If wet bulb temperatures exceed 31°C (88°F), people cannot consistently perform physical labor without endangering their lives; in temperatures that exceed 35°C (95°F), a healthy human can die within a few hours without access to water or shelter. The authors of the PNAS study analyzed "wet-bulb temperature thresholds across a range of air temperatures and relative humidities" using bias-corrected climate change models. Their conclusions were sobering.
"Some of the most populated regions, typically lower-middle income countries in the moist tropics and subtropics, violate this threshold well before 3°C of [global] warming," the authors write.