Check every hour. When one characteristic reaches zero, further dice are frost damage. When two characteristics reach zero, character is unconscious. 1D of stun damage. When one characteristic reaches zero, character is unconscious.
Further damage is from heat.
There are modifications of this rule for infants. Resolution in two weeks without scarring is anticipated. The dermis, or living layer of the skin, is partially involved. Blistering of the skin is evident on inspection. Pain is common and often intense.