A swimming pool, swimming bath, wading pool, or simply a pool, is a container filled with water intended for swimming or water-based recreation. There are many standard sizes; the largest and deepest is the Olympic-size swimming pool.[citation needed] A pool can be built either above or in the ground, and from materials such as concrete (also known as gunite), metal, plastic or fiberglass. The innovation of a composite construction of fibreglass, with an epoxy coating and porcelain ceramic tiles has led to the Pre-form, Composite-type with significant advantages over older methods; however, it also has increased sensitivity to metal staining.
Pools that may be used by many people or by the general public are called public, while pools used exclusively by a few people or in a home are called private. Many health clubs, fitness centers and private clubs have public pools used mostly for exercise. Many hotels and massage parlors have public pools for relaxation. Hot tubs and spas are pools with hot water, used for relaxation or therapy, and are common in homes, hotels, clubs and massage parlors. Swimming pools are also used for diving and other water sports, as well as for the training of lifeguards and astronauts.
Chemical disinfectants such as chlorine, bromine or mineral sanitizers, and additional filters are often used in swimming pools to prevent growth and spread of bacteria, viruses, algae and insect larvae. Alternatively, pools can be made without chemical disinfectants by using a biofilter with additional carbon filters and UV disinfection. In both cases, pools need to be fitted with an adequate flow rate.