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THERAPIES & TRADITIONS · GUIDE

Hot Springs and Thermal Baths: What They Are and What Is Known

How hot springs form, what mineral and thermal water bathing claims have been tested, and the burn, Legionella and amoeba cautions that public-health sources describe.

Content last updated . This page has not been medically reviewed.

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What a hot spring is

A hot spring is groundwater heated by the Earth and brought to the surface. The National Park Service says hot springs are heated by geothermal heat from the Earth's interior [1]. A US Geological Survey text explains that groundwater gets warmer as it sinks deeper because of the geothermal gradient, and that deeply circulating water can also be heated by cooling magma, which is molten rock [4].

A well-known example is Bath, in England. The Geological Society of London says around 1.3 million litres of water flow from its springs every day at around 40 degrees Celsius, and the largest spring rises inside the Roman Baths Museum [5]. The water there is dominated by calcium and sulphate, with sodium and chloride also high [5]. This shows that each spring has its own temperature and mineral mix, so claims about "hot springs" in general are weak.

How it is defined

There is no single worldwide definition in the sources we read. Japan's Hot Springs Act, as described by the Japan Onsen Association, counts water as an onsen when it is 25 degrees Celsius or more at the source, or when it contains a specified amount of at least one of 19 listed substances [6]. Other countries' rules are not described in the sources we read. "Thermal" and "mineral" water are also used loosely in marketing. The Cochrane reviewers define balneotherapy as bathing in natural mineral or thermal waters, using mudpacks, or both [7]. For the Japanese tradition see Japanese onsen.

Health claims checked

Hot spring and mineral bathing is a traditional practice. Studies mostly test spa-style programs, not a single dip, so they are covered in balneotherapy in Europe. The short version:
4 findings · 1 supported · 3 limited or unclearThe colour follows the evidence label on each card.

  1. Helps rheumatoid arthritis

    Insufficient evidence

    Cochrane (9 studies, 579 people, mostly unclear bias): evidence insufficient to show it works better than no treatment [7].

  2. Helps osteoarthritis

    Promising but limited

    An older Cochrane review (7 trials, 498 people) found silver-level evidence for mineral baths versus no treatment, but weak study quality [8]. A 2023 review of 17 studies found improvements in all, with quality often not high [9].

  3. The minerals are what matter

    Insufficient evidence

    For low back pain, reviewers say the role of mineral water remains uncertain [10].

  4. Cures or prevents disease, detoxifies

    No good evidence

    Proper evidence not found. We searched PubMed and the Cochrane records above for reviews of hot spring bathing and found none that tested these claims.

Baths cannot be blinded, and many trials measure self-reported pain, so comfort and expectation can shape results.

Infection and water quality

Natural does not mean germ-free. Public-health researchers report three concerns.
  • Legionella. In Taiwan, Legionella was detected at 21 of 91 spring water sites (23%) in seven hot spring recreation areas, and detection was not proportional to how often facilities were cleaned [11]. In Arkansas, 5 people developed Legionnaires' disease after visiting Hot Springs National Park, and 3 were potentially exposed in spa facilities using untreated hot spring water [12]. Testing found Legionella in piped spring water, including stagnant pipe water at 134 degrees Fahrenheit [12].
  • Free-living amoebae. Naegleria fowleri causes a devastating brain infection, primary amebic meningoencephalitis, which can happen when contaminated water enters through the nose, usually during recreational water activities [3]. It is rare worldwide [13]. In Taiwan, the first confirmed case had visited a thermal spring recreational area a week before hospitalisation, and the spring was suggested as the likely source [14]. US Geological Survey and CDC scientists detected N. fowleri in five hot spring sites in Grand Teton National Park [15].
  • Skin infections. See hot tub rash and skin infections.

The sources we read do not give prevention steps for amoebae. For facilities, ask about testing and maintenance; see spa and facility safety checks.

Related pages

Hammam, Turkish and Roman baths, hydrotherapy, balneotherapy and spa therapy terms and hydrotherapy safety.

Frequently asked questions

Are hot springs good for your health?

Traditional use is long, but trials are limited. Reviews of mineral bathing for arthritis found insufficient or weak-quality evidence [7][8].

Can hot springs be unsafe?

Yes. Very hot water can scald [1], and Legionella and amoebae have been found in some springs [11][14].

Do the minerals make the difference?

Not shown. Reviewers of low back pain trials call the role of mineral water uncertain [10].

How is a hot spring defined?

Japan's Hot Springs Act uses 25 degrees Celsius or more at the source, or listed substances in set amounts [6]. Other definitions were not described in the sources we read.

