What are the lowest and highest temperatures humans can survive?
Being in cold temperature air with proper shelter and clothing, humans can endure indefinitely, even if the temperature is below zero degrees. Though, a person in regular clothing immersed in water just above freezing will last only a some minutes.
As for heat: A sauna can reach temperatures of close to 200 degrees, but as the air is around 10% humidity, it actually feels comfortable and has a lot of healing properties. Though, if the air is 90% humidity, then the safe temperature is more like around 110 degrees.