Q: I'M always amazed at the numerous kinds of condoms available at the store. I never know which one to buy and, because I'm embarrassed to ask, I always end up picking the one with the nicest package. What should I look for when choosing a condom? Dr Rose writes: It really is confusing knowing which condom brand to buy. First of all, you should make sure that you choose one that is made from latex. Latex condoms can help to prevent the spread of AIDS, hepatitis, herpes and other transmitted diseases. The natural or lambskin condoms are more fragile and therefore less reliable. Make sure the condoms you buy are for preventing disease and are not ''novelty'' condoms. Novelty condoms do not say anything on their packaging about preventing disease and are intended only to enhance sexual pleasure. Whether you choose a condom that is ribbed or coloured depends on personal preference. Some condoms come with spermicides. There is some evidence that nonoxymol-9 spermicide can help to kill the germs associated with sexually transmitted diseases. Although this evidence is not conclusive, as an added precaution it is a good idea to buy a condom with spermicide or to use spermicide with the condom. Some condoms are lubricated with jellies or creams. This helps to prevent them from breaking during intercourse. If you buy an already lubricated condom you will not need to apply a separate lubricant. Some condom packages indicate when the condoms have expired. Do not buy a condom close to, or after the expiry date. Be careful not to store the condom in a warm place, such as in your car or in your pocket. Open the package gently so that you do not perforate or tear the condom. Check the tip of the condom to make sure there is no obvious damage such as holes or tears and that it is not sticky or gummy. Don't unroll it completely until you are ready to use it. Remember to use a new condom each time you have sexual intercourse. Q: My father has just started to exercise on an exercise machine because his doctor told him that he was too fat. On the machine, there's a sign that says: ''Stop exercising if you experience any pain, or feel faint or dizzy.'' My question is, what exactly do these symptoms indicate? Dr Rose writes: When you do any kind of demanding aerobic exercise, the body must increase the amount of oxygen it delivers to the tissues and muscles. To accomplish this, the hearts pumps faster, the blood pressure rises and the rate of breathing increases. When the body fails to keep up with the demand, there is a relative lack of oxygen and blood flow to the vital organs such as the brain and heart. Reduced flow of oxygen to the brain can result in dizziness or fainting spells. Reduced flow to the heart can lead to chest heaviness and chest pain (angina) or arrythmia (irregular heart beat) and even heart attack, especially if there already is a narrowing or blockage of the coronary vessels. Pain in extremities such as the thighs and calves is usually caused by an increase in lactic acid which is produced by the muscles during vigorous exercise. A build-up of lactic acid and other waste products leads to cramping. If your father is overweight and has just embarked on an exercise programme he should proceed slowly and cautiously under the doctor's supervision. Dr Rose Ong is a certified family physician licensed in the United States. She welcomes enquiries but cannot answer them individually. Specific questions should be addressed to your own physician. Additional enquiries: Peak Corporate Health Management, 525-6600, fax 525-8100.