Hypothermia can come on very slowly, making it difficult to notice that it's happening. When your body is cold, you try to protect yourself by shivering. When hypothermia sets in, the shivering stops as your body is now trying to conserve energy. This is one of the first warning signs of hypothermia. You might also begin to feel clumsy and disoriented as your body temperature drops. Other signs include forgetfulness, drowsiness, slurred speech, slow or irregular heartbeat, and shallow breathing.
One complication of hypothermia is that the heart becomes very sensitive and can be easily jolted into an irregular rhythm. If you try to rescue someone with hypothermia, you need to be as gentle as possible to prevent any sudden movements. As well, emergency personnel will monitor the heart during the re-warming process to watch for any irregularities.
How fast the signs of frostbite develop depends on the air temperature, wind-chill factor, and how well the affected body part is protected. It's possible to get frostbite within minutes during extremely cold conditions.
Mild frostbite (frostnip) makes the skin look white or waxy but the colour returns once the skin is warm again. Once thawed, the skin might turn red and it could take a while for the redness to go away.
Severe frostbite begins with white or waxy-looking skin, but the colour becomes grey or bluish as the damage progresses. The cold feeling disappears and is replaced with numbness, and blisters might form. At this point, it's very easy to cause other damage besides the actual freezing. For example, because you might have lost feeling in a foot that is frostbitten, you might not feel blisters forming and breaking. A frozen finger could break easily in a fall, and skin can burn very easily if it's not warmed up properly.
As a frozen body part warms up, the area may swell, itch, burn, and be extremely painful. Extreme frostbite will turn the skin black, and gangrene may develop, in which case amputation may be needed.