The onset time of hypothermia varies greatly depending on the individual and upon circumstance. It is possible for a dry person to be exposed to cold temperatures for many hours or even days before hypothermia sets in. In contrast, Minnesota Sea Grant reports that a person in water succumbs to hypothermia in as little as 30 minutes when submerged in temperatures between 32.5 and 40 degrees Fahrenheit.
WebMD defines hypothermia as a drop in body temperature from the normal average of 98.6 degrees Fahrenheit to under 95 degrees Fahrenheit. Hypothermia is usually associated with prolonged exposure to cold temperatures, but factors such as wind and wetness increase the chance of hypothermia, even in relatively warm conditions or waters.
A person's physical characteristics play a large role in the onset time of hypothermia. WebMD states that elderly persons or infants are at risk of developing mild hypothermia overnight in temperatures as high as 60 degrees Fahrenheit while sleeping. According to Minnesota Sea Grant, a person's level of exertion while exposed also changes hypothermia onset times. For example, a person treading water that measures 50 degrees Fahrenheit has half the expected survival time as someone wearing a personal floatation device. WebMD lists drug and alcohol abuse, severe trauma, and certain medical conditions, such as diabetes, as additional factors that increase a person's susceptibility to hypothermia.