Number of ‘dangerous humid heat days’ on the rise: Study

THe list of ‘ unsafe humid heat up years’ — those where the day-by-day maximum wet-bulb temperature, a measure that combines heat and humidity to capture how much cooling sweating can provide a person, was 25°C (77°F) or higher — has shot up across India, from an average of 101 such days a year in the 1970s to 141 days a year during 2016-2025, a new global study has found.Released on Wednesday by the US-based NGO Climate Central, the study attributed the increase primarily to human-induced climate crisis. Globally, it said, the number of such days has risen from 10 a year in the 1970s to 23 a year in the last decade (2016-25).The study covered 254 countries and territories and 961 cities worldwide, and found a sharp increase in dangerous humid heat days across Indian cities. In Delhi, the number rose from 96 days in the 1970s to 135 in 2016-25. In Mumbai, it rose from 136 to 206 over the same period, and in Chennai, from 205 to 257. The worst-affected Indian city assessed was Tirunelveli in Tamil Nadu, where the count rose from 119 to 273.Researchers analysed global weather conditions from 1970 to 2025 to determine how often dangerous humid heat occurred, and how much of it could be attributed to human-caused climate change. The study used data from the European Centre for Medium-Range Weather Forecasts’ (ECMWF) ERA5 dataset, a reanalysis record that reconstructs past weather conditions by combining observations with modelling.Nearly two-thirds (64%) of global dangerous humid heat days since 1970 can be attributed to human-caused climate change, the study said. It said these conditions put many people at risk of heat-related illnesses.“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 added.Dr Lisa Patel, a clinical associate professor of paediatrics at Stanford Children’s Health and executive director of the Medical Society Consortium on Climate and Health, said the numbers were a wake-up call.“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,” she said.