Legionella bacteria are found in both natural freshwater environments and human-made water systems, though they cause the most problems when they multiply in warm, stagnant plumbing and equipment. These bacteria thrive in water between 77°F and 113°F (25°C to 45°C), which is why they colonize so many of the water systems people interact with daily. Understanding where Legionella lives helps explain how people get exposed and what you can do to reduce risk in your own environment.
Natural Water and Soil
Legionella occurs naturally in freshwater lakes, rivers, streams, and wet soil. At low concentrations in the wild, the bacteria rarely pose a threat. The problem begins when water from these natural sources enters buildings and equipment where conditions favor bacterial growth: warm temperatures, stagnant flow, and surfaces where biofilm (a slimy layer of microorganisms) can accumulate.
One species, Legionella longbeachae, lives in soil rather than water. In an Australian survey, 73% of potting soil samples tested positive for Legionella, and 79% of those contained longbeachae specifically. A Japanese survey found the bacteria in nearly half of soil samples tested. Cases of Legionnaires’ disease linked to potting soil and compost have been reported in Australia, Japan, and the United States. Gardeners who open bags of potting mix can inhale dust or tiny particles carrying the bacteria.
Building Water Systems
The most common source of Legionella exposure is the plumbing inside buildings. The bacteria have been found in water distribution systems of hospitals, hotels, public buildings, homes, and factories. Any section of piping where water sits unused, warms to a moderate temperature, or accumulates sediment can become a growth site.
Large, complex buildings carry the highest risk because they have longer pipe runs, more dead-end branches, and more opportunities for water to stagnate. Hospitals are a particular concern: outbreaks have been linked to hospital water supplies, air conditioning systems, and cooling towers. Hotels present similar risks for travelers, especially through contaminated tap water and whirlpool spas.
Cooling Towers and HVAC Equipment
Cooling towers, the large units on rooftops that release heat from air conditioning systems, are one of the most well-known sources of Legionella outbreaks. They work by spraying water over a fill material while drawing air through it, creating a fine mist that can carry bacteria hundreds of meters downwind. Evaporative condensers work on a similar principle and carry the same risk.
These systems provide ideal conditions for Legionella: warm water, constant aeration, and large surface areas where biofilm can develop. When they’re poorly maintained, bacterial concentrations can climb high enough to cause community-wide outbreaks affecting people who never entered the building where the tower is located.
Household Fixtures and Devices
Legionella can colonize residential plumbing, particularly in parts of the system where water stays warm but not hot enough to kill bacteria. Your water heater is the starting point: setting it at or above 140°F (60°C) generally keeps temperatures high enough throughout the plumbing to prevent growth. Below that, water sitting in pipes between uses can drop into the ideal growth range.
Showerheads, sink faucets, and humidifiers create spray or small droplets that release bacteria into the air, which is the primary way people inhale Legionella. If you notice mineral buildup or slimy film on fixtures, that biofilm may be harboring bacteria. Removing and soaking showerheads in a cleaning solution periodically helps. Faucets with non-aerating designs, which don’t mix air into the water stream, produce fewer tiny droplets and may reduce exposure. Cold water systems in most climates are too cool for Legionella, though homes in warm regions where cold water temperatures reach around 80°F for extended periods can be an exception.
Other household items that create mist or spray need regular cleaning too. Hot tubs, humidifiers, and nebulizers all provide both the warm water and the aerosolization that Legionella needs to reach your lungs.
Less Obvious Sources
A surprising range of equipment can harbor Legionella when not properly maintained. The CDC identifies these as growth risks in the absence of regular control measures:
- Ice machines: internal water lines and reservoirs stay moist and can develop biofilm
- Produce misters: the fine spray used in grocery stores to keep vegetables fresh
- Lawn sprinklers and irrigation systems: water sitting in pipes between watering cycles warms to growth-friendly temperatures
- Safety showers and eyewash stations: rarely used, meaning water stagnates for weeks or months
- Pressure washing equipment: generates aerosols from water that may have been sitting in a tank or hose
- Evaporative air coolers (swamp coolers): pass air over wet pads, creating conditions similar to small cooling towers
In healthcare settings, bronchoscopes, CPAP machines, heater-cooler units used during surgery, and dental scalers all use water and can harbor the bacteria if their water reservoirs aren’t regularly disinfected.
Why Legionella Is So Hard to Eliminate
What makes Legionella unusually persistent in water systems is its relationship with single-celled organisms called amoebae. Freshwater amoebae are the natural hosts for Legionella. The bacteria don’t just float freely in water; they invade amoebae, multiply inside them, and get packaged into small protective vesicles that are highly resistant to disinfection. When certain amoebae form hard-shelled cysts to survive harsh conditions, Legionella inside those cysts becomes shielded from chlorine and other chemical treatments.
This relationship has practical consequences. Legionella associated with one common amoeba species can survive temperatures as high as 150°F to nearly 200°F, far above the range that kills free-floating bacteria. Infected amoebae have also shown resistance to chlorine concentrations over a thousand times higher than what would kill the bacteria alone. Both organisms embed themselves in biofilm on pipe walls, making them even harder to reach with disinfectants.
The bacteria can also enter a dormant state where they’re alive and potentially infectious but won’t show up on standard laboratory tests. This “viable but non-culturable” form means a water system can test negative for Legionella and still harbor bacteria capable of causing disease. This is one reason water management programs focus on controlling the conditions that allow growth, not just testing for the bacteria after the fact.
What Keeps Growth in Check
Temperature is the single most important factor. Legionella grows best between 77°F and 113°F but can grow at temperatures as low as 68°F. Keeping hot water stored above 140°F and ensuring it reaches fixtures above 124°F limits growth throughout a plumbing system. Cold water below 68°F is generally safe.
Beyond temperature, reducing stagnation matters. Flushing rarely used taps, draining and cleaning hot tubs, and clearing sediment from water heaters all remove the conditions Legionella depends on. For large buildings, ASHRAE Standard 188 establishes minimum requirements for water management programs, including building surveys, preventive measures, and monitoring protocols designed to keep Legionella under control. Healthcare facilities have additional requirements given the vulnerability of their patient populations.
For homes, the practical steps are straightforward: keep your water heater set to at least 140°F, run water through fixtures that don’t get used often, clean showerheads and faucet aerators when buildup appears, and maintain hot tubs and humidifiers according to manufacturer instructions. People with weakened immune systems may want to consider point-of-use filters on showerheads and faucets for an additional layer of protection.