What to Do When Your Facility Tests Positive for Legionella

It was a Tuesday morning when the call came in. A facility manager at a mid-size office complex — 400,000 square feet, a cooling tower on the roof, decorative fountains in the lobby — had just received results from a routine water test. One sample came back positive for Legionella pneumophila.

“I didn’t even know what to do first,” he told us later. “I just knew it was bad.”

That call is more common than most facility managers realize. In 2023 alone, the CDC recorded over 10,000 reported cases of Legionnaires’ disease in the United States — and most outbreaks are traced back to building water systems that had no active monitoring program in place.

Here is exactly what happened next — and what every facility manager should know before they ever make that call.

What Is a Legionella Positive Test — and What Does It Actually Mean?

A positive Legionella test means that Legionella bacteria were detected above reportable levels in one or more water samples from your building’s water system. It does not automatically mean an outbreak has occurred. But it does mean the conditions exist for Legionnaires’ disease — a serious, potentially fatal pneumonia — to develop, and immediate action is required.

Legionella is a waterborne bacterium found naturally in the environment. In building water systems, it amplifies when water temperature, stagnation, and low biocide residuals create favorable conditions — particularly in cooling towers, hot water systems, and decorative water features. It spreads through inhaled aerosols, not through drinking water or person-to-person contact.

What Should I Do If Legionella Is Found in My Building?

The first 48 hours are the highest-leverage window. Here is the response protocol we followed — and the sequence that applies to virtually every commercial building scenario.

1. Notify the right people immediately. In most jurisdictions, a positive Legionella result triggers a mandatory health department notification. Know your state requirements before you need them. Confirm whether your building falls under specific regulatory frameworks (healthcare, hospitality, high-rise residential) that carry additional reporting requirements.
2. Shut down or isolate the confirmed source. In this case, it was the rooftop cooling tower. Shutting it down prevents continued aerosolization while remediation is planned. For hot water systems, isolation depends on the building layout.
3. Engage a water treatment specialist immediately. This is not a job for a general maintenance contractor. The response requires water chemistry expertise — shock disinfection protocols, sampling design, chain-of-custody documentation for retesting, and regulatory coordination.
4. Conduct a full system walkthrough — not just the confirmed source. When our team arrived, we found two additional risk points that initial sampling had not flagged: a hot water recirculation loop running below 140°F and a stagnant decorative fountain. Both were remediated as part of the response.
5. Document everything in real time. If this escalates to a health department investigation or a legal matter, your response documentation is your protection. Timestamps, sample results, remediation steps, and temperature logs all matter.
6. Retest before returning systems to service. Negative results on post-remediation sampling close the active risk loop and provide documentation that the issue was resolved.