Hot tub

In contrast to a typical bathtub, a hot tub is designed to be used by more than one person at a time, with many models accommodating four or more people.

[2] The remains of wrecked marble bathtubs along with a prehistoric aqueduct that have been unearthed from this area bear ample testimony of the place's popularity in the ancient times.

[2] In 737 A.D., Japan's first onsen opened near Izumo, Shimane, and centuries later, the first ryokan (inns) were built, offering food, accommodations, and soaking tubs called ofuro.

[citation needed] California's portable electric hot tub listing[5] include R values of thermal insulation, and standby watts.

[citation needed] Since some hot tubs are not drained after each use it is necessary to treat the water to keep it attractive and safe.

For aesthetic reasons, and for the sanitizer to work properly, water should be neither too alkaline nor too acidic (low pH).

The ideal range of calcium hardness levels in a hot tub or spa water should be between 150 and 250 ppm (parts per million)[citation needed] Sitting in water above normal body temperatures can cause drowsiness which may lead to unconsciousness and subsequently result in drowning.

The U.S. Consumer Product Safety Commission (CPSC) recommends that water temperatures never exceed 40 degrees Celsius (104 °F).

The greater danger associated with electrical shock in the water is that the person may be rendered immobile and unable to rescue themselves or to call for help and then drown.

The Uniform Swimming Pool, Spa and Hot Tub Code is a model code developed by the International Association of Plumbing and Mechanical Officials (IAPMO) to govern the installation and inspection of plumbing systems associated with swimming pools, spas and hot tubs as a means of promoting the public's health, safety and welfare.

[25] Poorly sanitized hot tubs have been linked to a number of diseases, principally caused by facultative anaerobic bacteria.