A single day of extreme heat causes about 3,400 excess deaths across India, while a five-day heatwave causes nearly 30,000, a study has estimated.

Researchers Piyush Narang and Ashok Gadgil from the India Energy and Climate Center at the University of California Berkeley, US, said that even though global studies highlight a surging heat-related mortality, granular spatial-temporal data on how heatwaves affect mortality in India's districts remain inaccessible to common researchers.

Beat the heat with people-centric responses

The team adapted findings from a multi-city analysis of heat-related mortality across 10 cities in India to estimate excess deaths across all districts in the country.

'Excess deaths' is a public health metric that refers to the difference between total deaths over a specific period and deaths that could be expected to occur based on historical data.

The study, published in the journal Frontiers in Environmental Health, integrated district-level mortality rates from the Civil Registration System and population projections for 2024 to obtain district-level excess death estimates under one-day and five-day heatwave scenarios.

"We estimate that a single day of extreme heat causes approximately 3,400 excess deaths nationally; a five-day heatwave causes nearly 30,000," the authors wrote.

Heat mortality burden

Heatwave to severe heatwave conditions were prevailing in north, central and east India, with temperatures consistently exceeding 45 degrees Celsius in Madhya Pradesh, Rajasthan, parts of Uttar Pradesh and Haryana over past few days.

Mapping heat-induced mortality risk to individual districts revealed that the state of Uttar Pradesh alone accounts for about 8,100 excess deaths during a five-day heatwave, while excess deaths in districts, including Ahmedabad, Jaipur and Surat, each exceed 250 in a single event.

A 2.3-fold disproportion between mortality burden and economic capacity was found in the five highest death-burden states of Uttar Pradesh, Bihar, Madhya Pradesh, Rajasthan, and Gujarat, which together accounted for 66% of the country's excess deaths while contributing only 29% of India's GDP.

The result has direct and urgent implications for how India designs and funds its heat resilience architecture, the researchers said.

"The 2.3× GDP disproportion documented here provides a quantitative basis for arguing that federal adaptation investment, including funding under the National Disaster Management Authority and the National Action Plan on Climate Change, should be weighted toward high-burden, low-GDP states rather than allocated in proportion to population or administrative capacity," they wrote.

The top 100 districts, comprising nearly one-third of India's population, accounted for 44% of excess deaths projected for a five-day heatwave event, the researchers found.

Further, "heatwave mortality risk is not merely proportional to population size but is structurally concentrated in states with lower economic output (which are) precisely those with the least fiscal capacity to invest in adaptation," the authors said.

They added that the district-level estimates presented in the study align with findings from an increasing body of epidemiological and modelling evidence that suggest South Asia, especially India, is particularly vulnerable to heat-related mortality.