associated with a 4.6% rise in daily deaths on extremely hot days—substantially higher than the 0.8% increase observed on regular warm days. Similarly, the risk of death rose by 8.3% when temperatures shifted from warm to extremely hot at a pollution level of 20 μg/m³—but surged to 64% when PM_2.5 reached 100 μg/m³. These results highlight a concerning synergy between heat and air pollution, showing that their combined effects on health are significantly more harmful than either factor alone.

The study employed time-series analyses using generalized additive Poisson regression models to assess exposure-outcome associations. Interaction effects between air pollution and temperature were incorporated to understand their combined impact on mortality. This allowed us to estimate the effect of air pollution at different temperature levels, and vice versa.

"Our findings highlight the interaction between air pollution and extreme heat, especially in densely populated areas where these factors frequently coincide," says Jeroen de Bont, assistant professor at the Institute of Environmental Medicine. "To mitigate health impacts, immediate efforts are needed to reduce emissions from common sources of air pollution and climate change."

The study emphasizes the need for integrated strategies to address both air pollution and climate change in India, where global warming is expected to further exacerbate the situation.

"Given several common pollutants for air pollution and climate change, reducing air pollution levels will have almost immediate effects on reducing mortality from both air pollution and heat with co-benefits for health" say Petter Ljungman, principal researcher at IMM. "An important adaptation strategy is the development of integrated surveillance and warning systems that take into account both climate hazards and air pollution levels."