Hot tubs and spas are pools with hot water, used for relaxation or therapy, and are common in homes, hotels, clubs and massage parlours.
Whirlpool tubs first became popular in America during the 1960’s and 1970’s.
A spa is also called a "Jacuzzi" in USA with the word becoming generic after plumbing component manufacturer “Jacuzzi” introduced the "Spa Whirlpool" in 1968.
“Jacuzzi” is an Italian multinational corporation that produces whirlpool bathtubs and hot tubspas with its first product being a bath with massaging jets.
Air bubbles may be introduced into the nozzles via an air-bleed venturi pump that combines cooler air with the incoming heated water to cool the pool if the temperature rises uncomfortably high. There are different temperature control options like heat pumps and a pool heater.
Some spas have a constant stream of bubbles fed via the seating area of the pool or a footwell area.
This is more common as a temperature control device where the heated water comes from a natural (uncontrolled heat) geothermal source rather than artificially heated.
Water temperature is usually very warm to hot - 38 to 42 °C (100 to 108 °F) - so bathers usually stay in for only 20 to 30 minutes.
Bromine or mineral sanitizers are often recommended as sanitizers for spas because chlorine dissipates at a high temperature thereby heightening its strong chemical smell.
Ozone is an effective bactericide and is commonly included in the circulation system with cartridge filtration but not with sand media filtration due to clogging problems with turbid body fats.