The chart below shows the relative humidity and temperature and how they interplay to create lethal conditions such as heat stroke and death. See the second chart below which is in Celsius for recommendations to protect yourself.
National Weather Service health guidelines for extreme heat: https://www.weather.gov/safety/heat
Below is a similar chart in Celsius temperature readings. Scroll to the second page. SEE RECOMMENDED ACTIONS LISTED BY WET BULB TEMPERATURE.