Hot springs can absolutely make you sick, and the range of illnesses they cause is wider than most bathers realize. The warm, mineral-rich water that feels therapeutic also creates ideal growing conditions for dangerous microorganisms, releases toxic gases, and can push the cardiovascular system past safe limits. Infections from hot springs include everything from itchy skin rashes to a nearly always fatal brain infection, and the risks extend to chemical exposures that most visitors never think about.
The Brain-Eating Amoeba
The most alarming organism found in hot springs is Naegleria fowleri, a free-living amoeba that causes primary amoebic meningoencephalitis, or PAM. The infection enters through the nose when contaminated water is forced up the nasal passages during swimming, diving, or dunking your head. From there the amoeba travels along the olfactory nerve to the brain, where it destroys tissue rapidly. PAM has a fatality rate above 97 percent, and most victims are otherwise healthy children and young adults.
A recent survey of thermally influenced recreational waters in five western U.S. national parks found Naegleria fowleri in about a third of the samples tested. Some sites had cell concentrations higher than levels linked to past infections.1PubMed Central. Detection of Naegleria fowleri in Thermally Impacted Recreational Waters of Western United States National Parks The amoeba thrives in warm freshwater, which is exactly what hot springs provide. It is also found in lakes, rivers, and poorly maintained pools, but hot springs are a recurring source because their temperatures sit squarely in the amoeba’s preferred range.2PubMed. Real-Time PCR Confirmation of a Fatal Case of Primary Amoebic Meningoencephalitis in Turkey Caused by Naegleria fowleri or Brain-Eating Amoeba
PAM is extremely rare in absolute numbers, which is part of what makes it so insidious: most people have never heard of it. But it is almost universally fatal once symptoms appear. The simplest protective step is to keep your head above water or use a nose clip anytime you enter a natural hot spring. The amoeba cannot infect you through swallowed water or skin contact; it requires the nasal route.
What You Breathe In Near Hot Springs
Infections from hot springs are not limited to what touches your skin or enters your nose underwater. The steam and aerosol rising off the surface carry microorganisms directly into your lungs, and this is where two significant respiratory threats come in: Legionella bacteria and nontuberculous mycobacteria.
Legionella pneumophila, the bacterium responsible for Legionnaires’ disease, thrives in warm water and can become airborne in the fine droplets that bubble off an aerated hot spring or spa. The aeration, high water temperature, and variable organic load in these settings make disinfectant levels difficult to maintain, and modeling from documented hot-spring outbreaks has estimated Legionella concentrations in the air reaching hundreds of colony-forming units per cubic meter near contaminated pools.3PubMed. Quantitative microbial risk assessment model for Legionnaires’ disease: assessment of human exposures for selected spa outbreaks One laboratory study extrapolating from animal data estimated that the risk of Legionella infection from breathing aerosols at contaminated concentrations was roughly 65 to 76 percent depending on sex.4PubMed Central. Aerosolization of respirable droplets from a domestic spa pool and the use of MS-2 coliphage and Pseudomonas aeruginosa as markers for Legionella pneumophila Legionnaires’ disease presents as a severe pneumonia with fever, cough, and muscle aches, and it can be fatal, especially in older adults and smokers.
