A multi-author study published in Environmental International Journal in 2024 titled “ Impact of heatwaves on all cause mortality in India: A comprehensive multi-city study” has shown that there is strong evidence of the impact of heatwaves on daily mortality.
Ten cities from India, namely, Delhi, Varanasi, Shimla, Ahmedabad, Kolkata, Mumbai, Pune, Chennai, Hyderabad and Bengaluru were analysed by the researchers to find out the effect on mortality by high intensity heatwaves that continue consecutively for long durations in these cities.
The study found that there is an increase of 12.2 percent daily mortality if high mean temperature (referred to as temperatures that are higher than 97 percent days in a year) is recorded for a day. However, the daily mortality rate goes up by 14.7 percent, 17.8 percent and 19.4 percent, respectively, if such high temperatures persist for two, three and five days.
What World Meteorological Organization says
The study also noted that in case, when extreme heatwave conditions – referred to as temperatures that are higher than 99 percent days in a year —are recorded for five consecutive days, then heatwave related mortality can go up to 33.3 percent. The cities of Delhi, Ahmedabad, Hyderabad, Kolkata, Varanasi and Chennai observed such conditions this year.
According to the World Meteorological Organization (WMO), “A heat wave can be defined as a period where local excess heat accumulates over a sequence of unusually hot days and nights.”
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.
What the authors say
According to the authors of the study, “We estimated that around 1,116 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 three 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, whereas Shimla and Pune exhibited the lowest attributable mortality.
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Jeroen de Bont, researcher at the Institute of Environmental Medicine at Karolinska Institute and first author of the study says, “We wanted to find out how much the risk of death increases during heatwaves.”
What doctors say
Commenting on the study, Dr Pankaj Kumar, Senior Director, Critical Care, Fortis Hospital, Shalimar Bagh said, “Excessive hot weather causes heat stress, which may be associated with morbidity and mortality. It negatively affect mental health. Heat reduces physical work capacity and performance and adversely affects occupational health hazards.”
Dr Kumar added, “It is important as heat related deaths are largely preventable. This assumes more significance because of chances of climate change and global warming. There are documented reports about increase in deaths from cardiorespiratory diseases and adverse pregnancy outcomes. Heat stress affects extremely with age. Adults more than 65 years of age and children are more vulnerable.”
‘Patients with chronic diseases more susceptible’
“Patients suffering from other chronic diseases are also more susceptible. It has been shown that of all causes, cardiovascular illness is the primary cause of death during heatwaves. Dehydration caused by heat can lead to cardiac events and acute kidney failure. Heat derived lung damage causing acute respiratory distress syndrome and elevated air pollution during heat-waves is also known. Heat derived injuries may persist and continue damage even after cooling the body to normothermic temperatures. 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,” Dr Kumar said.
According to Dr M Wali, Senior Consultant, Department of Medicine, Sir Ganga Ram Hospital, “Extreme heat desaturates body cell proteins on increasing temperature, usually above 39 and completely paralyses neuronal functions above sustained body temperature of 40 degree Celsius or 104 degree Fahrenheit. The liver cells, brain cells and people with co-morbid conditions are more prone to higher mortality due to extreme heat waves.”
‘Diabetes, heart ailments a worry’
Dr Wali added, “Kidney cells, and the largest organ of the body that is skin, is affected. In some cases, due to halt in the sweating mechanism, the heat gets trapped in the body and thus heat stoke leads to coma. Many of such patients die having underlying diseases like diabetes, heart ailments, epileptic disorder and kidney ailments.”
“We have been seeing many deaths in North India every day, 5 to 10 per hospital, due to heat coma. Mortality is definitely increased due to decreased susceptibility to heat acclimatization, sudden or prolonged exposure and dehydration. Diabetics are more prone to heat stroke. Consumption of alcohol, and energy drinks also play a significant role. Chronic lung disease patients are too falling victim due to air pollution, complicated with heat stroke critical patients who are in ICU with neurological or cardiac disease are also adversely affected,” said Dr Wali.
Dr Wali added, “The stress of heat mediated through metabolic alterations complicated with dehydration can affect severely the vital organs’ functions like liver, kidney, brain and lungs leading to delirium and coma. Muscular system gets affected, leading to exhaustion and severe cramps.”
‘Extreme heat can cause organ failure’
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.”
Extreme heat waves also increase mortality rates, especially among existing critical patients with conditions such as cardiovascular diseases, respiratory disorders, and chronic illnesses. The physiological stress induced by extreme heat can exacerbate their conditions, leading to higher rates of complications, organ failure, and mortality.
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.
