Hot springs are naturally occurring bodies of water heated by geothermal activity. Because natural hot springs are unregulated and sourced from the Earth’s subsurface, they present unique risks that must be understood before entry. While soaking in these pools can be safe and enjoyable, it requires awareness of the physical, biological, and chemical hazards inherent in a wild environment.
Physical Dangers of Extreme Heat
The most immediate danger in a natural hot spring is the water temperature itself, which can easily exceed safe levels for human immersion. Water near the geothermal source often surpasses 140°F (60°C), a temperature that causes severe scalding in seconds. Injuries and fatalities have been reported near vents where water temperatures approach or exceed the boiling point.
Prolonged exposure to even tolerable heat can lead to a condition called hyperthermia, where the body absorbs more heat than it can dissipate. Initial symptoms include dizziness, nausea, and confusion, which can progress to heat stroke. Limiting soak time is necessary to prevent this and rapid dehydration.
Physical instability around the spring also poses a significant threat, especially in undeveloped areas. The ground surrounding a thermal feature can be fragile, unstable, or deceptively thin, concealing superheated water or mud just beneath the surface. Sudden shifts in water flow or eruptions from hidden vents can also rapidly change the temperature or stability of a seemingly safe soaking pool.
Biological Contamination Risks
The warm temperatures that make hot springs inviting also create an ideal habitat for specific types of pathogenic microorganisms. The most serious biological threat is Naegleria fowleri, a thermophilic amoeba known as the “brain-eating amoeba.” This organism thrives in warm freshwater environments, including hot springs, at temperatures between 95°F and 113°F.
Infection occurs when water containing the amoeba is forced up the nose, typically during activities like jumping, diving, or submerging the head. The amoeba then travels along the olfactory nerve to the brain, causing a rare but nearly always fatal infection called Primary Amebic Meningoencephalitis (PAM). Swallowing the water does not cause infection, but any activity that allows water to enter the nasal passages presents a risk.
Other heat-tolerant pathogens are a concern, especially in stagnant water or areas with high visitor traffic. Thermophilic bacteria, such as Legionella (which causes Legionnaires’ disease), can thrive in warm, aerosolized water sources. The lack of disinfection in natural settings means that bacterial and parasitic contamination from human or animal waste is always possible.
Geochemical Hazards and Gases
Hot springs derive their heat and mineral content from geological activity, which introduces chemicals that can be hazardous. Heavy metals, such as arsenic, mercury, and lead, are naturally dissolved in the geothermal water. Arsenic is a known carcinogen, and accidental ingestion of water with high concentrations can pose health risks.
While the skin absorbs these metals poorly, accidental swallowing of the water is a concern, as the body has difficulty eliminating them once they accumulate in organs like the liver and kidneys. Mercury, another potent neurotoxin, can damage the nervous and digestive systems if consumed.
The area around a hot spring vent may also contain high concentrations of toxic gases, which separate from the water as it reaches the surface. Hydrogen sulfide (H2S), a gas with a distinctive rotten-egg odor, is dangerous because high concentrations rapidly cause olfactory fatigue, making the gas undetectable. Odorless accumulation of H2S or carbon dioxide (CO2) in poorly ventilated or sunken areas can lead to respiratory distress, loss of consciousness, and asphyxiation.
Essential Safety Guidelines
Before entering any natural hot spring, always check for posted warnings, local advisories, and the water temperature, never relying solely on visual inspection. Avoid springs with temperatures exceeding 104°F (40°C), as this is generally considered the maximum safe limit for prolonged soaking.
To prevent infection from Naegleria fowleri, keep your head completely above the water and avoid any activities that might force water into the nasal cavity, such as splashing or diving. If you choose to submerge your head, using a nose clip or holding your nose shut can reduce the risk significantly.
Limit your soak time to 15 to 20 minutes at a time, especially in hotter water, to prevent overheating and dehydration. Drink plenty of fresh water before, during, and after soaking, and exit the spring immediately if you feel dizzy, nauseous, or confused. Never visit undeveloped or remote hot springs alone, and stay on designated paths to avoid unstable ground or hidden steam vents.