A separate respiratory condition called hot tub lung involves nontuberculous mycobacteria, particularly Mycobacterium avium complex. These bacteria colonize warm water and become aerosolized in the steam. When inhaled repeatedly, they trigger a granulomatous lung reaction that is thought to be more of a hypersensitivity response than a true infection.5PubMed Central. Hot Tub Lung: A Diagnostic Challenge Symptoms include a persistent cough, shortness of breath, fatigue, and sometimes fever. Early descriptions of the condition noted that the symptoms and lung findings were more suggestive of hypersensitivity pneumonitis than direct infection, though distinguishing between the two can be difficult clinically.6PubMed. Pulmonary illness associated with exposure to Mycobacterium-avium complex in hot tub water. Hypersensitivity pneumonitis or infection? Hot tub lung has been reported in people with healthy immune systems, and it typically improves once exposure stops, though some cases require steroid treatment.7Environmental Health Perspectives. Hypersensitivity Pneumonitis-like Granulomatous Lung Disease with Nontuberculous Mycobacteria from Exposure to Hot Water Aerosols
Hydrogen Sulfide Gas
Beyond biological aerosols, some geothermal hot springs release hydrogen sulfide, the gas responsible for the rotten-egg smell at many volcanic sites. At low concentrations, it is just unpleasant. At higher concentrations, especially in enclosed or poorly ventilated spaces, hydrogen sulfide can cause rapid loss of consciousness and death. Two fatal cases of acute hydrogen sulfide poisoning from thermal spring water were reported in a single clinical case series, both caused by inhaling gas that had evaporated off the water’s surface.8PubMed. Hydrogen sulfide toxicity in a thermal spring: a fatal outcome In Rotorua, New Zealand, two men were found dead in or near enclosed hot pools fed by geothermal water, and the coroner attributed both deaths to hydrogen sulfide poisoning.9PubMed. Deaths in Rotorua’s geothermal hot pools: hydrogen sulphide poisoning
The danger increases dramatically in enclosed spaces where the gas cannot disperse. If you are soaking in a natural hot spring and the sulfur smell suddenly gets much stronger, or if you feel dizzy, nauseated, or notice a headache coming on, move to fresh air immediately. At lethal concentrations, the gas actually deadens your ability to smell it, so a sudden disappearance of the odor in a geothermal area is itself a warning sign.
Skin and Eye Infections
The most common health complaint from hot spring and hot tub use is folliculitis caused by Pseudomonas aeruginosa. This shows up as itchy, red bumps or small pustules, often on areas covered by a swimsuit where contaminated water stayed in prolonged contact with the skin. It frequently appears as outbreaks, with multiple people who shared the same pool developing the rash within a day or two.10PubMed Central. Hot Tub-Associated Pseudomonas Folliculitis: A Case Report and Review of Host Risk Factors In most otherwise healthy people, Pseudomonas folliculitis resolves on its own within a week or two, though severe cases may need topical or oral antibiotics. More serious Pseudomonas infections, including pneumonia and mastitis, have also been linked to contaminated recreational water, though these are less common.11Journal of Health Science and Prevention. Health Risk in Hot Springs: A Literature Review
Fungal skin infections from dermatophytes are another documented risk. These are the same organisms behind athlete’s foot and ringworm, and they can survive in thermal water. Laboratory testing has shown that UV irradiation can fully inactivate common pathogenic dermatophytes in thermal water within two minutes of exposure, which points to UV treatment as a potential protective measure for managed facilities.12PubMed. Inactivation of pathogenic dermatophytes by ultraviolet irradiation in swimming pool thermal water But wild hot springs obviously have no such treatment.
Less well known is the risk to your eyes. Vittaforma corneae, a microsporidian parasite, has been identified in hot spring water in Taiwan and linked to cases of microsporidial keratitis, a painful eye infection that can damage the cornea. A survey of sixty water samples from six hot spring recreation areas found V. corneae-like organisms in a quarter of the sites, with cold spring water tubs and public foot baths showing the highest detection rates at 50 percent.13PubMed. Surveillance of Vittaforma corneae in hot springs by a small-volume procedure Wearing goggles or simply keeping your face out of the water substantially reduces this risk.
