Question: How long does it take for hypothermia to set in?

Answer: It Depends

I know that isn’t very satisfying, but hear me out. Hypothermia occurs when your character’s core temperature – usually somewhere between 97-99^oF – drops to below 95^oF. There are a lot of different factors that contribute to your character’s core body temperature. These include:

• The ambient air temperature
  • While hypothermia can theoretically occur at any temperature less than human body temperature, it usually isn’t a concern until the ambient air temperature drops below 50^oF.
• The windchill
• If your character is soaking wet, completely dry, or somewhere in between.
• If your character is submerged in water.
  • Water conducts heat really well, so your character will develop hypothermia more easily.
  • Hypothermia is possible in water temperatures as high as 70-80^oF.
• Your character’s clothing
• Your character’s body habitus
  • Fat characters will lose heat more slowly than super skinny ones.
• Your character’s age
  • Older characters (<65 years old) are more likely to develop hypothermia
  • Newborns, infants, and young children (<5 years old) are also more susceptible to hypothermia.
• Whether your character has recently drunk alcohol, taken drugs, or takes certain prescription medications.
• The surroundings, including their immediate surroundings (cold cement floor vs. carpeted floor)
• Your character’s body position
• Your character’s physical state
  • An exhausted or dehydrated character will be more susceptible to hypothermia.

It’s also important to note that not all hypothermia is equally dangerous. How low your character’s core temperature drops will determine the symptoms they exhibit.

• Mild hypothermia (90-95^oF): Shivering significantly. Awake and alert but unable to care for themselves.
• Moderate hypothermia (82-90^oF): May be conscious or unconscious. May or may not be shivering.
• Profound hypothermia (<82^oF): Unconscious. No longer shivering.

If your character’s body temperature drops too low (<75^oF), they may even appear dead. But there’s a saying in medicine: “you’re not dead until you’re warm and dead.” Cold temperatures have a way of preserving brain function. So even if your character is profoundly, devastatingly hypothermic, don’t let the other characters give up on them! With proper rewarming and resuscitation, they just might live.

Finally, if you’ve read through this whole blog and are still looking for some more concrete times, there is a formula you can use to calculate survival times in cold water. Developed by Hayward et. al. in 1975, this formula was developed by testing the core body temperatures of people – men and women of medium build wearing light clothing and life jackets – during submersion. The equation is:

Survival time (minutes) = 15 + 7.2 / (0.0785-.0035 x water temperature in celsius)

Hayward et. al. Feb 1975. 10.1139/y75-002

There isn’t an equivalent for air temperature – there are just too many variables – but you can check out this National Weather Service Windchill Chart to get an idea of how quickly frostbite will set in. Depending on how warmly your character is dressed, hypothermia may not be far behind.