June has arrived, and despite the onset of the Southwest monsoons in India, many parts of the country are still waiting for the rains to release us from the clutches of this relentless heat.
It has been a harsh summer for India. On a single day in April 2026, all 50 of the world’s hottest cities were in the country; by May 2026, it housed 97 of the world’s top 100 hottest cities. Andhra Pradesh alone recorded over 300 cases of heatstroke. Though consolidated data on heat-related illnesses are yet to emerge for 2026, India recorded over 7,000 heatstroke cases last year. A 2024 paper also affirmed the prevailing belief that heat-related deaths in India are massively underreported.
Studies suggest that India’s heat events are becoming more frequent and prolonged, though not more intense. From a public health perspective, this means exposure to heat stress for extended periods, without an opportunity to recover from the damage it causes. With recent studies suggesting that heat stress persists in the monsoons as well, it is more important now than ever to understand how the human body responds to heat.
Heat stress 101
Heat stress occurs when the body cannot adequately dissipate the heat it absorbs from the environment. It is not a single condition but a spectrum. Its earliest manifestations are heat rashes, sunburns, and painful limb cramps. These require only minimal medical intervention, if blisters form on the skin or the cramps become severe.
A heat stroke is the endpoint of heat stress. It is much harder to treat than to prevent
In the next stages, heat stress progresses to cause fainting, heat exhaustion, and finally, heat stroke. Heat exhaustion presents with heavy sweating, body temperature above 38°C (100.4°F), rapid breathing, blurred vision, a weak pulse, and nausea. While suffering from heat stroke, a person becomes dizzy, weak, and acts strangely; their body temperature rises above 41°C (105.8 °F), and their pulse quickens. Any stage after fainting requires medical attention, and heat stroke cases warrant calling an ambulance.
“Many people ignore early symptoms until the condition becomes serious. A heat stroke is the endpoint of heat stress. It is much harder to treat than to prevent,” says Dr. Keerthi Varman M, a General Physician and General Secretary of the Tamil Nadu Resident Doctors Association.
The best way to avoid heat stress is to avoid stepping out or doing strenuous activity during the hottest times of the day, wear loose-fitted clothes, and remain adequately hydrated. In case someone around you is displaying symptoms of heat stress, move the person to a cool area, offer them sips of water, loosen any tight clothing, and fan and spray them with water.
| Term |
Definition |
Characteristics |
| Heatwave |
A weather phenomenon: an unusually hot period lasting several days |
Atmospheric conditions; no universal definition exists |
| Heat Stress |
A physiological condition: when the body cannot rid itself of excess heat |
A combination of high temperature, humidity, physical exertion, and individual vulnerability |
| Heat Risk |
A composite assessment of danger: the interplay between heat hazard, exposure, and vulnerability |
Shaped by who is exposed, where they live, their health status, and their capacity to adapt and cope |
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Why the body’s cooling mechanisms fail
The human body constantly tries to maintain a stable internal temperature, usually between 36.2 C to 36.7 C (97.3 F to 98.2 F). The hypothalamus acts as the brain’s central thermostat, continuously monitoring core temperature via thermoreceptors in the blood and skin to maintain equilibrium. Under normal conditions, it triggers cooling mechanisms like sweating and vasodilation, or the widening of blood vessels, which allows more blood to flow to the skin, from where it is dissipated into the atmosphere.
When sweating does not work, vasodilation becomes the only way left to cool the body, which puts pressure on the heart vessels.
This system is insufficient in extreme heat events, and becomes inefficient when high temperatures are coupled with high humidity. If the air is already saturated with moisture, sweat will not evaporate. It is this act of evaporation—and not the act of sweating by itself—that cools the body. In just under a decade, India’s average relative humidity has increased from 67.1% to 71.2%, making humidity-related heat stress a rising concern. When sweating does not work, vasodilation becomes the only way left to cool the body, which puts pressure on the heart vessels. This (alongside ozone poisoning) is why excess heat is linked to cardiovascular stress. In fact, a 2025 study found cardiovascular failure to be the cause of most heat-related fatalities. Excessive heat is also linked with impaired kidney function and a worsening of chronic respiratory illnesses and diabetes.
Though high temperatures are usually associated with the daytime, factors like pollution and urban heat islands have contributed to warmer nights over the past decade. The body usually recovers from heat stress during the night, making warmer nights a compounding factor in heat-related illnesses. As a result, body temperature continues to rise and reaches a point that exceeds the normal temperature range, leading to heat stress.
