Q: When working outside or hiking during the summer how much water does a person need?

A: Drinking to quench thirst isn't enough. You need to drink past thirst. Studies have shown that drinking to satisfy thirst only replenishes two-thirds of the amount of water a person should drink when working or playing in the Arizona sun. Through perspiration, a person can loose about two quarts of water an hour. To replace that loss drink about a glass of water about every 10 minutes. That's a significant amount of fluid.

One way to tell if you're properly hydrated is urine output. If you urinate your normal amount and frequency, the body is well-hydrated, provided you don't have kidney disease. I think that's a better gauge than how thirsty you feel.

And what a person drinks may make a difference. In the long run, whether drinking a sports drink or just water the primary issue still is hydration. In general, it's better to drink plain water than any other fluid, like a sports drink. Sweat contains far more water than electrolytes, which many sports drinks contain. But if you loose a lot of sweat for hours then you'll need to replace the electrolyte solutions as well.

Harvey Meislin, M.D., director, Arizona Emergency Medicine Research Center at The University of Arizona College of Medicine

Q: I get a rash every time I try a new shampoo. Is there anything I can do to prevent this?

A: Common chemical substances that can cause allergic reactions when they come in contact with skin include: preservatives in shampoos and cosmetics; ingredients in mouth washes and hand lotions; latex in rubber gloves; materials in dental fillings; nickel in jewelry and clothing fasteners; and medications, such as the antibiotic neomycin, applied to the skin. A new contact allergies testing clinic at the UA Section of Dermatology now offers patch testing for 60 of these kinds of allergens.

People in certain occupations tend to be more susceptible to contact allergies. Hair dressers, mechanics, construction workers and people working in photo labs are among those who experience a greater number of contact allergies.

Contact allergy symptoms include an itchy, scaly, red rash on the skin that doesn't go away with the use of moisturizers or a change to a milder soap. Contact allergy symptoms also are not alleviated by the application of cortisone ointment typically used to treat skin irritations. The area of skin affected generally is confined to the area in contact with the irritating substance, although it can be widespread.

To test for contact allergies at the clinic, small amounts of the suspected chemicals are placed on the skin of the back and are kept in place with tape for a few days to see which, if any, produce a rash.

Once the irritating chemical is discovered, steps can be taken to avoid contact with it.

Nancy Silvis, M.D., assistant professor of dermatology, The University of Arizona College of Medicine; head, contact allergies testing clinic