Summer Legionella risks:Why does hot weather increase bacterial growth?

Every summer, the same conditions repeat across commercial buildings, hotels, healthcare facilities, and residential properties. 

Water warms. Occupancy shifts. Cooling systems push harder. And Legionella, which is manageable under the right conditions, finds exactly the environment it needs to multiply.

Legionnaires’ disease cases have increased significantly across Europe over the past two decades and continue to rise in multiple member states. That trajectory is not coincidental. It tracks closely with the seasonal and environmental patterns that summer creates in built water systems. 

Understanding the mechanism is the first step toward breaking it.

Why does summer increase Legionella risk?

Legionella pneumophila is a naturally occurring waterborne bacterium that becomes a clinical risk when it enters man-made water systems- plumbing networks, cooling towers, hot tubs, decorative fountains, storage tanks, and showerheads, where conditions allow it to multiply to infectious concentrations.

Transmission does not happen through drinking. It happens through inhalation of fine aerosolized water droplets carrying the organism into the respiratory tract.

Here is why summer creates those conditions across most commercial and institutional buildings:

1. Warmer water temperatures 

Legionella amplifies aggressively in the 25°C-45°C range. Summer drives building water directly into that band- cold lines warm from ambient heat, storage tanks lose their thermal differential, and remote fixtures drift furthest from the source temperature. 

2. Increased water stagnation

School closures, hotel wings offline, office floors at reduced capacity- all reduce flow through sections of the distribution system. Disinfectant residual depletes. Temperatures equalize with the ambient. Biofilm establishes in dead legs and infrequently used fixtures. When occupancy returns, that accumulated load moves through the system toward the point of use. 

3. Higher Cooling Tower Usage

HVAC systems run harder through peak summer heat, increasing aerosol output over longer daily cycles. A poorly maintained cooling tower does not just harbor Legionella; it disperses it. Contaminated drift can carry the organism across significant distances under the right wind conditions. 

4. Reduced Disinfectant Effectiveness

Heat accelerates chlorine depletion in bulk water. Summer biofilm growth also increases the organic load, consuming residual disinfectant before it reaches remote fixtures. Changes in pH can further reduce chemical effectiveness, so a system that is within parameters in spring can still have significant residual gaps at the point of use by July, even without any change to the treatment program. 

How to prevent Legionella risks in the summer?

Prevention is not a summer project. It is a year-round discipline that summer puts under the most pressure. The following are a few ways that can prevent Legionella risks in the summer. 

• Temperature control: Keep cold water below 18°C and hot water above 66°C at the heater. Monitor temperatures at remote and high-risk fixtures, not just at the source.

• Regular flushing: Flush infrequently used outlets weekly. Stagnant water cannot be treated for safety-  it has to move.

• Cooling tower maintenance: Clean and disinfect routinely according to the ECDC guidelines before peak season and at regular intervals throughout it. Do not treat cooling tower maintenance as a once-per-year event.

• Water Management Programs: A compliant WMP is not paperwork. It is a risk assessment, control plan, monitoring schedule, and corrective action framework, built for the specific conditions of your facility.

• Biofilm control: Chemical treatment addresses bulk water. Biofilm requires mechanical cleaning, sediment removal, and physical inspection, particularly after any period of reduced occupancy.

• Certified point-of-use filtration: Chemical disinfection and upstream controls cannot guarantee what reaches the terminal fixture. A certified physical barrier at the point of aerosolization is the control measure that accounts for what every other layer of the program cannot fully reach. 

  
  

How does MWT’s certified filtration reduce Legionella?

Each prevention layer above addresses a different part of the system. What none of them fully resolve is the terminal fixture- the exact point where aerosolized water contacts the respiratory tract.

MWT's 0.08 µm hollow fiber ultrafiltration technology provides a certified physical barrier at that final point. Certified Log 10 bacterial reduction and FDA-registered (No. #10094411) and EPA-FIFRA-registered (No. #105402-NLD-1), MWT filters reduce Legionella and other waterborne pathogens regardless of upstream residual fluctuations.

Handheld shower units carry a risk fixed showerheads do not, because the aerosol stream is directed at close range toward the face. In senior living settings where assisted bathing is routine, that proximity matters. PurGuard360® is an EPA WaterSense-certified handheld shower filter that combines water efficiency with certified filtration against waterborne pathogens. WaterSense focuses on water preservation; however, we are the only ones ensuring both water efficiency and certified filtration. 

Ice machines run continuously through summer, yet the water inlet feeding them is rarely included in a terminal fixture risk assessment. Legionella does not require hot steam- contaminated water contact at mucous membranes is a documented exposure route. The Inline PureFlow® Blue delivers certified inline filtration at the machine's water inlet, closing a risk point most WMPs leave open.

For facilities managing risk across a large or complex distribution system, Total Pure® addresses it at the building entry point. In summer, when temperatures are hardest to control across the full pipe run, starting with cleaner water at the source reduces the burden on every downstream control measure.

The risk is predictable. The response should be too. 

Summer does not create new risk- it reveals the gaps in programs that were never built to handle it. What separates a compliant summer from a reactive one is whether protection was in place before the water warmed, not after the first positive culture triggered the response.

Don't wait for a confirmed case to act. Explore MWT's certified filtration solutions or contact our team for a technical consultation.