Milder Heatwaves Deadlier In Long Term, Says Study

While the most extreme heatwaves have the greatest short-term impact on mortality, it is the mildest ones that kill most over time because they are more common, according to a study conducted in India.

As heatwaves are projected to become more common as the Earth's climate warms, the health risks of temporarily elevated temperatures must be investigated further, the researchers said.
"We wanted to find out how much the risk of death increases during heatwaves," said study first author Jeroen de Bont from Karolinska Institutet in Sweden.

The study, published in the journal Environment International, examined excess mortality in ten cities in different parts of India with different climate zones between 2008 and 2019. By selecting the days that were hotter than 95, 97 or 99 per cent of all days in the different regions, the researchers were able to create different definitions of heatwaves and examine the health risks associated with them.

The hottest and longest heatwaves, those that were hotter than 99 per cent of days and lasted at least five days, increased mortality the most—by over 33 per cent, they said. The study found that heatwaves that were hotter than 95 per cent of all days and lasted only one day increased mortality the least—by just over 10 per cent.

The mildest heatwaves were surprisingly the deadliest and the number of deaths was more or less inversely proportional to the intensity and duration of the heatwaves, the researchers said.
"This is because the milder heatwaves were so much more common than the hotter ones. In the end, the most extreme heatwaves turned out to cause the lowest number of deaths because they were so infrequent," said Mr De Bont.

"One consequence of this may be that heat warnings may need to be triggered at lower temperature thresholds to protect more people," the researcher said.

The team argues that policymakers and other stakeholders need to plan for both the relatively mild, short, and common, and the extreme, long, and uncommon heatwaves in order to offer relevant measures to protect public health in the future.

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