Heat Illness, Dehydration and Overheating in Saunas and Hot Tubs

Urgent Guidance First

Heatstroke is a medical emergency. In the US, call 911 if someone has a very high temperature, hot skin that is not sweating, confusion, a seizure, fast breathing or heartbeat, or loses consciousness. These are signs the NHS (UK) lists for heatstroke. [1] The NHS says not to drive yourself to hospital, and that while waiting you can wrap them in a cool, wet sheet or fan or sponge them with cold water. [1] If they lose consciousness, it says to put them in the recovery position and, if there are no signs of life, begin CPR. [1]

If a person in a hot bath or tub is confused, drowsy or cannot get out, treat it as an emergency: help them out of the heat safely and call for help. A Japanese review describes people who became unable to leave a bathtub because of impaired consciousness from overheating. [2]

Heat Exhaustion and Heatstroke

The NHS lists these signs of heat exhaustion: tiredness, dizziness, headache, nausea or vomiting, heavy sweating with pale, clammy skin, cramps, a high temperature, strong thirst and irritability. [1] It says heat exhaustion does not usually need emergency help if you can cool down within 30 minutes, but if it turns into heatstroke it needs emergency treatment. [1]

For heat exhaustion the NHS gives four steps: move to a cool place, remove unnecessary clothing, give plenty of water, and cool the skin. [1] It says to stay with the person, and that they should feel better within 30 minutes. [1] If they are still unwell after 30 minutes, the NHS lists that among the signs needing emergency help. [1]

In a bath, hot tub or sauna, the first step is to get out of the heat. This page does not set time limits. The NHS and the studies below do not give a universal safe duration.

Why Hot Water Can Overheat People

Bathing in hot water raises body temperature. Two Japanese papers on bath-related deaths argue that heat illness, not only blood pressure changes, plays a larger role than once thought. One reports that among people rescued after being unable to get out of a bathtub unaided, fewer than 10% were found to have an organic disease, and that confusion, high body temperature and fast heart rate improved after rescue and removal from the heat. [2] A prospective study of 4,593 bath-related emergency events in Tokyo, Saga and Yamagata found the main symptoms in survivors were disturbed consciousness and lethargy, and 30% of survivors had a body temperature above 38°C. [3] The authors concluded heat illness during hot water immersion causes drowning. [3]

These are observational data from Japan, where deep, hot, home bathing is common, mostly among older adults. They show a pattern, not how likely it is in a typical hot tub elsewhere. Proper evidence not found for rates of heat illness in US hot tubs; we searched PubMed for hot tub deaths and bath-related deaths and read the reviews listed here.

Another paper found that older adults who bathed after being in a cold room had higher skin temperature in the bath, which may drive larger blood pressure and heart rate changes. [4] The Japanese authors also note older adults may not perceive the water as too hot and bathe for long periods. [5]

Dehydration in Saunas and Hot Tubs

Heat makes people sweat, and the NHS lists being very thirsty and heavy sweating among heat exhaustion signs. [1] A review of sauna and sudden cardiac death says single case reports of deaths were attributed to dehydration, low blood pressure and heart rhythm problems from sauna combined with alcohol. [6] The sources we read did not measure how much fluid people lose in saunas or hot tubs, so we do not give amounts. Alcohol is covered in alcohol, dehydration and water safety and alcohol, drugs and hydrotherapy.