50 Indian Cities Face Invisible Stress; 'Dangerously Humid Heat' Doubles Worldwide: What Is Wet-Bulb Temperature?
Expert said India's heat risk is no longer driven by temperature alone, as rising humidity is making extreme heat far more dangerous, reports Surabhi Gupta.
Published : June 27, 2026 at 5:16 PM IST
New Delhi: Climate change is not only making summers hotter, but also far more dangerous by increasing humidity. A new report by US-based Climate Central has found that the number of dangerously humid days across the world has more than doubled since the 1970s, with India emerging as one of the countries most vulnerable to this growing threat.
The analysis of 961 cities worldwide, including 59 Indian cities, shows that the average number of dangerously humid days increased from 10 days a year during 1970-79 to 23 days annually between 2016 and 2025. The report estimates that 64 per cent of these dangerous humid-heat days since 1970 are directly linked to climate change caused by humans.
The findings are particularly worrying for India, where rising temperatures are combining with high humidity to increase the risk of heat stress. According to the study, 50 Indian cities are among those facing a sharp rise in dangerous humid heat, especially in coastal and densely populated regions.
Shailender Kumar, Assistant Professor and Senior Environmentalist at the University of Delhi, told ETV Bharat, “This is happening because climate change is simultaneously increasing temperature and atmospheric moisture. A warmer atmosphere can hold about 7 per cent more water vapour for every 1°C rise in temperature, making heat feel much more oppressive.”
He added, “Rapid urbanisation, loss of green cover, concretisation, shrinking water bodies, and the urban heat island effect are further trapping heat and preventing cities from cooling down, especially at night. Together, these factors are increasing wet-bulb temperatures, which reduce the body’s ability to cool itself through sweating and significantly raise the risk of heat-related illnesses and deaths. This is no longer just a climate issue, it is a public health, urban planning, and occupational safety challenge.”
What is wet-bulb temperature?
Unlike the regular air temperature, wet-bulb temperature (WBT) measures the combined effect of heat and humidity. It indicates how effectively the human body can cool itself through sweating.
When humidity is high, sweat evaporates slowly, making it difficult for the body to lose heat. This can quickly lead to heat exhaustion, dehydration, heat stroke and, in severe cases, death. Scientists say that a wet-bulb temperature of 35°C is the limit of human survival, as the body can no longer cool itself effectively.
Humid heat rising rapidly worldwide
The report shows a dramatic rise in dangerously humid weather over the past five decades. While the world recorded an average of 10 such days annually in the 1970s, that number has climbed to 23 days a year during 2016-2025.
In 2025 alone, the world experienced an average of 23 dangerously humid days, of which 19 days (83 per cent) were attributed to climate change. The study also notes that 2024 recorded the highest number at 32 dangerously humid days globally, highlighting the accelerating impact of global warming.
Coastal India among the worst affected
The report identifies Tamil Nadu, Andhra Pradesh, Maharashtra and West Bengal as some of the states witnessing the sharpest increase in dangerous humid heat.
Among the most affected cities:
- Tirunelveli saw dangerously humid days rise from 119 to 273 days a year.
- Chennai increased from 205 to 257 days.
- Tiruchirappalli rose from 129 to 251 days.
- Madurai jumped from 57 to 200 days.
- Vijayawada and Visakhapatnam now experience dangerous humid conditions for more than two-thirds of the year.
- The study also places Tirunelveli (34th), Chennai (39th), Tiruchirappalli (43rd) and Vijayawada (45th) among the 50 cities worldwide projected to experience the highest number of dangerously humid days.
- Mumbai region, North India also witnessing sharp increase
The impact is no longer limited to coastal areas. In Maharashtra, Mumbai and Navi Mumbai have seen dangerously humid days increase from 136 to 206 annually, while Thane and Dombivli have recorded an increase from 182 to 222 days.
Across North India, cities including Delhi, New Delhi, Najafgarh and Faridabad now experience around 30-40 dangerously humid days every year. Similar increases have been recorded in Agra, Lucknow, Kanpur, Prayagraj, Varanasi, Ghaziabad, Jaipur, Jodhpur, Kota, Bhopal, Indore, Ranchi and Jamshedpur.
Researchers say the combination of rising temperatures, monsoon moisture and the urban heat island effect is making humid heat increasingly dangerous in many inland cities.
However, the study also found that geography plays an important role. Bengaluru and Srinagar recorded no dangerously humid days during the study period, while higher-altitude cities such as Pune experienced relatively fewer such days.
Health risks likely to grow
Scientists warn that dangerously humid heat, once considered rare, is becoming increasingly common. The elderly, children, outdoor workers, pregnant women and people with chronic illnesses are particularly vulnerable, especially in areas with limited access to cooling, healthcare and heat-resilient infrastructure.
Upasona Ghosh, Associate Professor at the Public Health Foundation of India, said India needs to move beyond issuing heatwave advisories for outdoor workers and develop community-based heat adaptation plans, particularly for people living in informal settlements. She said measures such as cool roofs, flexible working hours and heat-health wage insurance should be expanded and evaluated for wider implementation.
Kumar said, “India is entering an era of "invisible heat stress," where humidity makes high temperatures far more dangerous than what thermometers alone indicate.
He concluded, “Wet-bulb temperature should become a key parameter in India's heat action plans. Climate adaptation must now extend beyond weather forecasting to include urban planning, occupational safety, public health preparedness and ecosystem restoration to protect vulnerable communities.”
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