DEAR DR. DONOHUE: My granddaughter, ever since she was little, has had two or three nosebleeds a year. Now she’s 18 and has had a bad one. She was working out at the gym when it occurred. The bleeds don’t occur because of bumping or blowing the nose, but just come on all of a sudden.

During this last one, blood flowed out her nose and globs came out her mouth. Ice packs stopped the bleeding. What causes this, and what can be done for it? — C.W.

ANSWER: Most nosebleeds, more than 90 percent, arise in the lower third of the nostril, slightly above the nasal opening. When bleeding occurs, apply firm pressure on both sides of the nose by squeezing the lower part of the nose between the forefinger (index finger) and thumb for five to 15 minutes. The person should tilt the head forward to keep blood from running down the throat. Ice packs are another good way to stop the bleeding. QR Powder, available without a prescription, also works well.

The deliberate or inadvertent insertion of a finger into the nose is the most frequent cause of bleeding. Drying and flaking of the nasal lining is another common cause. Your granddaughter might have been straining in the gym, which raised her pressure some and caused a tiny vessel to break.

Keeping the nasal lining moist prevents bleeds. A saline (saltwater) spray is a good way of achieving that. You can make a spray by adding a quarter teaspoon of salt to a cup of warm water. Coating the lowermost part of the inner nose with a thin film of petroleum jelly is another way to keep it moist. People always write to me about how dangerous it is to put petroleum jelly in the nose. It is dangerous to put large globs in the nose, but a thin coat is safe.

If bleeding doesn’t stop in 20 minutes, the person needs emergency treatment.

Frequent nosebleeds require a search for possible clotting problems. The family doctor can start the search.

DEAR DR. DONOHUE: I am an avid blood and platelet donor. I heard that we donors are putting ourselves at risk of vein damage. The article I read suggested that every time you receive a puncture, it creates a small amount of scar tissue in your veins and a place for plaque to build up. Have studies been done on this? — J.B.

ANSWER: I can’t find any studies on that topic. From personal experience, I know people who have had many, many needle punctures of veins both for treatment and for diagnostic reasons. Occasionally, a vein becomes occluded, but health hasn’t been put in danger. The usual veins used for obtaining blood are “superficial” veins, ones near the surface. A large network of veins you can’t see takes over the job of returning blood to the heart when one of the superficial veins is no longer patent.

Continue your blood and platelet donation without fear. You’re not putting yourself in harm’s way. You’re doing a greatly needed and appreciated service for people who might not survive without the generosity of people like you.

DEAR DR. DONOHUE: We have five children, all of whom spend most of their time outdoors. Our oldest boy, age 16, attracts mosquitoes. The rest of us are never bitten. How can I help him? What makes him such a magnet for mosquitoes? — H.B.

ANSWER: Body heat, odor and visual cues draw mosquitoes to certain people. The emission of carbon dioxide and lactic acid is a big attractant for these creatures. So is sweat.

Repellents with DEET are effective in keeping mosquitoes at a distance. A 10 percent concentration is safe for young children. Older children and adults can use a 30 percent preparation. One application lasts about four hours. Permethrin is another good repellent. It’s put on the clothes, not the skin. When used in conjunction with DEET, it provides a fairly impenetrable barrier for mosquitoes, and it lasts a very long time.

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. Readers may also order health newsletters from www.rbmamall.com.