Extreme heatwaves responsible for rise in mortality in India: Study

Ten cities from India – Delhi, Varanasi, Shimla, Ahmedabad, Kolkata, Mumbai, Pune, Chennai, Hyderabad and Bengaluru – were analised by the researchers to find out the effect on mortality by high intensity heat waves that continues consecutively for long duration in these cities.

The study found that there is an increase of 12.2 % daily mortality if high mean temperature (referred to as temperatures that are higher than 97% days in a year) is recorded for a day . However the daily mortality rate goes up by 14.7 % ,  17.8% and 19.4% respectively if when such high temperatures persist for two, three and five days respectively.

The study also noted that when extreme heat wave conditions – referred to as temperatures that are higher than 99% days in a year- are recorded for 5 consecutive days then heat wave related mortality can go up to 33.3 %. The cities of Delhi, Ahmedabad, Hyderabad, Kolkata, Varanasi and Chennai observed such conditions.

In the study, data from largest cities such as Delhi, Chennai, Kolkata, and Mumbai covered larger periods from 2008 till 2015-2019 while data sets from other smaller cities ranged mainly between 2008 and 2012.

“We estimated that around 1116 deaths annually across the 10 cities were attributable to heatwaves defined as 2-consecutive days above 97th percentile. In contrast, 58 deaths were attributed to heatwaves defined as at least 3 consecutive days above the 99th percentile. This trend was consistently observed across all cities. Higher annual attributable mortality was observed in Delhi, Ahmedabad and Chennai, where Shimla and Pune exhibited the lowest attributable mortality”, said the lead researcher.

Commenting on the study, Dr Pankaj Kumar, Senior Director, Critical Care, Fortis Hoapital, Shalimar Bagh said, “Excessive hot weather causes heat stress, which may be associated with morbidity and mortality. It negatively affects mental health. Heat reduces physical work capacity and performance and adversely affects occupational health hazards. It is important as the heat related deaths  are largely preventable.  This assumes more significance because of chances of climate change and global warming.  Dehydration caused by heat can lead to cardiac events and acute kidney failure. Use of alcohol and narcotics is associated with increased mortality during heat waves. Patients with many neurological diseases show inability to adequately control body temperature. So besides heat stroke and heat exhaustion, there is mortality due to heat stress, which still remain largely preventable.”

According to Dr Rajeev Gupta, Director – Internal Medicine at the CK Birla Hospital, Delhi, “Extreme heat affects human health by causing heat-related illnesses such as heat stroke, heat exhaustion, and dehydration. It puts strain on the cardiovascular system, kidneys, and other organs. High temperatures can lead to dehydration, electrolyte imbalances, renal failure, metabolic acidosis, respiratory distress, and cardiac dysfunction. In severe cases, extreme heat can cause organ failure and death, particularly in vulnerable populations”.

The reason for high mortality during heat waves is that extreme heat stress can cause organ failure and death. The strain placed on the body during periods of high temperatures can lead to cardiovascular events (such as myocardial infarction), renal failure, respiratory distress, and neurological complications like heat stroke. Prolonged exposure to extreme heat can disrupt vital organ functions and, if untreated, may result in severe health outcomes including death.