How a Cooling Tower Sparked a Legionnaires’ Outbreak in Toronto
A cooling tower at a Toronto Transit Commission maintenance yard became the source of a local Legionnaires’ disease outbreak this summer. Public health investigators traced six confirmed cases back to the industrial equipment after matching bacterial samples from the tower with a patient’s sample. The outbreak centered on the Bathurst and Dupont streets area, with cases appearing between July 2 and July 26. Four of the cases were TTC employees. No deaths were reported.
The Bacteria That Lives in Water
Legionnaires’ disease is a severe form of pneumonia caused by Legionella bacteria. The bacteria occur naturally in freshwater environments but thrive in man-made water systems. Cooling towers, hot tubs, sprinkler systems, humidifiers, and decorative fountains can all become breeding grounds if not properly maintained. People catch the disease by inhaling tiny water droplets containing the bacteria. The disease does not spread from person to person, and the risk to the general public remains low.
How Investigators Found the Source
Toronto Public Health launched an investigation after six people living, working, or visiting the one-kilometre radius around Bathurst and Dupont fell ill. The agency collected environmental samples from potential sources in the area. At the same time, laboratories analyzed clinical samples from the confirmed cases. A match between a sample from a cooling tower at the TTC’s Hillcrest Complex and a patient sample provided the critical link. Public health officials said the epidemiological and environmental findings together gave them confidence that the tower was the source. The affected equipment was taken offline immediately, and the TTC contracted a deep-cleaning service to remediate the systems.
Investigators also identified a possible epidemiological link between this outbreak and five earlier Legionnaires’ cases at Castleview Wychwood Towers, a long-term care home adjacent to the Hillcrest complex. That connection is still under review.
The Role of Cooling Towers
Cooling towers are large heat-exchanger units used in commercial buildings and industrial facilities. They release heat by evaporating water, which can create a fine mist of droplets. If Legionella colonizes the tower’s water, that mist can carry bacteria into the surrounding air. Wind can spread the contaminated aerosol over several hundred metres. This is why outbreaks often cluster around a single building or facility.
Regular maintenance is key to preventing growth. That includes keeping the water clean, using biocides, and monitoring bacterial levels. Ontario’s public health guidelines recommend that building owners develop water management plans for cooling towers. The TTC said it had already taken steps to identify and mitigate risk before the outbreak was confirmed, but the presence of bacteria in two building systems showed the plan needed reinforcement.
What This Means for the Public
This was the second Legionnaires’ outbreak in Toronto in 2026. A May outbreak sickened at least 12 people in the city’s southeast. Ontario typically reports 200 to 300 cases each year. While the disease is rare, it can be deadly, especially for older adults, smokers, and people with weakened immune systems.
For the general public, the immediate risk from this outbreak was low. Public health officials did not issue any broad warnings beyond the affected area. The key takeaway is that cooling-tower outbreaks are preventable with proper maintenance and rapid response. The TTC’s cooperation with public health and the swift shutdown of the tower likely limited the number of cases.
Building owners should ensure their water systems comply with local regulations and undergo regular testing. Residents near large commercial facilities should be aware of the symptoms of Legionnaires’ disease—cough, fever, chills, and muscle aches—and seek medical attention if they develop pneumonia-like illness after potential exposure.