How hot is too hot? A data-driven approach can help India beat the heat.

Various factors influence heat stress, including acclimatization, weather, activity levels, age and pre-existing health conditions. (PTI)
Various factors influence heat stress, including acclimatization, weather, activity levels, age and pre-existing health conditions. (PTI)


  • Heat stress looms as the country’s foremost health hazard but scientifically updated heat-action plans that consider variables like mortality, morbidity and ambulance calls could help reduce vulnerability. We must shift focus to long-term heat risk mitigation.

As India’s new government takes office, it does so in a blisteringly hot and record-breaking summer. The capital alone saw a maximum temperature of 49.9° Celsius and has seen several heatwaves this season. While the monsoon has started impacting many regions, it is crucial to understand that there is a new normal when it comes to heat, and it affects all—politically, economically and socially. So, how hot is too hot?

The year 2024 has confirmed predictions of the Intergovernmental Panel on Climate Change (IPCC) that extreme heat-related health risks will be the biggest climate threat for South Asian countries. In April, over eight Indian states, particularly those on both coasts, experienced severe heatwave conditions. 

The northern and central regions faced intense heatwaves in May, with more than 37 cities continuously recording daytime highs above 45° Celsius. Cities such as Delhi grappled with the combined effects of extreme heat and air pollution. Since March, these temperatures have led to nearly 25,000 reported heatstroke cases, going by government data, record-breaking power demand, lower water levels in reservoirs, and damage to fruit crops.

Also read: Heatwave: 56 heat-related deaths, 25,000 heatstroke cases reported in India from March to May

With heatwaves becoming more intense and frequent due to climate change, and a recent study showing that Indian cities are warming at twice the rate as the rest of India, how do we build resilience against the varying degree of vulnerability arising from heat risks across the country?

Understanding the difference between heatwaves and heat stress is crucial for strengthening resilience. Heatwaves are declared by agencies like the India Meteorological Department (IMD) and World Meteorological Organization (WMO) based on specific weather criteria such as departures of air temperature from normal levels for 2 to 5 consecutive days, whereas heat stress is when the human body absorbs heat beyond its tolerance limit. 

Various factors influence heat stress, including acclimatization, weather, activity levels, age and pre-existing health conditions. It essentially determines how hot is too hot. Take, for instance, the 2003 European heatwave that resulted in about 15,000 deaths in France, with temperatures hovering around 37° Celsius for a few days. 

In India, heatwaves are declared only when temperatures cross 40° Celsius in the plains, 30° Celsius in hills and 37° Celsius in coastal areas. These thresholds are based on the normal summer climate of the past to which the local population has acclimatized.

However, an analysis conducted by the Council on Energy, Environment and Water (CEEW) for the heat action plan for Thane, a coastal city in Maharashtra, unveils concerning trends. One, the increase in extreme warm nights over the last decade surpasses that of extreme hot days, which poses serious health concerns. 

Second, we noticed that ‘felt heat,’ which takes into account both temperature and humidity, was actually 3-4°C higher than just dry heat. This means even if the temperature is 35° Celsius, you might feel like it is 38° Celsius, as was seen in the months of May and June.

To build resilience against the evolving risk due to heat, city-level heat action plans (HAPs) must address three key aspects.

Also read: Heatstroke vs Heat Exhaustion: 7 essential tips for staying safe as temperatures soar

One, when to take action: Unlike floods or cyclones, heatwaves vary in their impacts, which are largely influenced by individual health vulnerability. Hence, HAPs should establish local city-level heat stress thresholds, indicating temperatures and meteorological conditions hazardous to human health. Cities like Thane and Ahmedabad have already implemented them. However, currently, more than 67% of Indian HAPs still lack these crucial thresholds.

Analysing past occurrences of daily heat extremes and correlating data-sets of variables such as mortality, morbidity and ambulance calls is one way to establish these thresholds. However, this is challenging due to a scarcity of health data linked to heat-related illnesses and fatalities, with only a few HAPs, such as the Ahmedabad HAP, having been able to do so. 

The IMD’s forthcoming Indian context-specific heat index for forecasts from next year and the National Centre for Disease Control’s initiatives on heat-health data collection (under the National Programme on Climate Change and Human Health) are positive steps to strengthen thresholds based on the impact of temperature on heat-related mortality and illnesses.

Two, where to take action: Extreme heat affects diverse populations differently, influenced by socio-economic and health factors. However, nearly 97% of Indian HAPs lack vulnerability and risk mapping, which leaves decision-makers with very little information that can guide a priority order for action when a heatwave strikes. 

It is imperative to conduct granular mapping in HAPs to identify vulnerable groups, such as outdoor workers, slum residents and the elderly, along with assessing the degree of heat risk they face.

Three, how to take action: For the strategies laid out in HAPs to succeed, timely implementation is critical. HAPs play a crucial role in coordinating inter-agency efforts by assigning roles and responsibilities for heat action. The involvement of gram panchayat-level decision-makers in Odisha’s heatwave management mechanisms exemplifies this. Such mechanisms help streamline implementation plans for local administrations.

Also read: How heat stress has increased in urban India this year

A 42° Celsius day in Mumbai will impact someone working in a grocery shop differently than a food delivery driver or an IT professional working in a large company. Then you add humidity. India has demonstrated leadership in reducing extreme heat impacts through proactive measures like HAPs. As these plans enter their second generation, they should shift focus from response to preparedness and long-term heat risk mitigation.

Heat stress looms as the foremost health hazard in India and South Asia. Yet, united and powered by data, we can beat the heat.

These are the authors’ personal views.

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