The study will compare the rewarming effectiveness of heat donation through the head or the torso during rewarming of mildly hypothermic subjects. Warming will be accomplished through either forced-air warming or using a charcoal heater; both units are commonly used for warming. The investigators hypothesize that head warming will be as, or more, effective compared to

Dr. Gordon Giesbrecht is studying/comparing the effectiveness of heat donation through the head or the torso in rewarming a mildly hypothermic individual. A rigid forced-air warming cover for the head or the torso will be used to rewarm the hypothermic individuals.

Subjects will be asked to participate in three experimental trials, separated by at least 48 hours. Each trial will last about 4 hours (1 hour for setup, 1 hour for cooling, and 2 hours for rewarming and removal of instrumentation).

On each of the three trials, subjects will undergo immersion to the level of the sternal notch in a tank of 8˚C water for up to 60 minutes. They will then exit the water, be dried off and lie in a hooded sleeping bag with the head inside the hood where one of the three warming procedures will be administered for 60 minutes or until core temperature returns to normal values ( ̴ 36.5-37˚C).

A. Spontaneous rewarming (Shivering Only) - In this control condition, no external heat will be provided and you will rewarm spontaneously with the heat produced from shivering.

B. Head Warming (Head): Either of these rewarming methods will be used:

• Forced-Air Warming to the Head (FAW-H) - A modified rigid forced-air warming cover will be placed over your head. Warm air ( ̴43˚C) will blow into the cover through an inlet on top of the cover and will escape from the bottom of the cover. You will be breathing warm air (̴ 43˚C).
• Charcoal Heater to the Head (CH-H) - A charcoal heater will be placed on right side of your face/head with ducts wrapping around the dorsum of the head, anteriorly over the forehead, nose, chin and the neck, not covering the eyes or the mouth. You will be breathing ambient air (̴ 22˚C).

C. Torso Warming - Either of these rewarming methods will be used:

• Forced-Air Warming to the Torso (FAW-T) - A specially designed portable rigid forced-air cover (PORIFAC) will be placed over your torso and the upper thighs. Warm air (̴ 43˚C) will blow into the cover through an inlet above the abdomen and will escape from the bottom of the cover. Participant will be breathing ambient air (̴ 22˚C).
• Charcoal Heater to the Torso (CH-T) - The charcoal heater will be placed on your anterior chest with a towel in between. The flexible ducts will be applied to the areas of high heat transfer i.e. over the shoulders, neck, and then anteriorly under the axillae to cross over the lower anterior chest. You will be breathing ambient air at room temperature (~22˚C).

After 60 minutes of warming, subjects will be placed in a warm water bath (40-42˚C) until they are comfortable and core temperature returns to normal values (̴ 36.5-37˚C).

Outcome measures include rate of core cooling, afterdrop amount, rate of rewarming, skin heat flux, oxygen consumption and shivering intensity.

Inclusion Criteria:

• 18-45 yrs old, males or females, healthy

Exclusion Criteria:

• cardiopulmonary disease
• any symptoms caused by cold exposure