Researchers have developed a new modeling approach to assess the likelihood of human survivability and liveability during exposure to extreme heat. The new model can be applied in any climate regime and customized with populations that have potential co-morbidities. The approach integrates well-established principles from thermal physiology and human biophysics. The study’s results include current and future extreme heat across very hot and dry, and very hot and humid conditions, with risks increasing over most of the world.