The People Closest to a Cooling Tower Are the Ones Cleaning It

Legionnaires' disease is usually discussed as a public health story, with building occupants and passers-by as the people at risk. For a maintenance crew the picture is different.

The worker who opens a cooling tower, disturbs the biofilm and pressure washes the fill is generating aerosol from the exact material the bacteria live in, at arm's length, deliberately. Cleaning a tower is the most aerosol-generating thing anyone does to one.

What happened, and where

The scale of the Canadian experience is worth stating because it explains why the regulation looks the way it does.

Canadian environmental health research records the Quebec City outbreak of 2012 as affecting over 180 people with 14 deaths, traced to a cooling tower on a downtown building.

It also records outbreaks closer to home: Surrey, British Columbia in 2018 with over ten hospitalizations, Moncton in 2019 with sixteen cases and fifteen hospitalizations, and London, Ontario in 2019 with six hospitalizations.

The transmission route is inhalation of contaminated water droplets, and cooling towers are efficient at producing them. Plumes arising from towers carry the bacteria well beyond the roof they sit on.

The conditions that let it grow are mundane: warm stagnant water, biofilm, scale and sediment. A tower that is running and poorly maintained is a better incubator than one that is shut down.

The same conditions occur in hot water tanks, water storage systems, whirlpool baths, hot tubs and spas, so cooling towers are the headline rather than the whole exposure.

Vancouver has its own standard

Most building managers in British Columbia do not know this one exists.

Vancouver operates a cooling tower registry through annually renewable operating permits, and its plumbing bylaw sets a Legionella pneumophila standard of fewer than 10 colony forming units per millilitre in cooling towers.

That is a numeric compliance threshold at the municipal level, in a country where most jurisdictions have none.

Surveillance results from Vancouver in 2021 showed the vast majority of reporting towers, around 94.6 percent, meeting that standard. The towers that exceeded it were concentrated: thirty exceedances across just sixteen facilities, with two hospital campuses between them accounting for twelve of the towers involved.

That concentration is the useful finding. Legionella in cooling towers is a highly localised, building-scale problem rather than a general ambient one, which means the answer is specific buildings getting their maintenance right rather than a sector-wide panic.

Hamilton was the first Canadian municipality to require mandatory cooling tower registration, following outbreaks in 2006 and 2008, so the municipal route has precedent going back nearly two decades.

Quebec sets the strictest regime in Canada

After 2012, Quebec moved further than anywhere else in the country.

Owners of evaporative cooling systems must register their towers with the Régie du bâtiment du Québec. They must implement a preventive maintenance program developed by a professional, maintain a register of information about the towers, and operate a certified control plan for each one.

Since July 2014 the regulation has also required Legionella sampling at thirty-day intervals.

Federally, Public Services and Procurement Canada developed standard MD-15161, Control of Legionella in Mechanical Systems, applying to Crown-owned buildings it manages. It sets requirements for design, installation, maintenance, operation and bacterial testing, with weekly, monthly and longer-interval inspection requirements and a site-specific Legionella Bacteria Control Management Program for each building.

The National Building Code of Canada contributes design requirements, including the provision that cooling tower blow-down and discharge be kept away from outdoor air intakes, which is the detail that turns one building's problem into its own rather than its neighbour's.

Outside Quebec and the municipalities with their own bylaws, there is no general mandatory registry, which is why industry groups continue to recommend one as a first step.

The worker exposure nobody plans for

Here is the part that belongs on a workplace safety page rather than a public health one.

Every control that reduces risk to occupants involves somebody going to the tower and disturbing it. Inspection, cleaning, descaling, fill replacement, drift eliminator checks and water treatment all put a person in the aerosol.

Pressure washing is the worst case. It takes a biofilm that was sitting still and converts it into a breathing-zone mist, which is precisely the mechanism that infects people at a distance, applied at close range.

Respiratory protection matters here and it has to be selected for the exposure rather than grabbed from the van. Fit decides whether a rating means anything, which is covered in our guide to respirator fit testing under CSA Z94.4.

Eye protection belongs with it, since aerosol reaches mucous membranes, covered in our safety glasses guide.

And the water treatment chemicals are their own hazard entirely. Biocides, oxidizers and acids used in tower treatment need glove selection by substance rather than by habit, covered in our guide to choosing chemical resistant gloves.

Everything else about working on a roof plant deck

The tower is rarely the only hazard in the job, and the setting adds several.

Most towers sit on roofs, which brings fall exposure at the edge, at skylights and at hatches, along with the access route to get there.

The deck around a running tower is wet, and in winter it is icy. Slip resistance matters more on a wet plant deck than almost anywhere else, covered in our guide to slip resistance at work, and footwear for the trade generally is covered in our guide to work boots for industrial maintenance workers.

