India's deadly heatwaves are getting worse: Here's what extreme heat actually does to your body

Most people think of a heatwave as just a stretch of uncomfortable days that includes sweat, irritability, maybe a bad night's sleep. But ask any emergency physician working through an Indian summer and they will tell you it's far more than that. In India, where temperatures are rising faster and staying higher for longer, understanding what extreme heat does to your health isn't optional anymore.

“Heat Waves typically occur between March and June, and in some rare cases even extend till July. The extreme temperatures and resultant atmospheric conditions adversely affect people living in these regions as they cause physiological stress, sometimes resulting in death,” Dr. Nethra N, Senior Specialist – Internal Medicine, KIMS Hospitals, Electronic City, Bengaluru told TOI Health.

The India Meteorological Department (IMD) defines a heatwave as a condition where the maximum temperature of a station reaches at least 40°C for plains and 30°C for hilly regions. A severe heatwave is when that departure exceeds 6.4°C.

The numbers behind the crisis

According to NCRB data, more than 20,000 people died from heatstroke between 2001 and 2020. On average, more than 1,000 Indians die every year due to heat waves, and some estimates suggest that a single day of a heatwave across the country can result in 3,400 deaths. A peer-reviewed study published in Frontiers in Environmental Health in 2026 modelled this further, estimating that a five-day heatwave causes nearly 30,000 excess deaths nationally.

The Lancet's 2025 Countdown report found that India recorded an average of 19.8 heatwave days last year, of which 6.6 days would not have occurred without human-induced climate change. Heat exposure in 2024 also resulted in the loss of 247 billion potential labour hours, mostly in the agriculture and construction sectors, amounting to an economic loss of about $194 billion. So the damage isn't just medical — it's systemic. “Compared to 1990-1999, in 2024, people were exposed on average to 366 more hours during which ambient heat would have posed a moderate or higher risk of heat stress if undertaking moderate outdoor physical activity. This was a record high,” the report said.

What extreme heat does to the body

Dr. Nethra N explains that heat-related illness sits on a spectrum. It starts with heat cramps, swelling and fainting, usually with a fever below 39°C. Then comes heat exhaustion: fatigue, dizziness, nausea, muscle cramps, heavy sweating. And at the most severe end, heatstroke, body temperature at or above 40°C, potentially accompanied by delirium, seizures, or coma. That last one is potentially fatal and demands immediate emergency care.

But the hidden damage is just as concerning. Research has identified multiple physiological mechanisms triggered by heat exposure, including ischemia, heat cytotoxicity, inflammatory response, disseminated intravascular coagulation, and rhabdomyolysis, that can critically impact vital organs including the brain, heart, intestines, kidneys, liver, lungs, and pancreas. Essentially, sustained extreme heat doesn't just make you feel terrible. It can quietly injure organs that have no obvious symptoms until something goes seriously wrong.

Among the indirect effects, heat exposure stresses underlying physiological systems and results in conditions like renal insufficiency, acute cerebrovascular and cardiovascular disease, and exacerbations of pulmonary disease. People with pre-existing heart disease, high blood pressure, or kidney conditions are especially vulnerable, because their systems are already under load, and the added stress of heat can push them over the edge.

Who's most at risk

The elderly, infants and young children, outdoor workers, pregnant women, and people with chronic illnesses are in the highest-risk category.

What you should actually do

Dr. Nethra N's advice is practical and non-negotiable.

• Stay hydrated and don't wait until you're thirsty to drink water, because thirst is a poor indicator of dehydration.
• ORS, buttermilk, lemon water with salt aren't just home remedies, they're medically sound options for replacing electrolytes lost through sweat.
• Wear loose, light-coloured cotton clothing.
• Cover your head when stepping out.
• Avoid going outside between noon and 3pm, when the heat is at its most intense.
• And if someone around you is showing signs of heat exhaustion move them to shade immediately, cool them down with wet clothes, and call a doctor if symptoms worsen or they lose consciousness.
• Don't give alcohol, tea, coffee, or carbonated drinks to someone who's overheating.
• Don't leave children or pets in parked vehicles, ever. The temperature inside a closed car in the Indian summer sun can become dangerous within minutes.

India's summers are no longer just an inconvenience to push through. Extreme heat is a medical emergency in slow motion, and it's getting worse every year.

Medical experts consulted This article includes expert inputs shared with TOI Health by:

Dr. Nethra N, Senior Specialist – Internal Medicine, KIMS Hospitals, Electronic City, Bengaluru

Inputs were used to explain the how heatwaves are affecting human health and what can be done to stay safe.