Can a hot spring cause Legionnaires' disease?

Yes, in some outbreaks, including 5 cases linked to Hot Springs National Park, Arkansas [12].

More details: evidence, limits and cautions

The details behind this page: cautions, limits of the evidence and where sources disagree. Open any note to read it.

Safety first
Some hot springs are dangerously hot. The US National Park Service says hot springs in active volcanic zones may produce superheated water, so hot that immersion can result in injury or death [1]. A case report describes a visitor who suffered partial- and full-thickness immersion scald burns of both ankles and heels at a hot spring [2]. For a serious burn or a medical emergency in the US, call 911. See scalds and burns from hot water. If you become seriously unwell after warm fresh water or spa use, seek urgent care and tell clinicians where you were. Doctors are encouraged to consider an amoeba infection when meningitis follows warm freshwater exposure [3], and Legionnaires' disease has its own warning signs. Information only; this is not medical advice.
Who should check first
Heat adds strain to the heart and blood vessels. People with heart or blood pressure conditions, pregnancy, diabetes or reduced skin sensation, kidney disease, open wounds, or sedating medicines, and older adults and young children, should talk with a clinician first. Read hydrotherapy with heart conditions, hydrotherapy for older adults and alcohol, drugs and hydrotherapy. We give no temperatures or times because safe limits depend on the person and the spring.
Regulation and what we could not read
How hot springs are inspected or licensed is not described in the sources we read, and it differs by place. We did not use CDC pages (listed as unreadable for this project) and could not read the Naegleria prevention guidance there. We list no springs or spas.
References
  1. US National Park Service. Hot Springs/Geothermal Features. Read 2026-10-22.
  2. Baruchin AM. Hot spring burns (case report). Burns 1996;22(2):156-157. PubMed 8634128 Read 2026-10-22.
  3. Cope JR, Ali IK. Primary amebic meningoencephalitis: what have we learned in the last 5 years? Curr Infect Dis Rep 2016;18(10):31. PubMed 27614893 Read 2026-10-22.
  4. US Geological Survey. Ground Water Atlas of the United States, HA 730-I, geothermal water text. pubs.usgs.gov Read 2026-10-22.
  5. Geological Society of London. Bath spa geosite (text courtesy of the UK Groundwater Forum). Read 2026-10-22.
  6. Japan Onsen Association. What is onsen? Read 2026-10-22.
  7. Verhagen AP et al. Balneotherapy (or spa therapy) for rheumatoid arthritis. Cochrane Database Syst Rev 2015;(4):CD000518. PubMed 25862243 Read 2026-10-22.
  8. Verhagen A et al. Balneotherapy for osteoarthritis: a Cochrane review. J Rheumatol 2008;35(6):1118-1123. PubMed 18464302 Read 2026-10-22.
  9. Protano C et al. Balneotherapy for osteoarthritis: a systematic review. Rheumatol Int 2023;43(9):1597-1610. PubMed 37301799 Read 2026-10-22.
  10. Forestier R et al. Crenobalneotherapy for low back pain: systematic review of clinical trials. Int J Biometeorol 2022;66(1):13-23. PubMed 34625843 Read 2026-10-22.
  11. Hsu BM et al. Legionella prevalence in hot spring recreation areas of Taiwan. Water Res 2006;40(17):3267-3273. PubMed 16928391 Read 2026-10-22.
  12. James AE et al. Potential association of Legionnaires' disease with hot spring water, Hot Springs National Park and Hot Springs, Arkansas, USA, 2018-2019. Emerg Infect Dis 2022;28(1):44-50. PubMed 34932451 Read 2026-10-22.
  13. Siddiqui R, Khan NA. Primary amoebic meningoencephalitis caused by Naegleria fowleri: an old enemy presenting new challenges. PLoS Negl Trop Dis 2014;8(8):e3017. PubMed 25121759 Read 2026-10-22.
  14. Tung MC et al. Identification and significance of Naegleria fowleri isolated from the hot spring which related to the first primary amebic meningoencephalitis patient in Taiwan. Int J Parasitol 2013;43(9):691-696. PubMed 23665128 Read 2026-10-22.
  15. Barnhart EP et al. Naegleria fowleri detected in Grand Teton National Park hot springs. ACS ES&T Water 2024;4(2):628-637. PubMed 38356928 Read 2026-10-22.
Education, not medical advice. This page is for general information. It does not diagnose or treat any condition and is not a substitute for care from a qualified health professional. Natural does not automatically mean safe. Ask a clinician or pharmacist before changing your care, especially if you take medication, are pregnant or breastfeeding, or have a health condition.