Preventing Legionnaires’ disease in air conditioning systems comes down to one core principle: don’t let water sit in warm, stagnant conditions where Legionella bacteria thrive. The bacteria grow best between 77°F and 113°F (25°C to 45°C), with the highest risk at 100°F to 110°F (38°C to 43°C). Any AC system that holds, recirculates, or mists water can become a breeding ground if it isn’t properly maintained.
Not all air conditioners carry the same risk, though. Understanding which type of system you’re dealing with changes everything about what prevention looks like.
Which AC Systems Actually Pose a Risk
Standard split-system and window air conditioners are low risk for Legionnaires’ disease. These units cool air by passing it over refrigerant coils, and the condensation they produce drains away rather than recirculating. There’s no reservoir of warm, standing water for bacteria to colonize. If you have a typical home AC, Legionella is not a realistic concern as long as condensate drains freely.
The systems that do pose a meaningful risk are those that use water as part of the cooling process:
- Cooling towers: Large rooftop units common in commercial and industrial buildings. They recirculate water and produce aerosol mist that can spread bacteria over significant distances.
- Evaporative coolers (swamp coolers): Common in dry climates, these pull air through wet pads, creating exactly the warm, moist environment Legionella loves.
- Air washers and humidifiers: HVAC components that add moisture to circulated air using water reservoirs.
If your system falls into one of these categories, prevention requires active, ongoing maintenance rather than a set-it-and-forget-it approach.
Temperature: The Single Most Important Factor
Legionella’s growth potential tracks closely with temperature. Below 77°F (25°C), the bacteria don’t grow. At 80°F (27°C), growth potential is low. At 90°F (32°C), it becomes moderate. Between 100°F and 110°F (38°C to 43°C), growth potential is very high. Above 131°F (55°C), there is no growth at all.
For cooling towers and evaporative systems, you can’t simply heat the water to kill bacteria the way you would in a hot water tank. Instead, the strategy is to prevent stagnation and use chemical treatment to compensate for the fact that the water sits squarely in the danger zone during normal operation.
Preventing Legionella in Evaporative Coolers
Evaporative coolers, whether portable units or rooftop systems, need hands-on cleaning at specific intervals throughout the cooling season. The water reservoir, filter pads, and internal waterways are all places where bacteria and biofilm (a slimy layer of microorganisms) can build up.
Before the Cooling Season
Disconnect power to the unit. Remove external covers and pull out the filter pads. Clean the pads thoroughly with a hose and replace them if they’re worn or heavily soiled. Scrub all internal waterways, including the sump and any bleed-off system. Close the drains, fill the system with clean water, and disinfect with chlorine-based household bleach before running the unit for the season.
During the Cooling Season
Portable evaporative coolers should be completely drained and cleaned at least once during hot weather. For fixed units, check the water level and cleanliness regularly. Don’t let water sit undisturbed for extended periods, especially if the unit goes unused for a week or more. Stagnant, warm water in an idle evaporative cooler is an ideal Legionella incubator.
At the End of the Season
Disconnect power. Use a brush to loosen any sediment and slime inside the unit. Drain all water from the tank and pipes completely. Clean the tank and pump with a cloth soaked in chlorine-based bleach, then flush with clean water. Remove the filter pads, hose them clean, and let them dry. Leave drains open and fit weatherproof covers to exposed outdoor units. The goal is to leave the system bone dry through the off-season so nothing has a chance to colonize.
Preventing Legionella in Cooling Towers
Cooling towers are the system most frequently linked to Legionnaires’ disease outbreaks. They recirculate large volumes of water at temperatures perfect for bacterial growth and generate aerosol droplets that can travel considerable distances from the building. Prevention here requires a formal, documented approach.
ASHRAE Standard 188 establishes minimum requirements for legionellosis risk management in buildings with these systems. It calls for a building survey, a written water management program, preventive measures, and documentation of all maintenance activities. Many jurisdictions now require compliance with this standard for commercial buildings.
The practical elements of a cooling tower maintenance program include:
- Chemical disinfection: Oxidizing disinfectants like chlorine or bromine should maintain measurable residuals throughout each day of operation. The specific concentration depends on manufacturer recommendations and the system’s water chemistry.
- Routine inspections: After reopening or during normal operation, weekly checks of the HVAC system are recommended to ensure everything runs properly. This frequency can gradually decrease to monthly or quarterly once the system is stable.
- Startup and shutdown procedures: Follow manufacturer guidelines and industry best practices for seasonal startup and shutdown. These transitions are high-risk moments because water has often been sitting stagnant.
If Legionella is detected or an outbreak is suspected, emergency disinfection is more aggressive. The CDC recommends achieving a disinfectant residual of at least 20 parts per million (ppm) as free available oxidant, then maintaining 10 ppm for at least 24 hours. After physical cleaning, another round at 10 ppm for one hour follows. This level of treatment should be handled by a water treatment professional.
After a Prolonged Shutdown
Buildings that have been vacant or operating at reduced capacity for weeks or months present elevated Legionella risk. Water that sat stagnant in pipes, cooling towers, and reservoirs during the shutdown has had time to reach ambient temperatures and develop biofilm. Biofilm is particularly problematic because it shelters bacteria from disinfectants and provides nutrients for continued growth.
Before bringing these systems back online, the CDC recommends flushing the entire water system, inspecting and cleaning cooling towers, and replacing HVAC filters. Once the building is reoccupied, weekly HVAC checks help catch problems early. This was a widespread concern during pandemic-era building reopenings, but it applies any time a system has been idle for an extended period, including seasonal properties, vacation homes with evaporative coolers, or commercial buildings between tenants.
Recognizing the Symptoms
Legionnaires’ disease typically appears 2 to 10 days after exposure to Legionella bacteria. Early symptoms include headache, muscle aches, and fever that can reach 104°F (40°C) or higher. By the second or third day, the illness progresses to a cough (sometimes producing mucus or blood), shortness of breath, chest pain, gastrointestinal symptoms like nausea and diarrhea, and confusion or other mental changes.
It’s a form of pneumonia, and it can be severe, particularly for people over 50, current or former smokers, and anyone with a weakened immune system or chronic lung disease. You cannot catch it from another person. The only route of infection is breathing in contaminated water droplets or mist, which is exactly what makes poorly maintained cooling systems dangerous.
A Quick Reference for Prevention by System Type
- Standard split or window AC: Keep condensate drains clear. No special Legionella prevention needed.
- Portable evaporative cooler: Drain and clean at least once during summer. Leave completely dry when not in use.
- Fixed evaporative cooler: Full cleaning and bleach disinfection before and after each cooling season. Regular checks during use.
- Commercial cooling tower: Written water management program per ASHRAE 188. Continuous chemical disinfection. Regular inspections. Professional water treatment.
The common thread across every system is simple: don’t let warm water sit. Whether that means draining a portable swamp cooler, scrubbing a rooftop unit with bleach, or maintaining continuous chemical residuals in a cooling tower, the bacteria need stagnant, warm water and biofilm to multiply. Remove those conditions, and you remove the risk.