What degree of coldness is considered life threatening?

Answer 1

The external temperature is not usually a measure of "life threatening" but the core temperature is. Loss of heat to the environment is due to the temperature gradient (this occurs only as long as the ambient temperature is below 98.6). It also depends on the amount of clothing and, of course the amount of shelter.
Core temperature is the temperature that is essential to the overall metabolic rate of the body. The temperature of the periphery is not critical. So this is what is used to determine how a person doing in the weather they are in. There are so many variables (wind, rain, snow, clothing and shelter) that the core temperature is what is used.
Basically below freezing, 40 degrees in wind and rain, 60 degrees in hurricanes can cause hypothermia. If the core temperature is below 97, then problems will begin to occur and these can be life threating.