Why Hot Tub Temperatures Max Out at 104 Degrees

WHY HOT TUBS MAXMany people question why their spa will only heat to 104 ºF. Throughout history public bathing was done at higher temperatures. Even today, in Japan, many public bath houses have water up to 115 ºF.

Hot tubs & spas in the United States, Canada and most parts of the world have been regulated to obtain a maximum temperature of 104 ºF since 1980.  On Dec 31, 1979, the Consumer Products Safety Commission, CSPC, released advisory #79-071.

The advisory warned that heat strokes could be caused by water temperatures of 106 °F or higher.  The CSPC recommended a maximum temperature of 104°F for both public and private hot tubs and spas.

The advisory was adopted by Underwriters Laboratory (UL) and lists it in their controlling document (#UL 1563) for home spas, equipment assemblies and associated equipment. The relevant portion of this document as it pertains to max temp regulations for home tubs and spas is section 32; Temperature Regulating Controls…

32.1 – A unit shall be provided with a water temperature regulating control that has a maximum set point of 40 °C (104 °F) in the tub.

32.2 – The temperature regulating control shall be adjustable and shall have marked settings, but it shall not have any settings marked hotter than 40 °C (104 °F).

32.3 –  A digital temperature regulating control that displays the selected temperature, but no higher than 40 °C (104 °F), may additionally display the actual temperature of the water.

UL has strongly advised that any hot tub or spa manufacturer that produces a control, or modifies a standard control, to exceed 104 °F is in violation of their UL listing and can have their listing pulled by UL.

The actual 1980 CSPC cover letter that accompanied advisory #79-071 is listed below:

U.S. Consumer Product Safety Commission Warns of Hot Tub Temperatures
Release # 79-071
December 31, 1979

WASHINGTON, D.C. (December 31, 1979) — Safety officials frequently warn the public about the dangers of drinking and driving. Now the U.S. Consumer Product Safety Commission is cautioning that drinking and hot tubing do not mix well either.

According to CPSC staffers, the use of hot tubs at water temperatures above the normal body temperatures can cause drowsiness which may lead to unconsciousness and subsequently result in drowning.

The risk of drowning is significantly heightened if individuals consume alcoholic beverages while, or prior to, soaking in hot water, CPSC staff warns. The Commission has been informed of 10 deaths recorded so far in 1979, three of which involved alcohol-related drowning in hot tubs heated to approximately 110 degrees Fahrenheit.

Even if no alcohol is consumed, extremely hot water during hot tub use can threaten life, CPSC reports. Soaking in a hot tub with water heated to 106 degrees Fahrenheit, for example, can raise human body temperature to the point of heat stroke (or impairment of the body’s ability to regulate its internal temperature). These conditions can be fatal even to fully healthy adults.

Hot tub sales in the U.S. have increased about 125 percent in the past two years, from approximately 40,000 tubs sold in 1977 to approximately 90,000 sold this year. Based on industry projections, CPSC estimates that sales could climb as high as 135,000 tubs next year, a 50 per cent increase over 1979 sales.

As hot tubs have gained in popularity throughout the nation, so have concerns at CPSC that consumers learn how to use these products safely. Accordingly, CPSC staff strongly urges consumers to observe the following safety rules for hot tub use.

Safety Rules For Hot Tubs

  • Hot tub water temperatures should never exceed 104 degrees Fahrenheit. A temperature of 100 degrees is considered safe for a healthy adult. Special caution is suggested for young children.
  • Excessive drinking during hot tub use can cause drowsiness which could lead to unconsciousness and subsequently result in drowning.
  • Pregnant women beware! Soaking in water above 102 degrees Fahrenheit can cause fetal damage during the first three months of pregnancy (resulting in the birth of a brain damaged or deformed child). Pregnant women should stick to the 100-degree maximum rule.
  • Before entering the hot tub, users should check the water temperature with an accurate thermometer; hot tub thermostats may err in in regulating water temperatures by as much as four degrees.
  • Persons with medical history of heart disease, circulatory problems, and diabetes or blood pressure problems should obtain their physician’s advice before using hot tubs.
  • Persons taking medications which induce drowsiness, such as tranquilizers, anti-histamines or anti-coagulants, should not use hot tubs.

CPSC staff currently is working with staff from the Spa and Tub Association ( a division of the National Swimming Pool Institute) and the International Spa and Tub Institute (both of Santa Ana, California) as they develop voluntary safety standards for the manufacture, installation, and use of hot tubs. These standards, which are expected to reflect many of the above safety warnings, may take effect as soon as spring, 1980.