Has COVID Increased Legionella Risk in Commercial Buildings?

COVID-19 has changed how commercial buildings are used. While full closures were a short-term disruption, the long-term shift toward hybrid and remote working has resulted in a quieter, less consistently used built environment.

For facilities managers, this poses a subtle but serious problem: water systems that are underused or intermittently operated can become breeding grounds for Legionella bacteria. Understanding this ongoing risk and how to manage it is now more important than ever.

How Lower Occupancy Affects Water Safety

Reduced Flow in Pipework and Outlets

When fewer people are working in a building each day, demand on the water system drops. Taps, showers, toilets, and even communal kitchen sinks may go days without being used. This reduced draw causes water to sit still in the pipes for extended periods.

Stagnant water is a key contributor to Legionella growth, particularly when water temperatures drift into the critical range of 20°C to 45°C - warm enough for bacteria to multiply but not hot enough to kill them.

Infrequent Use of Water Tanks and Cylinders

It’s not just taps that go quiet. When occupancy falls, hot and cold water storage systems may no longer turn over water at the rate they were designed for. As a result, stored water can stagnate, sediment can build up, and temperatures can fluctuate - all of which create ideal conditions for biofilm and Legionella to take hold.

Maintenance Gaps and Oversight

Some buildings have adjusted maintenance regimes in line with reduced usage, but many have not. Without a proactive strategy, it’s easy for rarely used outlets or sections of pipework to be overlooked entirely.

In large buildings, especially those with zoned plumbing or multiple risers, some areas may now receive water flow only sporadically. This introduces a patchy risk profile, where Legionella could be thriving in one zone even if others remain compliant.

What the Regulations Say

The duty to control Legionella applies at all times, regardless of how many people occupy a site. Under the Health and Safety at Work etc. Act 1974, and COSHH Regulations, you are required to assess and manage risks from biological hazards in the workplace - including Legionella.

The HSE's ACOP L8 guidance makes it clear that even small changes in building use, layout, or staffing levels should prompt a review of your Legionella risk assessment.

How to Manage Legionella Risk in Partially Occupied Buildings

1. Revisit Your Risk Assessment

Even if your building has remained open throughout the pandemic, reduced staffing or hybrid working models may have changed the water usage pattern. Your risk assessment should be updated to reflect:

• Outlets that are now used less often or not at all

• Pipework or systems serving underused areas

• Flow rates through calorifiers and storage tanks

2. Introduce a Proactive Flushing Schedule

Flushing underused outlets is one of the simplest and most effective control measures. Identify all areas of low usage and create a weekly flushing programme to:

• Draw fresh water through pipework

• Remove stagnation and sediment

• Maintain effective disinfection levels

Every flush should be documented in a logbook or digital record to demonstrate compliance.

3. Check Storage Volumes Against Demand

If usage has dropped significantly, consider whether your current tank sizes or cylinder volumes are still appropriate. Oversized storage can slow turnover and cause water to linger, particularly in cold water tanks.

Assess whether tank capacity can be reduced or reconfigured - or whether changes in system design are warranted for the long term.

4. Maintain Temperature and Biocide Control

Ensure that cold water stays below 20°C and hot water above 60°C at all sentinel outlets. If you’re using chemical biocides for disinfection, verify that residual levels remain within effective ranges, especially at the furthest points of the system.

5. Sample and Test Strategically

Regular Legionella testing isn’t a substitute for control, but it can provide valuable insights. Focus sampling on outlets or system zones where flow is lowest, and use results to inform adjustments to your maintenance routine.

Frequently Asked Questions

Does reduced occupancy really increase Legionella risk?

Yes. The risk is tied to water stagnation, not just full shutdowns. Even moderate drops in daily usage can lead to low-flow conditions, stagnant water, and a favourable environment for Legionella.

Can we shut off unused outlets instead of flushing them?

Only if they are formally decommissioned and removed from the system. Otherwise, water in those outlets can still stagnate and pose a risk.

Do hybrid working patterns require a new Legionella strategy?

Often, yes. Especially in large or older buildings, changes in occupancy can create uneven flow and temperature distribution. Risk assessments should be updated to reflect these new usage patterns.

Summary: Legionella Risk Has Evolved - Your Controls Should Too

COVID didn’t just change how we work - it changed how our buildings behave. In many commercial premises, lower footfall means less water use, more stagnation, and heightened risk of Legionella.

Facilities managers should be actively reviewing risk assessments, flushing schedules, and water system configurations to adapt to this new normal. A water system designed for full daily use may now need tighter controls to stay safe under part-time operation.

To explore strategies and practical tools for keeping your systems compliant, visit our Learning Centre and learn more about managing water hygiene in a post-COVID world.