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As the summer of 2026 approaches, the India Meteorological Department (IMD) has issued a critical warning. Between April and June, regions in eastern, central, and peninsular India are expected to experience more heatwave days than usual. However, scientists emphasize that the real danger lies not just in rising temperatures but also in the humidity levels in the air. While we often fear dry heat at 45°C, the humid heat of 35°C in coastal areas can be a 'silent killer'. Central India is already witnessing temperatures soaring between 42-45 degrees Celsius, making headlines. Yet, there are lesser-known factors that influence how millions perceive heat differently. Notably, humidity is a crucial element that scientists believe requires our attention.

Why is Humid Heat More Dangerous?

Sweating is the body's primary defense mechanism, but it fails in humid conditions. The human body relies on sweat to cool itself. When sweat evaporates from the skin, it carries heat away from the body. However, in humid heat (like that in Mumbai, Chennai, or Kerala), the air is already saturated with moisture, preventing sweat from evaporating. This leads to an inability to release internal heat, resulting in increased heart rates, elevated blood pressure, and in severe cases, organ failure or even death.

Understanding Wet-Bulb Temperature

In such scenarios, the air is so humid that sweat cannot evaporate. This inability to cool down can cause heart rates to spike, blood pressure to rise, and in extreme cases, bodily functions to cease. Scientists measure this combined threat using the 'wet-bulb temperature' (WBT), which accounts for both heat and humidity, unlike standard thermometers that only measure air temperature. For decades, it was believed that the human body could withstand a WBT of up to 35 degrees Celsius. This threshold was based on theoretical assumptions rather than actual human data.

New Findings on Human Tolerance to Heat

This understanding shifted with the 'Human Environmental Age Thresholds' (HEAT) project, a groundbreaking study where volunteers were placed in specially designed environmental chambers. They ingested small sensors to monitor temperature while gradually increasing heat and humidity levels. Researchers found that even young, fit adults began losing their ability to regulate core body temperature at a WBT of just 31 degrees Celsius. To put this into perspective, a combination of 38 degrees Celsius and 60% humidity is sufficient to push the human body beyond its cooling limits.

Vulnerable Populations at Greater Risk

For the elderly, those on medication, or individuals with pre-existing health conditions, the danger threshold is even lower. Professor W. Larry Kenney, who led the project, stated, "If we can determine the upper limits of temperature and humidity, we can better prepare people—especially those at higher risk—before heatwaves occur."

The Impending Threat to India

A study led by the IMD analyzing data from 1981 to 2020 revealed that wet-bulb temperatures have been rising along India's coasts. The monsoon, once considered a relief, is now a significant contributor to this threat. During active monsoon periods, the risk of humid heat in northern India can increase by up to 125%. When the monsoon stalls, this risk shifts southward, leaving regions like Kerala and peninsular India particularly vulnerable.

Future Projections and Concerns

Moreover, projections indicate that by the end of this century, nearly 70% of Indians may face wet-bulb temperatures of 32 degrees Celsius or higher. In such conditions, about 2% of the population—millions of people—could exceed the 35 degrees Celsius threshold, deemed potentially lethal. These threats are becoming increasingly alarming as the summer of 2026 is expected to be one of the hottest on record. The IMD has already forecasted more heatwave days than usual across eastern, central, northwestern, and southeastern peninsular India, with experts warning that it could surpass the record heat of 2024.

The Need for Awareness

Scientists have made it clear that monitoring temperature alone is insufficient. In a country like India, which is humid, coastal, and monsoon-prone, the most lethal heat often appears deceptively mild. Anjal Prakash, an IPCC author, remarked, "It seems that temperatures rising from 40 degrees Celsius to 50 degrees Celsius can become deadly. When wet-bulb temperatures exceed 35 degrees Celsius due to climate change, surviving without air conditioning becomes impossible. Stay vigilant."