Being in cold temperature air with proper shelter and clothing, humans can survive indefinitely, even if the temperature is below zero degrees. However, a person in regular clothing immersed in water just above freezing will last only a few minutes.

As for heat: A sauna can reach temperatures of close to 200 degrees, but since the air is around 10% humidity, it actually feels comfortable and has many healing properties. However, if the air is 90% humidity, then the safe temperature is more like around 110 degrees.

So maybe a better question to ask is at what temperature and humidity combinations are humans capable of surviving?