Bengaluru: A single-day heatwave can cause around 3,400 excess deaths every year in India, and that number can climb to around 30,000 during a prolonged, five-day heatwave, according to a study published on May 26.These numbers are far higher than figures quoted by the Indian government on multiple occasions. Researchers studying heat and its impacts in India told The Wire that even the estimates listed by the study could be conservative. Despite this, the study is an important start to a much-needed conversation on the real impacts of heatwaves – impacts that will otherwise remain invisible, because India undercounts heat deaths drastically, they said.Heatwaves and deathsHeatwaves are periods of unusually hot weather that can impact human health adversely. According to the India Meteorological Department (IMD), a region in India is classified as witnessing a heat wave when the maximum temperature reaches at least 40º C or more if it is located in the plains, or at 30º C or more in the hills. The IMD also declares a region to be experiencing a heat wave when its temperature departs from normal by 4.5º 6.4º C. If the departure from normal is more than 6.4º C, the region is said to be witnessing a ‘severe’ heat wave. Parts of northwest and central India including Delhi have been reeling under a heatwave since May 17.Heat can also kill. Hot days, unusually warm nights, high humidity and different combinations of these can create an environment that can be fatal for people, especially vulnerable sections such as the elderly. In 2024, a large team of scientists affiliated with 14 institutions across the world including the Delhi-based Centre for Policy Research, Ashoka University and the National Research Development Corporation India analysed the association between deaths and daily mean temperatures across ten Indian cities from 2008 to 2019. They defined an extreme heatwave as two consecutive days with an intensity above the 97th annual percentile: meaning, that for two consecutive days, the daily temperature was hotter than 97% of all historically recorded temperatures for that specific location. They found that such a two-day heatwave was associated with a 14.7% increase in daily mortality. Heatwaves caused around 1116 deaths annually in these ten cities alone.Huge increase in mortality at national levelScientists Piyush Narang and Ashok Gadgil of the University of California Berkeley, United States, built on this study. They extrapolated the 2024 study to all districts in India using publicly-available district-level population and mortality data (data provided by the Civil Registration System of India for the year 2020), and combined these with a climate classification system to ascertain how heat led to excess deaths in specific climatic zones. Estimating excess deaths is important because they often give a better estimate by capturing both the direct and indirect deaths in a population, primarily due to the methods it uses (such as the use of all-cause mortality data in estimating these numbers). The team calculated heat-associated excess deaths across each district based on two heatwave scenarios, as defined in the 2024 study: a single-day heatwave (a short duration heatwave), and a prolonged five-day heatwave. They found that a one-day heatwave could cause approximately 3,400 excess deaths across India, while a single prolonged five-day heatwave could cause around 30,000 excess deaths.“This finding indicates that even a one-day exposure to extreme heat can produce a significant increase in mortality at the national scale, much larger than the all-India heat-related excess mortality reported in the press and by government agencies, which remains about 800 per year,” the study said. Government estimates cite low numbers of heat deaths annually; and there are discrepancies across records. According to a statement by the Ministry of Earth Sciences in August last year, 3,798 people across India died between 2018 and 2022 due to heat-related causes. The National Crime Records Bureau (NCRB) reported only 20,615 heatstroke deaths during a 20-year-period, according to the Indian Express. An online data gathering exercise by HeatWatch (a nonprofit working on heat stress mitigation and adaptation) found that between March and June 2024, 733 deaths occurred due to heatstroke in 17 states in India. However, the data presented by the Ministry of Health and Family Welfare in the Lok Sabha session that year listed only 360 deaths between March 1 and July 25 for these 17 states.This is why the estimates provided by the new study matter: because heat deaths are “chronically underreported” in India, said Apekshita Varshney, founder and executive director of HeatWatch. “Most heat deaths never appear in official “heatstroke” statistics, so the true impact of heatwaves is invisible. By using excess‑mortality methods, this study helps fill that gap and offers a more realistic, if still conservative, picture of how many people are dying,” she told The Wire.These estimates may be conservative because the scientists use the 97th percentile, and the dry bulb definition of heatwaves, commented Varshney. This means that coastal heatwaves which don’t breach these records would be missed in this estimate; the authors have also accepted this, she said.Some regions, districts are hardest-hitThe team also found that heat hit some regions harder than others. For instance, Uttar Pradesh alone accounted for approximately 8,056 excess deaths during a five-day heatwave, followed by Bihar (approximately 3,615 deaths), Madhya Pradesh (approximately 2,964 deaths), Rajasthan (approximately 2,664 deaths) and Gujarat (approximately 2,354 deaths). These five states alone, comprising 43% of India’s population, accounted for more than 60% of total national excess mortality under the five-day heatwave scenario. The scientists who authored the study call this “a profound and troubling environmental injustice”.“The five states bearing the highest heatwave mortality burden account for 66% of national excess deaths while contributing only 29% of India’s Gross Domestic Product (GDP). This 2.3-fold disproportion between mortality burden and economic capacity means that the states least able to finance adaptation are precisely those facing the greatest heat mortality risk,” they wrote. There were clear district-level trends too. The team estimated that in a five-day heat wave, the highest estimated excess deaths are expected for Ahmedabad (approximately 307 deaths), Jaipur (approximately 265 deaths) and Surat (approximately 261 deaths). Other vulnerable districts include Prayagraj, Patna, Lucknow, Kanpur Nagar, Azamgarh, Agra and Bareilly, each with expected excess deaths exceeding 180.“The district‑level granularity is crucial,” Varshney said. “It shows that heat is not a rare disaster, but a frequent and rising killer, concentrated in specific regions that are often poorer and less able to adapt…like how a handful of states like Uttar Pradesh, Bihar, and Madhya Pradesh carry a disproportionate mortality burden. That shifts heatwaves from being treated as occasional weather events to being recognised as a recurring public‑health emergency.”An important “first step”Per the study, its findings “indicate that even short-duration heatwaves can result in thousands of excess deaths nationally, while prolonged heat events pose risks comparable to large-scale public health emergencies”.And with climate change making extreme heat events more frequent and intense, “failure to act on such evidence is likely to result in continued large and avoidable loss of life”, the study warned.“Strengthening mortality surveillance, improving access to high-resolution temperature data, and integrating heatwave preparedness into district-level public health and disaster management systems are critical steps toward reducing preventable deaths from extreme heat in India,” it noted.While the authors of the study had written about these findings in an article for Down To Earth last year, their results were published as peer-reviewed research in the journal Frontiers in Environmental Health on May 26.The study is an important “first step”, Varshney commented.“A finding that a single intense heat day in India can generate on the order of 3,400 excess deaths nationally can appear frightening, especially when compared to very low official annual figures. But we must read these numbers as an order‑of‑magnitude signal of severe undercounting, not a precise tally.”Such evidence‑based estimates for heat can help us argue for investment in heat‑resilient infrastructure, systems and processes – just like how the Odisha Super Cyclone in 1999 prompted serious investments in preparedness, relief and rehabilitation, Varshney said.“They [evidence-based estimates] also strengthen the case for national‑level disaster declarations, dedicated finance, and capacity‑building of disaster management and inter‑departmental coordination,” she added.Currently, the Indian government does not recognise heatwaves as a notified national disaster, which limits the funding that states and departments can avail of to implement actions to tackle heat.