Legionella
L. pneumophila·Affects lungs
The bacterium behind Legionnaires' disease, a severe form of pneumonia contracted by inhaling aerosolized droplets from contaminated showers, faucets, and cooling systems.
L. pneumophila
EPA limit
Treatment Technique under SWTR
Most at risk
Adults over 50, smokers, immunocompromised, chronic lung disease patients, hospital and nursing-home residents
Found in
Well water · City water
AquaOx filters
YesViqua UV sterilizer (downstream pairing)
What it does to you
Health effects
Legionella causes two distinct illnesses. Legionnaires' disease is a severe pneumonia with high fever, cough, shortness of breath, muscle aches, and confusion appearing two to ten days after exposure. It requires hospitalization in most cases and is fatal in roughly 10 percent of community-acquired cases and 25 percent of hospital-acquired cases.
Pontiac fever is the milder form: a self-limiting flu-like illness that resolves in two to five days without antibiotics. Most exposures actually produce no illness at all; whether infection takes hold depends on the dose, the strain, and the host's age and immune status.
Critically, Legionella is not contracted from drinking the water. The bacteria colonize human lungs only when inhaled as fine aerosols. Showers, faucet sprays, decorative fountains, hot tubs, cooling towers, and medical respiratory equipment are the documented transmission routes. Person-to-person transmission is extremely rare.
Where it comes from
How it gets into your water
Legionella lives naturally in lakes, streams, and soil at low concentrations that pose no risk. The problem starts when it enters human-built warm-water systems where it can amplify to dangerous levels. The bacteria thrive between 77°F and 113°F, with peak growth around 95 to 115°F: exactly the temperature range of underused or undersized residential water heaters.
Conditions that promote Legionella growth include stagnant water in seldom-used fixtures, sediment and scale buildup inside pipes and tanks, loss of disinfectant residual in long pipe runs, and biofilm colonies that protect the bacteria from chlorine. Older buildings, hotels, hospitals, and nursing homes are the classic outbreak settings.
Once colonized, Legionella enters human lungs when fixtures generate aerosols: a running shower, a faucet, a misting system, a humidifier, or an HVAC cooling tower drifting downwind onto a building's air intake.
Water heaters below 140°F
The 95-115°F range is peak Legionella growth zone
Stagnant plumbing runs
Unused guest bathrooms, dead-leg piping
Shower heads & aerators
Aerosolize droplets into inhalable mist
Cooling towers & HVAC
Major source of community outbreaks
Find out for yourself
Is it in your water?
Legionella is not routinely tested in municipal supplies. EPA regulates it under the Surface Water Treatment Rule as a Treatment Technique standard, meaning utilities must filter and disinfect their source water adequately, but the practical risk arises after water enters your building plumbing.
If you live in an older home, have a water heater set below 130°F, leave fixtures unused for weeks at a time, or are in a higher-risk household (elderly residents, recent organ transplant, chronic lung disease), targeted testing is reasonable. Certified labs offer culture and qPCR testing of shower head and tap samples.
CDC reported case counts of Legionnaires' disease have risen roughly nine-fold since 2000. Most cases are sporadic and traced back to building plumbing rather than the municipal supply itself.
The fix
How AquaOx filters it
Be direct: Legionella is mostly a building-plumbing and water-heater problem, not strictly a water-quality-at-the-meter problem. No point-of-entry filter, including AquaOx, fully eliminates Legionella risk on its own, because the bacterium can colonize and amplify inside the hot-water system downstream of any filter.
That said, the AquaOx whole-house system reduces inputs that fuel Legionella growth. The sediment stage filters scale precursors and biofilm food sources, and the carbon stages strip chlorine that would otherwise have provided some residual disinfection (so customers who want belt-and-suspenders should add the Viqua UV sterilizer downstream of the carbon). The Viqua UV unit inactivates Legionella as water enters the home, lowering the seed population that would colonize the heater.
The single most important Legionella control in any home is keeping the water heater set to at least 140°F at the tank and using a mixing valve at the tap to prevent scalding. This is a building-management measure that no filter can replace. If anyone in your household is immunocompromised, combine high tank temperature, periodic flushing of unused fixtures, and the UV sterilizer pairing.