Dissolved Chemicals in Geothermal Water
Geothermal water picks up whatever it passes through on its way to the surface, and that often includes arsenic, fluoride, mercury, boron, and other heavy metals. This has been recognized as a public health concern for decades; geothermal fluids have long been noted to contain fluoride, boron, arsenic, and smaller amounts of other heavy metals at concentrations that can require treatment before the water is safe.14Geothermics. Geothermal fluid chemistry and human health
Modern analyses continue to confirm the problem. A study of hot springs in China’s Tengchong Geothermal Field found very high concentrations of fluoride, mercury, and arsenic, with fluoride levels far exceeding safe drinking water thresholds.15PubMed. Geochemical characteristics and environmental implications of heavy metals and fluoride of the hot spring waters in China’s Tengchong Geothermal Field Similarly, geothermal waters in eastern Shandong Province were found to have fluoride concentrations universally above China’s national drinking water standard, and some had arsenic above the limit as well. Health risk assessments from that study concluded that ingestion exposure posed the dominant danger, with the vast majority of samples reaching alert or unacceptable risk levels for non-carcinogenic and carcinogenic effects. Dermal contact risk was negligible for most samples, though a few high-arsenic sites reached alert levels even for skin exposure.16PubMed. Driving forces and health risks of fluoride and arsenic in geothermal water: insights from hydrogeochemical analysis, deterministic and Monte Carlo models
What does this mean for a bather? If you are soaking in a hot spring and not drinking the water, your risk from heavy metals and fluoride is much lower than if you were drinking it. Skin absorption is a less efficient route for most of these substances. But swallowing even small amounts of the water, or soaking in high-arsenic springs repeatedly over months or years, could accumulate meaningful exposure. Some traditional practices involve drinking hot spring water for supposed health benefits, and that is where the chemical risk becomes genuinely serious.
Radon
Some hot springs also contain dissolved radon, a radioactive gas that enters the water from uranium-bearing rocks underground. When you soak in a radon-containing hot spring, the gas escapes into the air above the water surface and can be inhaled. Measurements at hot springs in China found indoor radon concentrations in bathrooms during water use that exceeded both China’s indoor limit and the World Health Organization’s reference value, resulting in an additional annual effective dose of about 0.09 millisieverts for regular users.17PubMed Central. Effects of Radon From Hot Springs on Lymphocyte Subsets in Peripheral Blood Measurements of hot spring water in Indonesia found radon activity concentrations that translated to annual effective doses within the range of normal background radiation exposure for the general public.18PubMed Central. Radon Activity Concentrations in Natural Hot Spring Water: Dose Assessment and Health Perspective For an occasional bather, the radon dose from a single visit is tiny. For workers at hot spring facilities who are exposed daily in enclosed spaces, cumulative exposure over years deserves attention.
Cardiovascular Stress and Thermal Burns
Even without any microbes or chemicals in the water, the heat itself poses risks. Immersion in hot water causes blood vessels to dilate, which can drop blood pressure substantially. A review of heat therapy interventions, including mineral hot spring bathing, found that systolic blood pressure reductions of roughly 11 to 27 mmHg within 24 hours were reported across different modalities, and few studies specifically targeted adults over 65 or systematically tracked background blood pressure medications.19PubMed Central. Acute blood pressure responses and safety considerations in heat therapy interventions: A narrative review For a healthy young person, a temporary blood pressure drop might just make you dizzy when you stand up. For someone on blood pressure medication, or someone with a heart condition, that same drop can cause fainting, falls, or more serious cardiovascular events. Alcohol compounds the danger by further dilating blood vessels and impairing judgment about how long to stay in.20PubMed Central. Unveiling Lethal Risks Lurking in Hot Tub Baths: A Review of Tragic Consequences
At the extreme end, some natural hot springs are hot enough to cause scald burns. Water temperatures at geothermal sites can reach boiling or near-boiling, and even springs that look calm can have localized spots of extreme heat. One case report described a visitor who received partial- and full-thickness scald burns on both ankles and heels during a hot spring visit.21Burns. Hot spring burns Testing the water temperature before getting in, and knowing the specific conditions of the spring you are visiting, can prevent these injuries.
Pregnancy and Hot Springs
Pregnant women are frequently warned about hot tubs and hot springs, and the concern is well grounded. The core issue is hyperthermia: raising the body’s core temperature above normal levels during early pregnancy has been associated with increased risk of neural tube defects. Human studies linking hot tub or spa use in early pregnancy to neural tube defects and spontaneous abortion are limited in number, but they are consistent with a broader body of research showing that high maternal fever in early pregnancy raises the same risks.22PubMed. Risks of hyperthermia associated with hot tub or spa use by pregnant women
A larger case-control study looking at 17 categories of birth defects found that mothers who reported using a hot tub in early pregnancy had significantly higher odds of having infants with gastroschisis and anencephaly. Among mothers who used a hot tub more than once during the exposure period and stayed in for more than 30 minutes per session, the odds ratios for several defects climbed to roughly double or higher.23PubMed. Maternal use of hot tub and major structural birth defects The combination of frequency and duration mattered: brief, infrequent exposure carried less risk than longer, repeated soaking sessions. If you are pregnant or trying to conceive, the standard advice to limit hot water immersion to 10 minutes and keep the water below 102°F (about 39°C) reflects this evidence.
