Cooling towers can grow Legionella bacteria and release them in fine water droplets that people may breathe in, sometimes far from the building. The risk is managed with a written water management plan that covers maintenance, water treatment, monitoring and what to do when results are out of range. Owners are responsible for making sure that plan exists and is followed.
What Legionella is
Legionella are bacteria found naturally in fresh water. At low levels they are generally not a concern, but in building water systems they can multiply, especially in warm, stagnant water with sediment, scale and biofilm. When people inhale contaminated aerosols, they can develop Legionnaires' disease, a serious pneumonia, or a milder illness called Pontiac fever. Older adults, smokers and people with weakened immune systems are at higher risk.
Legionnaires' disease is not spread from person to person. It comes from water systems, which is why building owners play a central role in prevention.
Why cooling towers are a particular risk
Cooling towers reject heat from chillers and other equipment by evaporating water. That process combines the conditions Legionella likes:
- Water temperatures that can support bacterial growth.
- Sediment, scale and biofilm on surfaces and in the basin.
- Nutrients from airborne dirt, leaves and debris drawn into the tower.
- Periods of low use or stagnation, such as seasonal start-up.
- Drift: small droplets carried out of the tower by the air stream.
Evaporative condensers, fluid coolers and some humidifiers and decorative fountains share similar risks. Domestic hot and cold water systems can also grow Legionella, particularly where water is kept lukewarm or sits unused.
Rules vary across Canada
Requirements for cooling towers differ by province and municipality. Some jurisdictions require registration of cooling towers, regular sampling and reporting, and written maintenance programs, while others rely on general public health and building maintenance obligations. Check with your provincial and local public health authorities for the rules that apply to your building.
The water management plan
A water management plan is a written, site-specific program. Guidance from ASHRAE Standard 188 and public health agencies sets out the general approach. A good plan includes:
- A team with named responsibilities, including the owner, operators and the water treatment provider.
- A description and diagram of the water systems, showing where risk exists.
- Control measures, such as biocide treatment, scale and corrosion control, and blowdown.
- Control limits for each measure and how often they are checked.
- Corrective actions to take when results are outside limits.
- Routine Legionella sampling where required or recommended, done by an accredited laboratory.
- Procedures for start-up after shutdown, seasonal operation and cleaning.
- Records of all monitoring, treatment, cleaning and corrective actions.
- Periodic review of the plan, and review after any change to the system or any positive results.
Physical condition matters too
Water treatment cannot overcome a tower in poor physical condition. Regular inspection should look at:
- Drift eliminators: in place, undamaged and fitted without gaps.
- Basin and fill: free of sludge, scale, algae and debris.
- Dead legs and unused piping where water stagnates.
- Chemical feed equipment and controllers working as intended.
- Location of the tower relative to outdoor air intakes, operable windows and public areas.
Protect the workers. People cleaning or inspecting cooling towers can be exposed to contaminated aerosols. Work should follow written procedures, with disinfection where required beforehand, suitable respiratory protection and lockout of fans and pumps. Some towers also meet the definition of a confined space. See our guide on respiratory protection programs.
If Legionella is detected
Follow the corrective actions in your plan and any instructions from public health authorities. Typical steps include increasing treatment, cleaning and disinfecting the system, investigating the cause and resampling to confirm control. If there are cases of illness associated with the building, public health will lead the response and owners must cooperate fully.
How ehZee can help
Our air and water filtration system inspections look at the physical condition of cooling towers and related equipment, review water treatment records and check whether a water management plan exists and is being followed. We do not perform laboratory analysis; we help you interpret results and act on them, and coordinate the contractors involved. For wider plant condition, see mechanical system inspections.
Common questions
Does chemical treatment alone control Legionella?
Treatment is essential, but it works best with clean surfaces, good physical condition and proper flow. Sludge and biofilm protect bacteria from biocides.
How often should cooling towers be sampled for Legionella?
It depends on local requirements and your water management plan. Some jurisdictions set mandatory frequencies. Where none apply, your plan and water treatment provider should set a routine.
What about towers that are shut down for winter?
Drain and clean them as your plan requires, and follow a start-up procedure that includes cleaning and disinfection before returning them to service.