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Measuring heat stress
It is this layered understanding of how humans experience heat that led to the development of heat indices. The simplest one is the Heat Index which considers the air temperature and humidity to give a more realistic understanding of how easily the body can cool itself through sweating.
However, the amount of heat the body absorbs and how easily it cools itself depends on many other environmental factors. So, two more advanced indices were developed: the Wet Bulb Globe Temperature (WBGT), and the Universal Thermal Climate Index (UTCI). The WBGT assesses heat using more parameters, like wind speed and solar radiation, while the UTCI takes a more interdisciplinary approach and integrates areas like thermal physiology, occupational medicine, and mathematical modelling. There are also specialised indices like the Thermal Work Limit (TWL), which is used mainly for workplace safety. It helps determine how much physical work a person can safely do in hot conditions by also considering clothing and air pressure. Though UTCI is fast gaining favour globally, India continues to rely on the Heat Index and WBGT.
The prevailing understanding is that the human threshold is a wet bulb temperature of 35°C.
All of these indices map the heat stress they measure to the corresponding physiological responses it results in, and the kind of adjustment in activity it might require. Naturally, one might wonder: how hot is too hot?
The prevailing understanding is that the human threshold is a wet bulb temperature of 35°C. Beyond this limit, even a young, healthy person seated in the shade and having access to enough drinking water would experience a continual increase in core body temperature, which will escalate to heatstroke and death within a few hours. However, some recent studies found minimally active young adults to reach thermoregulatory failure at wet bulb temperatures as low as 26°C or 31°C. More importantly, research scholars from Harvard say that—even from a purely physiological viewpoint—there is no straightforward answer. Factors like age, income, and preexisting health conditions change one’s capacity to withstand heat, posing the question: ‘too hot for whom?’.
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Prevention and planning
The questions of ‘how hot is too hot?’ and ‘too hot for whom?’ similarly transfer to a sociological study of heat stress. Roughly three-quarters of India’s workforce—about 380 million people—work in occupations like agriculture and construction which have high heat exposure. Over 90 percent of the labour force works in the informal sector, with no guarantee of basic employee protections or employer liability.
This puts certain sections of society most at risk of heat stress: those who work outdoors, and the rural and urban poor. This includes construction workers, sanitation workers, street vendors, agricultural labourers, salt pan workers, delivery workers, and traffic police personnel. The rural poor, who cook on firewood stoves and walk long distances to fetch water, face prolonged, compounded heat exposure with little relief. A case study in Yavatmal, Maharashtra found that in a district where 64% of houses have tin roofs, indoor temperatures reached up to 45.8°C during peak summer hours, exceeding outdoor temperatures—meaning the home meant to shelter them only worsened their exposure. The urban poor in informal settlements face near identical conditions, where building materials trap heat and make staying indoors as dangerous as working outside. Certain occupations are also inherently more at risk, like those who work in glass or rubber factories, experiencing additional heat stress due to their workplace. Caste-based occupations, similarly, involve higher exposure to heat stress, as quantified by a 2024 IIT-B study.
The rural poor, who cook on firewood stoves and walk long distances to fetch water, face prolonged, compounded heat exposure with little relief.
Without clearly defined employee-employer relationships, it is hard to articulate how best to protect informal workers. But they are also the ones hit hardest by the absence of policies. A 2026 paper by Harvard University’s Salata Institute for Climate and Sustainability proposes coordination between various stakeholders, including worker unions, employers, governments and civil society institutions, to fasten progress. They do emphasise, however, that it would be unreasonable to expect employers to be able to fund and deliver these protections alone, and that sustainable funding mechanisms must be explored.
Environmental engineer and activist Prabhakaran Veeraarasu points out that these variations in how heat stress affects people does not reflect in the Heat Action Plans (HAPs) that cities prepare. This happens at two levels. India currently relies primarily on absolute temperature thresholds defined by the Indian Meteorological Department (IMD) for its Heat Action Plans (HAPs). Policies built on the cruder surface temperatures are working with incomplete data and will produce incomplete solutions. Notable exceptions are Thane city (since 2024), and Tamil Nadu, since earlier this month, which have both adopted a basic Heat Index instead.
The more serious concern is structural. Heat Action Plans worldwide have a tendency to lack legislative and financial backing, resulting in committees, but not change. India’s HAPs, specifically, suffer from short-sightedness; immediate measures like water coolers are not accompanied by long-term measures to increase shaded areas.
Until heat is understood not just as a weather condition but as a lived physiological and social burden, many of its worst effects will continue to go unrecognised and unprevented.