Why “Natural” Does Not Mean Safe
One of the most persistent misconceptions about hot springs is the belief that because the water is natural, it is inherently safe or even healing. A qualitative study of people who used natural hot springs for musculoskeletal pain found that about three-quarters believed the water had natural healing power, many believed the water was free of chemicals, and more than two-thirds thought that natural hot spring treatment has no side effects.24Journal of Environmental and Public Health. A Qualitative Study of Patients’ Beliefs and Perception on Medicinal Properties of Natural Hot Spring Bath for Musculoskeletal Problems The reality is that the very properties that make hot springs “natural” also make them hazardous: untreated water at elevated temperatures, fed by underground geology that dissolves metals and radioactive elements into solution, creating an environment where specific pathogens flourish.
Managed hot spring facilities face a genuine dilemma. The mineral composition and therapeutic uses of natural spa water often prevent standard chlorine-based disinfection, because the chemicals can react with the minerals or alter the water properties that bathers are paying for.25PubMed. Untouchability of natural spa waters: Perspectives for treatments within a personalized water safety plan Facilities that do chlorinate their pools effectively transform the experience into a regular heated pool, which defeats the purpose for many visitors. UV treatment offers a middle path for some pathogens but does not address everything. Wild, unmanaged hot springs have no treatment at all, and the diversity of potential pathogens in their water can be substantial.26PubMed Central. Exotic Tourist Destinations and Transmission of Infections by Swimming Pools and Hot Springs—A Literature Review
Temperature, Pathogen Load, and the Extremes
Not all hot springs carry the same risk. Water temperature is one of the strongest predictors of what lives in it. A study of five hot springs in Eritrea, ranging from warm to boiling, found that the boiling spring at 100°C had by far the lowest relative abundance of potential human pathogens, at just over 4 percent of the microbial community. The cooler springs hosted substantially more organisms capable of infecting humans.27PLOS ONE. Potential human pathogenic bacteria in five hot springs in Eritrea revealed by next generation sequencing This makes intuitive sense: extremely hot water kills most things, including the organisms that could infect you. The danger zone for most waterborne pathogens is roughly 25°C to 45°C, which is exactly the temperature range where people like to soak. Springs that are too hot to enter comfortably are, paradoxically, biologically safer; the comfortable ones harbor more threats.
This temperature relationship does not help you much in practice, since the springs you actually want to bathe in are the ones in the dangerous range. But it does explain why the source of the water matters less than where the water ends up. A spring that emerges at near-boiling temperatures and cools into a bathing pool downstream may pick up organisms as it cools, and the bathing pool itself becomes the riskiest spot.
Cyanobacteria in Hot Springs
One hazard that receives surprisingly little attention is cyanobacteria, sometimes called blue-green algae. These organisms form mats and biofilms on rocks and sediments in warm water, and some species produce potent toxins. An analysis of public hot springs in Saudi Arabia found cyanobacterial mats containing microcystins at concentrations of 468 to 512.5 micrograms per gram, and the microcystin levels detected in the spring water itself exceeded the World Health Organization’s provisional guideline value for drinking water. Lipopolysaccharide endotoxins were also detected at high concentrations in both the mats and the water. The authors concluded that hot springs worldwide should be screened for toxic cyanobacteria to protect people who bathe in them.28PubMed. Toxic cyanobacteria and cyanotoxins in public hot springs in Saudi Arabia
Microcystins are liver toxins, and at high enough doses they can cause nausea, vomiting, diarrhea, and liver damage. Skin contact with cyanobacterial mats can also cause rashes and irritation. You probably know to avoid swimming in a lake with a visible algal bloom, but most people would not think to look for the same thing in a hot spring. The slimy mats clinging to rocks around a geothermal pool deserve more wariness than they typically get.