While the most extreme heat waves 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 heat waves 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 heat waves," 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 heat waves and examine the health risks associated with them.
The hottest and longest heat waves, those that were hotter than 99 per cent of days and lasted at least five days, increased mortality the mostby over 33 per cent, they said.
The study found that heat waves that were hotter than 95 percent of all days and lasted only one day increased mortality the leastby just over 10 per cent.
The mildest heat waves were surprisingly the deadliest and the number of deaths was more or less inversely proportional to the intensity and duration of the heat waves, the researchers said.
"This is because the milder heat waves were so much more common than the hotter ones. In the end, the most extreme heat waves turned out to cause the lowest number of deaths because they were so infrequent," said 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 heat waves in order to offer relevant measures to protect public health in the future.