Americans are bringing the benefits of steaming, natural hot springs home by installing spas or hot tubs. To relax tense muscles and reduce stress, soothing bubble baths can't compete with the pleasure of a long, luxurious soak.

According to the National Spa and Pool Institute, an Alexandria-based trade group, there are an estimated 2 million residential spas nationwide, about 57,000 of which are in the Washington area.

A spa is a generic term for a vessel containing warm, bubbling water. Spas are made of fiberglass, concrete or ceramic tile and have a heater, pump and filter attached to keep the water warm and clean. Hot tubs, like spas, use similar heating and filtering equipment but are usually made of wood. Jacuzzi is a brand name for a company that makes spas.

Spas, considered a glamorous and expensive home addition -- an average-sized fiberglass or wood tub that seats four to six people can cost as much as $8,000 -- are gaining in popularity. Among the primary reasons people install them are to reduce stress and minimize pain from aching joints and muscles, according to the spa institute.

For people with arthritis, applying heat to inflamed joints helps them relax and feel better, said Arthur Gravzel, senior vice president for medical affairs at the Atlanta-based Arthritis Foundation. "We also instruct people to combine being in a hot tub with doing range-of-motion exercises so they don't completely lose the ability to move."

Soaking in hot water can also give people a sense of psychological well-being. Being removed from the external stresses of everyday life has a calming effect, said Gravzel.

Before taking out a second mortgage to install the muscle-pleasing device, there are a few matters to consider. Like swimming pools, if the water is not properly maintained, the results can be harmful to both the bathers and the equipment.

Too little chlorine or bromine -- disinfectants used to keep water clean -- can cause bacteria to grow. A common form -- Psuedomonas aeruginosa -- can become trapped in the skin's hair follicles, causing a very irritating, itchy rash, said John Puetz, vice president of research for Great Lakes Biochemical Co., a Milwaukee-based firm that manufactures chemicals for spa use.

Because frequent spa use can strip the skin's natural oils, dry skin is a common complaint. Some people are also sensitive to chlorine.

Physicians recommend that all hot tub users shun alcohol. They also advise people with heart conditions or high blood pressure and pregnant women to avoid extreme changes in temperature. They should consult a physician before soaking in a hot tub.

Measuring exactly what impact sitting in a spa would have on an unborn fetus, for example, is difficult. "The theory is that if you warm the body up, the baby inside is captive to that heat and does not have a way to cool itself," said John Larsen, professor of obstetrics, gynecology and genetics at George Washington University Medical Center. "We are not saying don't use the tub at all. We are saying you know your own hot tub and you know your own body. Stay in for a short period of time and get out."