The number of “dangerous humid heat days” — those in which the maximum daily humid temperature, a measure that combines heat and humidity to capture how much cooling sweating can provide a person, is 25 degrees Celsius (77 degrees Fahrenheit) or higher — has risen 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.
The study, published on Wednesday by the US-based NGO Climate Central, attributed the increase primarily to the human-caused climate crisis. Globally, the number of such days has increased from 10 days per year in the 1970s to 23 days per year in the past decade (2016-25).
The study covered 254 countries and regions and 961 cities across the world 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 days in 2016-2025. In Mumbai, the number rose from 136 to 206 during the same period, and in Chennai from 205 to 257. The worst affected Indian city assessed was Tirunelveli in Tamil Nadu, where the number rose from 119 to 273.
The researchers analyzed 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 Center for Medium-Range Weather Forecasts’ (ECMWF) ERA5 dataset, a reanalysis record that reconstructs past weather conditions by combining observations and modelling.
Nearly two-thirds (64%) of dangerous humid heat days globally since 1970 can be attributed to human-caused climate change, the study said. She said these conditions put many people at risk of heat-related illnesses.
The study added: “While high temperatures pose risks to everyone, older adults, children, pregnant women, individuals with underlying health conditions, and people without access to refrigeration face disproportionately greater risks. Humidity only accumulates the risks, making even seemingly mild days much 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.”
Dr. Lisa Patel, associate professor of pediatrics 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 temperatures have more than doubled since the 1970s,” she said. “And 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 no coincidence. This kind of data is exactly the tool doctors and public health officials need to predict where heat-related illnesses are emerging and who is most at risk before people end up in the emergency room.”
