Illustrations by SayoStudio “I can’t do this,” the woman in her 60s said as she lowered herself into the 40° Celsius water that filled a big silver tub at the end of a dimly lit room in the University of Ottawa’s Human and Environmental Physiology Research Unit. It was only after the temperature dropped a degree that she immersed herself, shoulder-deep, for the next two hours. Then she did it again — and again and again — every day for a week. “I’m going to be straight up: People hated doing this,” says Caroline Li-Maloney, a Ph.D. candidate at the university who studies how the human body adapts to extreme heat. …