Fans and drives mean energy isolation before anyone reaches into the housing, and a tower basin can meet the definition of a confined space depending on its configuration.

Changeable outer clothing keeps contaminated water out of a vehicle and a home, covered in our comparison of coveralls and bib overalls.

We do not sell water treatment chemicals, sampling kits, biocide dosing equipment or the laboratory services behind any of this. Those come from water treatment specialists, and the testing regime is theirs rather than ours.

Shutdown and restart is the risk window

A tower that has been sitting idle is more dangerous to bring back than one that has been running, and the seasonal pattern in Canada makes this a recurring event rather than an exception.

Warm stagnant water is the growth condition, and a tower taken out of service in the autumn and left with water in it through a mild spell has had months to develop what nobody was monitoring.

The federal control program addresses this through design, installation, maintenance and operation requirements together with a building-specific control management program, rather than treating maintenance as a seasonal task.

The practical rule is to drain rather than idle where the system allows it, and to treat a restart as a cleaning and disinfection event rather than switching the fan on.

The same applies to any part of a building water system that has been out of use: a wing that was closed, a shower nobody ran, a hot water loop isolated for a renovation.

And the worker doing the restart is the first person exposed to whatever grew, which puts them back at the front of the risk picture rather than the occupants.

Building a program that holds

The common shape across the Canadian frameworks is a written, building-specific water management plan rather than a generic maintenance contract.

Know what you have. A surprising number of building owners cannot say how many cooling towers, hot water systems and storage tanks they operate, which is exactly the gap a registry closes.

Assign responsibility to a named person rather than to a department, and make sure the water treatment contractor's scope and the mechanical contractor's scope do not leave a gap between them.

Follow the inspection intervals the applicable standard sets rather than the ones that fit the schedule. The federal standard works on weekly and monthly cycles for a reason, and a tower inspected when someone remembers is a tower with months of unobserved growth.

Sample on a defined interval where that is required or sensible, and act on an exceedance quickly rather than resampling to see whether it goes away.

Keep the records. They are the evidence that the program ran, and in an outbreak investigation they are the first thing requested. The same logic applies here as in our guide to CSA certified versus CSA compliant: a claim without a record is just a claim.

For the wider programme see the complete PPE checklist for Canadian workers, our guide to PPE requirements for Canadian construction sites, our guide to who pays for PPE in Canada and our guide to CSA Z1220 first aid kit requirements.

Frequently Asked Questions

Who is most at risk from a cooling tower?

Occupants and passers-by are the public health concern, but the people closest to the hazard are the maintenance workers who open, clean and service the tower. Pressure washing a tower converts settled biofilm into breathing-zone aerosol, which is the same transmission mechanism applied at close range.

Have there been outbreaks in British Columbia?

Yes. Surrey recorded an outbreak in 2018 with over ten hospitalizations. Canada's largest recent outbreak was Quebec City in 2012, affecting over 180 people with 14 deaths, traced to a cooling tower on a downtown building. Moncton and London, Ontario both had outbreaks in 2019.

Is there a registry requirement in Canada?

Not nationally. Quebec requires province-wide registration with the Régie du bâtiment du Québec, Vancouver operates a registry through annually renewable operating permits, and Hamilton was the first Canadian municipality to require registration following outbreaks in 2006 and 2008. Elsewhere there is generally no mandatory registry.

What standard applies in Vancouver?

Vancouver's plumbing bylaw sets a Legionella pneumophila standard of fewer than 10 colony forming units per millilitre in cooling towers, alongside the operating permit registry. Surveillance in 2021 found roughly 94.6 percent of reporting towers meeting it, with exceedances concentrated in a small number of facilities.

How often does Quebec require sampling?

Every thirty days, a requirement in force since July 2014. Quebec also requires registration, a preventive maintenance program developed by a professional, a register of tower information and a certified control plan for each tower, which together make it the most stringent regime in Canada.

What protection does a maintenance worker need?

Respiratory protection selected for the aerosol exposure and properly fit tested, eye protection because aerosol reaches mucous membranes, and gloves selected against the specific water treatment chemicals in use rather than by habit. Energy isolation before reaching into fan housings, and confined space procedures where a basin meets the definition.

This guide is general workplace safety information, not water treatment, engineering or medical advice. Legionella control requirements differ substantially between provinces and municipalities, with Quebec and some municipalities imposing registration, maintenance and sampling obligations that do not apply elsewhere. Confirm the requirements for your building with your municipality, your provincial authority and the occupational health and safety authority that governs your workplace, and engage qualified water treatment professionals. Anyone with respiratory symptoms following possible exposure should seek medical assessment and mention the exposure.