The Hidden Winter Risk: Why Cooler Weather Does Not Lower Legionella Threats

Legionella prevention is not a summer-only job. In healthcare and long-term care environments, winter operations can quietly increase Legionnaires’ disease risk because of stagnation, tepid water zones, and residual decay in building plumbing. Infection prevention leaders who treat winter like a lower-risk season often miss early warning signals that precede colonization and clinical cases.

Why winter can raise Legionella risk in healthcare water systems

Lower occupancy, holiday slowdowns, and wing closures can extend water age, which means more stagnant water in distal lines and dead legs. Stagnation drives disinfectant loss and allows temperatures to drift into the Legionella growth range. CDC notes that stagnant or standing water can reduce disinfectant levels and let hot water cool into the 77 to 113 °F range, conditions that promote biofilm and Legionella growth. CDC

Temperature control remains a cornerstone of Legionella control. Evidence reviews emphasize maintaining hot and cold systems outside the 25 to 43 °C growth range. Warm water leaves systems vulnerable to colonization, while stable elevated temperature supports control.

Cold water is not immune in winter. When cold loops sit or travel through warm spaces, temperatures can creep above 20 °C, especially where use is intermittent. This increases risk when stagnation is present or loops are rarely used.

Clinical stakes in hospitals and long-term care

For healthcare-associated Legionnaires’ disease, severity is higher than community-acquired cases. CDC reports a case fatality rate around 25 percent for definite healthcare-associated infections. This underscores the need for proactive water management programs, testing strategies, and rapid response.

Compliance expectations do not take winters off

Regulators and accreditors expect year-round control. ASHRAE Standard 188 establishes the framework for comprehensive water management plans that identify hazardous conditions and control measures across building water systems.

For Joint Commission accredited facilities, EC.02.05.02 codified in the R3 Report requires organizations to maintain a documented water management program with team oversight, analysis of hazardous conditions, and verification and validation activities. The standard became effective January 1, 2022, and remains an active expectation going into 2026.

In sterile processing and device reprocessing, AAMI ST108 aligns water quality management with the organization’s broader water management program team structure. This is an opportunity for infection prevention, facilities, and sterile processing to close winter gaps together.

Winter infection prevention checklist for water systems

Use this practical checklist to harden your Legionella prevention posture before and during the winter season.

1. Refresh water system mapping and seasonal use patterns Update your diagrams and fixture inventories to reflect closures, reduced use areas, and construction tie-ins. ASHRAE 188 calls for a graphical depiction of water systems that supports risk analysis at endpoints where aerosolization can occur.

   
   

2. Prioritize temperature management Audit hot water generation and return temperatures. Verify that distal outlets achieve target setpoints quickly. Literature supports keeping systems outside the 25 to 43 °C range that favors Legionella growth.

   
   

3. Fight stagnation with planned flushing Create winter-specific flushing schedules for low-use wings, clinics, and procedure rooms. CDC emphasizes the role of stagnation in disinfectant loss and temperature drift. Document flow times to achieve hot and cold stabilization, not just quick purges.

   
   

4. Trend disinfectant residuals and water age indicators Track residuals, temperatures, and flow events as leading indicators. Correlate these metrics with any culture or qPCR data for early signal detection. Recent research continues to evaluate how WMP metrics predict Legionella detection.

   
   

5. Tighten verification and validation routines Verification confirms the program is implemented as designed, while validation confirms controls are effective. Joint Commission’s R3 describes the requirement for measurable activities. Use winter as a trigger to review logs, alarms, and corrective action records.

   
   

6. Reassess patient risk and testing policies Healthcare-associated cases carry higher mortality. Ensure diagnostic pathways, environmental sampling triggers, and response playbooks reflect winter occupancy and procedure changes.

Communicating winter risk to executives and the front line

Leadership often assumes winter means safer water because outside air is colder. Your message should be simple. Winter changes how people use the building. Less use means more stagnation. Stagnation reduces disinfectant and pushes water temperatures toward the Legionella growth zone. This is when infection prevention and facilities need the most coordination, not the least.

Build visibility with a one-page winter water safety dashboard that includes: • Distal temperature pass rates by unit • Residual trends at sentinel outlets • Percentage of low-use fixtures successfully flushed to targets • Time to hot setpoint at high-risk outlets • Any culture or qPCR hits with corrective actions

A smarter approach: continuous monitoring plus targeted treatment

The old model was sample, wait, then react. Today, facilities can combine continuous monitoring for temperature and flow with controlled secondary disinfection strategies that maintain residuals between service visits and keep biofilm pressure lower. This combination is especially valuable in winter when variable use patterns are the norm. CDC and ASHRAE guidance support programs that maintain stable temperatures and disinfectant levels while minimizing stagnation.

Where you already use mineral oxychloride programs or similar technologies, ensure the winter flushing plan and monitoring thresholds are tuned to occupancy and staff schedules. Align your verification and validation documentation with Joint Commission expectations to show year-round control.

Bottom line for infection prevention

Winter is not a break for Legionella control. It is a shift in risk drivers that demands the same rigor as summer. Keep water moving, keep temperatures out of the growth zone, keep residuals stable, and keep your water management program verified and validated. The clinical stakes remain high. Healthcare-associated cases average a 25 percent fatality rate, so proactive winter management is not optional.