Dangerous Humid Heat Days Surge Across India, Study Finds

India is experiencing a dramatic surge in 'dangerous humid heat days,' according to a new global study released by the US-based NGO Climate Central. These days are defined by a wet-bulb temperature of 25°C (77°F) or higher, a measure combining heat and humidity to indicate the body's reduced ability to cool itself through sweating.

Nationally, the average number of such days has climbed from 101 per year in the 1970s to an estimated 141 days annually between 2016 and 2025. This trend mirrors a global increase, where dangerous humid heat days have more than doubled from 10 a year in the 1970s to 23 a year in the last decade. Climate Central attributes this rise primarily to the human-induced climate crisis.

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While high temperatures pose risks to everyone, older adults, children, pregnant people, individuals with underlying health conditions, and people without access to cooling face disproportionately greater dangers. Humidity only piles on to the risks, making even seemingly mild days far more dangerous than they appear. As global temperatures continue to rise due to the burning of fossil fuels, dangerous humid heat is becoming more frequent and widespread.

The study, which analyzed weather conditions from 1970 to 2025 using data from the European Centre for Medium-Range Weather Forecasts, found sharp increases across Indian cities. Delhi saw its dangerous humid heat days rise from 96 to 135, Mumbai from 136 to 206, and Chennai from 205 to 257. Tirunelveli in Tamil Nadu recorded the most significant increase, jumping from 119 to 273 dangerous humid heat days per year.

Researchers estimate that nearly two-thirds (64%) of global dangerous humid heat days since 1970 are linked to human-caused climate change. Experts warn that these conditions pose significant risks, particularly to vulnerable populations like the elderly, children, pregnant individuals, and those without access to cooling. Dr. Lisa Patel of Stanford Children’s Health described the findings as a "wake-up call," emphasizing the need for public health officials to anticipate and prepare for heat-related illnesses as global temperatures continue to climb due to fossil fuel consumption.

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Dangerous humid heat has more than doubled since the 1970s. We’re already seeing the consequences play out in real time. Fans are fainting at World Cup matches in cities like Houston, and that’s not a coincidence. This kind of data is exactly the tool clinicians and public health officials need to anticipate where heat-related illness will strike and who is most at risk before people end up in the ER.