Hot springs are pools of water heated geothermally, often containing a unique mix of dissolved minerals. While soaking in these warm waters is a popular therapeutic activity, these natural features are not sterile environments. The combination of warm water and organic material creates conditions that harbor various microorganisms and present physical and chemical hazards. Hot springs can potentially pose health risks to bathers, ranging from minor skin irritations to severe infections.
Infectious Agents in Geothermal Water
The primary biological hazard in hot springs comes from thermophilic (heat-loving) organisms that thrive in the elevated water temperatures. A major concern is the free-living amoeba, Naegleria fowleri, commonly known as the “brain-eating amoeba.” This organism is naturally found in warm freshwater and sediment, and it is capable of surviving in temperatures up to about 113°F (45°C).
Infection occurs when water containing the amoeba is forced up the nasal passages, such as through splashing or submerging the head. From the nose, the amoeba travels along the olfactory nerve to the brain, causing a disease called Primary Amoebic Meningoencephalitis (PAM). Although PAM is extremely rare, Naegleria fowleri has been confirmed in geothermal features across various natural hot springs.
The warm environment of hot springs also supports the growth of pathogenic bacteria. Legionella pneumophila, the bacterium responsible for Legionnaires’ disease, is a common concern because it flourishes in water temperatures between 77°F and 113°F (25°C and 45°C). Infection typically occurs when the bacteria are aerosolized and inhaled in mist or steam, leading to a severe form of pneumonia.
Bathing in contaminated water can also result in skin infections, particularly from the bacterium Pseudomonas aeruginosa. This organism causes folliculitis, often called “hot tub rash,” an itchy, inflamed skin rash. While less severe than amoebic or Legionella infections, this is a common consequence of soaking in poorly maintained warm water. Other organisms, like Giardia and E. coli, which cause gastrointestinal illness, are also found in some undeveloped hot springs.
Non-Infectious Health Concerns
Beyond microscopic organisms, the physical environment of hot springs presents several non-infectious dangers. The most immediate risk is thermal injury, as water temperature can fluctuate rapidly and reach scalding levels. Temperatures above 104°F (40°C) are considered unsafe for prolonged soaking and can cause burns or heat-related illnesses.
Extended exposure to high temperatures can result in hyperthermia that may progress to heat stroke. The heat causes blood vessels to dilate, which can lead to a drop in blood pressure and subsequent dizziness or fainting. This risk is amplified by dehydration, a common issue when soaking, which impairs the body’s ability to regulate its core temperature.
Many hot springs contain elevated concentrations of minerals and chemicals. Heavy metals, such as arsenic and mercury, may be present, though absorption through the skin is minimal. However, ingesting the water, even accidentally, can lead to internal exposure to these substances. Additionally, some springs emit gases like hydrogen sulfide, which can be toxic if inhaled in high concentrations.
Identifying High-Risk Hot Springs
The level of risk in a hot spring is strongly correlated with its physical characteristics and maintenance status. Unregulated, wild springs carry a higher risk of infection compared to commercially maintained facilities. Commercial operations typically monitor and treat the water to control pathogen levels, although they are not entirely risk-free from issues like chemical imbalances or equipment failure.
Water quality indicators can help assess the danger of a specific natural spring before entering. Water that appears murky, stagnant, or has a foul odor signals high biological activity and increased pathogen concentration. Springs with a high flow rate and clear water are safer because continuous movement prevents the buildup of dangerous microorganisms and organic matter.
Temperature is another factor in risk assessment, especially regarding amoebic and bacterial growth. Springs that are consistently between 80°F and 113°F (27°C and 45°C) are within the ideal proliferation range for organisms like Naegleria fowleri and Legionella. Conversely, springs that are extremely hot, approaching or exceeding 120°F (49°C), pose a greater risk for immediate burns or heat-related illness.
Safety Measures for Soaking
Individuals can take several steps to minimize risk when visiting a hot spring. The most important precaution is preventing water from entering the nasal passages, particularly in natural or untreated springs. This means avoiding submerging the head, splashing, or jumping into the water, which can force water and amoebae up the nose.
To avoid heat-related illness, limit soak time to a maximum of 15 to 20 minutes and take breaks to cool down. Staying well-hydrated by drinking plenty of water before and during the soak is necessary to compensate for fluid loss from the heat. Verify the water temperature before full immersion, especially in undeveloped areas where temperatures can be unpredictable.
Certain groups should exercise caution or avoid hot springs due to increased vulnerability. This includes young children, pregnant women, and people with compromised immune systems or open wounds, as they are more susceptible to infection and the effects of heat. If a person has open cuts or abrasions, they should cover them with a waterproof dressing or wait until they have healed before soaking.