DEAR DR. DONOHUE: My brother-in-law has invited my son and me to go ice fishing. My son is thrilled. I am not. When I was a boy, my right little finger was frostbitten. It still feels funny. Am I more susceptible to frostbite because I had it once before? How can I protect my son and myself from it? – M.M.

ANSWER: The damage from frostbite comes from ice crystals that form in the tissues beneath the skin. It can happen even when temperatures are above freezing. Wetness, wind and high altitude can lead to frostbite when the thermometer registers greater than 32°F (0°C).

Frostnip is the first stage of frostbite. The affected skin feels very cold and is painful. Warming frostnipped skin brings it back to full function without any aftermath.

When frostnipped skin becomes numb, an alarm should sound. Numbness indicates that nerves are frozen and that tissue damage has reached a danger point. Other signs of advanced frost damage are pale, waxy skin that’s swollen.

Naturally, warming is the appropriate treatment. Warming should not be attempted if there is a chance that the skin might refreeze. Refreezing induces the formation of giant ice crystals that cause even greater damage.

Immerse the frozen part, if immersion is possible, in water with a temperature between 104 and 108°F (40 to 42°C) for 15 to 30 minutes. Taking ibuprofen, the common anti-inflammatory drug, relieves pain and lessens tissue damage. Applying aloe vera (as long as it’s not in an alcohol solution) to the skin is also helpful.

To prevent frostbite, wear layers of clothes that conserve body heat and protect against moisture and wind. Mittens are better than gloves, since the bunched-up fingers provide warmth to each other.

DEAR DR. DONOHUE: While skiing I hurt my thumb. I fell and jammed it against the ski pole. I can bend it and use it, but it hurts. What should I do for it? – N.R.

ANSWER: The most likely diagnosis is skier’s thumb, a common skiing injury. It happens just as you described your accident. A skier falls, and the thumb is levered away from the rest of the hand by the ski pole. That stretches the ligaments at the base of the thumb. Skier’s thumb, therefore, is a sprain.

You treat it in the same way you treat other sprains, such as a sprained ankle. For the first 24 hours, ice the area. You are past that stage. From the second day on, begin warm soaks, 15 to 20 minutes at a stretch, three times a day.

Wearing a splint prevents further damage to the ligaments and hastens healing.

If the pain hasn’t gone away in another week, then you have to see a doctor. You could have a broken thumb bone, something only an X-ray can reveal.

Or you could have a third-degree sprain. That’s a sprain where the ligaments are completely severed. If the thumb is very lax and can be pushed far beyond its usual range of motion, that suggests that the ligaments are torn. If that is the case, surgical correction is needed.

DEAR DR. DONOHUE: How does cross-country skiing compare with other exercises? I have taken it up for the first time in my life – at age 71 – and love it. I come back exhausted and sweating, so I figure it must be giving me a good workout. – G.P.

ANSWER: Cross-country skiing is a demanding sport. A 155-pound man burns between 490 and 590 calories in each hour of cross-country skiing. That’s the equivalent of biking 14 miles in an hour or playing a full hour of basketball without any halftime break.

DEAR DR. DONOHUE: I haven’t gotten an answer to the letters I have written you. I am trying to get a point across that is very serious. I have seen on television that, if you are laid up for a period of time, it can be dangerous if you don’t walk around. You can have a blood clot that can travel to your lungs and can cause death. When I was hospitalized, I insisted on walking to the bathroom to prevent such a clot. How long is the danger period of being on your back? Please answer. – T.P.

ANSWER: The topic is quite important. More than 50,000 North Americans die each year because of a lung blood clot.

When people are inactive, blood stagnates in their leg veins. That happens when they are forced to lie in bed for a protracted period, when they sit in a car or on a plane for hours on end, or as a consequence of some illnesses, such as cancer. When blood does not circulate, it does what standing blood always does: It clots. The clot is a thrombus. Pieces of a thrombus can break off, be swept into the circulation and travel to the lung, where they lodge in blood vessels and block blood flow. Those pieces are called emboli, and such lung clots are pulmonary emboli. They can kill.

Dr. Donohue regrets that he is unable to answer individual letters, but he will incorporate them in his column whenever possible. Readers may write him or request an order form of available health newsletters at P.O. Box 536475, Orlando, FL 32853-6475.