Whether hot yoga is dangerous remains a contentious question. There have been a few alarming incidents reported during and after hot-yoga classes, including the 2016 case of a 35-year-old woman who experienced sudden cardiac arrest during a session. And a 2015 study reported that people's core temperatures during Bikram classes could rise above 39 degrees. But that research was flawed, says Tracy, whose 2014 study showed a maximum core temperate of 38.7, with an average rise to 37.9. The danger zone, he says, begins at 38.9 degrees.