A property management company in Augusta reopened a mostly vacant office building after a slow leasing period left two floors unoccupied for nearly five months. Water service had never been shut off, but almost nothing had run through the pipes on those floors: no faucets, no showers in the tenant break rooms, minimal flow through the cooling tower serving that section of the building. When a new tenant moved in and flagged a musty smell from the water fountain, testing found Legionella bacteria in the building’s hot water system at levels well above what’s considered safe for occupied space.
Nothing about the water supply itself had changed. What changed was that the water sat still for months, and stagnant water is exactly the condition Legionella needs to establish itself.
Why Unused Water Systems Are a Specific Risk
Legionella is a bacteria that occurs naturally in freshwater environments and becomes dangerous when it grows to high concentrations in human-made water systems, then gets aerosolized into breathable droplets through showers, cooling towers, decorative fountains, or plumbing fixtures. People who inhale those droplets can develop Legionnaires’ disease, a serious and sometimes fatal type of pneumonia, or the milder Pontiac fever.
Legionella needs three conditions to multiply: warm water (the CDC identifies 77°F to 108°F as the range where it grows best), stagnation, and a nutrient source like biofilm, sediment, or scale buildup inside pipes. A building in regular use disrupts all three. Water moves through the pipes, temperatures fluctuate with demand, and disinfectant residual (typically chlorine or chloramine from the municipal supply) keeps circulating and doing its job.
Take away regular use, and all three risk factors get worse at once. Water sits at ambient building temperature, often squarely in Legionella’s preferred range. Biofilm builds up undisturbed on interior pipe surfaces. And disinfectant residual decays over time with no fresh water bringing more in.
Buildings and Spaces Where This Comes Up
This isn’t limited to obviously vacant properties. It applies anywhere water use drops off significantly for an extended period:
- Seasonal buildings, including schools during summer break, vacation rental properties between guests, and recreational facilities that close for part of the year.
- Vacant commercial space, including office floors between tenants, retail units awaiting new occupants, and buildings undergoing renovation with water service still connected but fixtures not in regular use.
- Portions of larger buildings, such as an unused wing of a medical office, a mothballed hotel floor, or a building section closed off during a phased renovation while the rest stays occupied.
- Properties coming out of foreclosure or extended vacancy, where water service may have been on the entire time but nobody was using it.
- Facilities that reduced operations, including businesses that shifted to remote work and left office water systems running with minimal use.
In each of these cases, the water supply entering the building was never the problem. The risk built up inside the building’s own plumbing, cooling equipment, and fixtures during the period of low use.
What Reopening Without Testing Misses
The instinct when reopening a building is often to run the water for a few minutes and consider the system flushed. That’s not sufficient for Legionella specifically. A short flush clears visibly discolored water from the tap but doesn’t necessarily disrupt biofilm colonies established deeper in the plumbing, in water heaters, or in cooling tower components, and it doesn’t tell you whether bacteria levels are actually within a safe range.
OSHA’s guidance on Legionella notes that building water systems with any history of stagnation should be evaluated before full occupancy resumes, particularly where vulnerable populations (older adults, immunocompromised individuals, smokers) will be present. For healthcare facilities specifically, the CDC recommends testing as part of a water management program review whenever a facility resumes water use after an extended shutdown.
What a Pre-Reopening Assessment Involves
A Legionella risk assessment for a building coming back into use typically covers:
- Water sampling from multiple points in the system, including hot water tanks, cooling towers if present, and fixtures furthest from the main water entry point, where stagnation is usually worst.
- Temperature checks at hot water tanks and points of use, since maintaining hot water above 122°F and cold water below 68°F, where feasible, significantly limits Legionella growth.
- Review of flushing procedures already performed, to identify gaps before testing rather than after.
- Inspection of cooling towers, decorative water features, and other aerosol-generating equipment for visible biofilm, scale, or sediment that indicates prolonged stagnation.
What to Do
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If a building, floor, or system has sat with minimal water use for more than a few weeks, test before full reopening. This applies to seasonal properties, vacant commercial space, and any facility resuming normal water use after a gap.
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Don’t treat a flush as equivalent to a test. Running water clears visible discoloration but doesn’t confirm bacteria levels are safe, especially in hot water systems and cooling equipment.
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Build reopening testing into your water management plan going forward. If your building has any history of seasonal closure or partial vacancy, a documented testing step before reoccupancy protects both occupants and the property owner.
If you’re reopening a building, a floor, or a water system after an extended period of low use, EnviroPro 360 can test it before people move back in. Reach out here and we’ll help you figure out what to check.
