Hot tubs are large tubs or small pools full of warm water that people use for relaxation, hydrotherapy or massage. There are simple wooden-staved soaking tubs and plastic tubs, known also as spas. Natural gas, electric or solar heaters are the most popular devices to heat hot tubs.
Wooden tubs are made with wooden staves and steel bands. Cedar, teak and redwood are most commonly used; inside the barrel there is a wooden bench. Spas are made in a large variety of shapes, with a variety of seating arrangements. Some models are equipped with water jets. The water jet can be aerated, with moving and rotating features, to provide a massage effect. Spas are cheaper than wooden hot tubs, easier to install and clean, and less energy consuming. Usually, spas have a circulator pump to heat and filter water and another pump to drive the hydrotherapy jets.
As many small organisms thrive in warm water, sanitation is vital in hot tubs. The hot tubs water composition must be maintained to keep the proper sensitization and prevent any damage to the tub. Water filters are used, also chlorine and bromine, ozone generator are the most common sanitation solutions. People can use also UV sterilization, silver or copper ion generators. Periodically, specialists recommend shocking the hot tub to remove microorganisms, using potassium monopersulfate or granulated chlorine. After the shock treatment, the hot tub should not be used for 9 hours. Total alkalinity and pH must be maintained, to maintain the water chemistry within acceptable limits.