From @beigegat’s article it says that from real expieriences it’s 31.5C

The oft-cited 35C value comes from a 2010 theoretical study. However, research co-authored by Kenney this year found that the real threshold our bodies can tolerate could be far lower. “Our data is actual human subject data and shows that the critical wet-bulb temperature is closer to 31.5C,” he says.