A heat index is a measure that combines air temperature and relative humidity to estimate how hot it feels to the human body. It reflects the reduced efficiency of sweating when humidity is high, making the same air temperature feel significantly hotter.
To determine safety during a heat wave, though, scientists use the wet-bulb temperature, which is a more direct measure of the body’s ability to cool itself through evaporation. It represents the lowest temperature that can be reached by evaporating water into the air and is calculated using temperature, humidity, and air movement.
While the heat index is a perceived temperature used for public weather warnings, the wet-bulb temperature is a physiological limit: sustained exposure to wet-bulb temperatures around 35°C (95°F) can be fatal even to healthy people in the shade, because the body can no longer shed heat through sweat.
In short, the heat index tells you how hot it feels, while the wet-bulb temperature tells you whether the body can survive.