DEAR DR. DONOHUE - I am very concerned about my grandson. He is a healthy 18-year-old, of average weight, a non-smoker, non-drinker. What has me worried is the nosebleeds he gets at times, and rather heavy, too. Of course, he doesn't want to see a doctor about this. Should we be worried? Mrs. S.A.T.

ANSWER - Most nosebleeds in healthy young men your grandson's age are due to such things as dryness of nasal linings or irritation from chronic rubbing and itching, as from allergy, for example. A little questioning and observation relative to that might be informative here.Rarely do nosebleeds in children signify any serious condition. If they are repeated, prolonged or heavy, then you cannot ignore them, for obvious reasons. Blood clotting problems or weak nasal blood vessels are just two considerations. A somewhat unusual tumor, called angiofibroma, also might come to mind in frequent nosebleeds in a young person.

We're speaking in subjective generalities here about such things as frequency, heaviness, and so on. How often is frequent? I can't give you a definition, but any degree of frequency that gives rise to concern is "frequent" for our purposes. You are concerned. You start out your letter telling me that. That's enough to insist on some answers.

It won't take the doctor long to get a clue to any of the dire causes of nosebleeds. And the visit will put your mind at ease, if nothing else.

DEAR DR. DONOHUE - A while back, my married daughter in another city sent me an article you had written on Barrett's esophagus. I had told her I was having terrible gastric acidity and difficulty in swallowing. I never had connected the two before. My doctor is now treating my condition. P.S. - A biopsy showed there was no malignancy there, thank God. Thanks for the tip. - Mrs. L.H.

ANSWER - Good news. For other readers: Barrett's esophagus, which I mentioned, is a swelling of the lower part of the esophagus from the constant irritation of stomach acid splashing upward. A symptom, of course, is Mrs. H.'s swallowing difficulty. In some, the tissue growth can be premalignant, so it has to be investigated. This is usually done via a gastroscope with biopsy tissue retrieved with the same instrument.

DEAR DR. DONOHUE - What causes water to get into the lungs? This person I know has low thyroid problems. I wonder if there is a connection between the two? What other problems can be caused by low thyroid? I will appreciate any information you can give me. - R.M.

ANSWER - If a person's thyroid production has been very low for a very long time, it is possible for fluid to accumulate in the lung covering sacs, the pleura. The name for that is pleural effusion.

Now, having said that, I must add that a person with low thyroid production would have more common and more obvious signs of the problem. I mean such symptoms as dry skin, extreme fatigue, cold intolerance and constipation. More common causes of pleural effusion are heart failure, pneumonia or other lung infections.

DEAR DR. DONOHUE - Can you answer this for me? Does the wearing of a hat cause baldness? - T.

ANSWER - Sure I can answer that. There is no reason I can think of under the sun why the wearing of a hat would cause baldness. I have had this question asked of me before. Perhaps someone can enlighten me as to where the idea originated.

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DO YOU HAVE PROBLEM HAIR? Dr. Donohue's booklet No. 14, "Good Health for Your Hair," shows how proper treatment can result in healthier, more attractive hair. Send your request to Dr. Donohue/No. 14, P.O. Box 19660, Irvine, CA 92713-0660. Enclose a long, self-addressed, stamped envelope and $1.00.

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Dr. Donohue welcomes reader mail but regrets that, due to the tremendous volume received daily, he is unable to answer individual letters. Readers' questions are incorporated in his column whenever possible.