Is Climate Change Increasing Your Legionella Risk?
| The UK’s climate is changing faster than many building managers realise. Spring and early Summer 2026 have already seen record-breaking temperatures. On 25 May 2026, Kew Gardens recorded 34.8°C. The last three years all fall within the five hottest years ever documented in the UK. Rainfall patterns have become increasingly erratic, with Autumn 2023 to Spring 2024 recorded as the wettest period on record, followed by November 2025 seeing rainfall approximately 31 percent above average across England and Wales. |
For estates and facilities teams responsible for water hygiene compliance, these are not abstract environmental statistics. They are factors that directly affect how Legionella bacteria behaves in water systems, and how much harder it is becoming to keep those systems within safe, compliant temperature ranges.
A recent article published in Building Better Healthcare, drawing on analysis from Pete Gunn, Senior Consultant at the Water Hygiene Centre, examined how increasing pressures on the UK’s water supply infrastructure could create new Legionella management challenges for healthcare estates. The implications extend well beyond healthcare, affecting any building where water systems are managed under ACOP L8 obligations.
Why Warmer Temperatures Create Greater Legionella Risk
Legionella bacteria thrive in specific conditions. Understanding these conditions is the foundation of effective Legionella control under ACOP L8 and HSG274. The three core principles of Legionella management are well established:
These principles are straightforward in theory. In practice, rising UK ambient temperatures are making them increasingly difficult to maintain, particularly for cold water systems.
Cold water storage tanks located in roof spaces, loft voids, or plant rooms are especially vulnerable. When external temperatures rise, the ambient temperature in these spaces can far exceed the safe threshold, pushing stored cold water above 20 degrees Celsius even with normal turnover and flushing in place.
This is not a theoretical risk. It is a scenario that water hygiene engineers are encountering with increasing frequency across the UK, particularly during the warmer months that are now becoming more common and more prolonged.
The Infrastructure Pressure Behind the Risk
Beyond individual building systems, the wider water supply infrastructure in the UK is also coming under strain. Reservoir pressures and long-term supply concerns are growing as demand increases and rainfall patterns become less predictable. When mains water temperature itself is elevated during warm periods, the incoming water that would normally help cool a system and draw down temperatures can arrive at temperatures already approaching or exceeding safe thresholds.
Water suppliers in the UK are typically required to supply water below 25 degrees Celsius. However, 25 degrees is still above the 20 degree threshold required for effective Legionella control. In a building where cold water temperatures are already struggling, warm incoming mains water compounds the problem rather than solving it.
What This Means for Your Legionella Management Programme
For building managers and duty holders, the implications of a warmer, more unpredictable climate are practical and pressing. A Legionella management programme that was designed for the temperature profiles of a decade ago may not be adequate for the conditions being experienced now.
The following areas deserve particular attention as UK temperatures continue to rise:
Cold Water Storage Tanks
Any cold water storage tank located in a space that is exposed to elevated ambient temperatures, including roof voids, loft spaces, and external plant rooms, should be assessed for its ability to maintain water below 20 degrees Celsius during warm periods. Insulation, location, and shade all play a role in whether tanks can maintain safe temperatures.
Calorifier and Hot Water System Performance
Hot water systems must store water at or above 60 degrees Celsius to control Legionella risk. In buildings with ageing or undersized plant, achieving and maintaining this temperature consistently can become more challenging as the baseline ambient temperature rises. Regular temperature profiling is essential to identify shortfalls before they become compliance failures.
Little-Used Outlets and Stagnation
Warmer temperatures accelerate the growth of Legionella bacteria in stagnant water. In buildings with seasonal fluctuations in occupancy, such as schools during holiday periods, hotels in low season, or office buildings with hybrid working patterns, little-used outlets represent a higher risk than they did in cooler conditions. Flushing regimes should be reviewed and, where necessary, increased in frequency.
Seasonal Variation in Risk
Legionella risk is not static throughout the year, and risk assessments that treat it as such are not reflecting the current reality. A building that maintains compliant temperatures easily in October may struggle significantly in July. Seasonal risk assessments and increased monitoring during warmer months are becoming increasingly important.
Climate Change and Legionella Risk: Key Factors at a Glance
| Risk Factor | Why It Matters | Recommended Action |
|---|---|---|
| Rising ambient temperatures | Cold water storage tanks may exceed 20 degrees Celsius | Review tank insulation, location, and temperature monitoring frequency |
| Elevated mains water temperature | Incoming water may not cool systems as expected | Increase monitoring; consider water sampling as a risk assessment tool |
| Erratic rainfall and reservoir pressure | Potential for reduced flow rates and increased stagnation risk | Review flushing regimes and dead leg management |
| Extended warm periods | Prolonged conditions favour Legionella growth | Seasonal risk reviews and increased summer monitoring |
| Increased building vacancy patterns | Hybrid working and seasonal closures increase stagnation risk | Update written scheme of control to reflect current occupancy patterns |
The Legal and Compliance Position
The duty holder’s obligations under ACOP L8 and the Health and Safety at Work Act 1974 do not change because the climate does. The legal requirement to assess and manage Legionella risk remains in place regardless of the environmental pressures a building faces.
What climate change does change is the context in which those obligations must be met. A Legionella risk assessment that accurately reflected risk five years ago may now be insufficient if it does not account for the temperature conditions that the building actually experiences during warmer months.
ACOP L8 requires that the written scheme of control is reviewed regularly and whenever there is reason to believe it may no longer be valid. Consistently elevated cold water temperatures during summer months are precisely the kind of change that should trigger a review.
Is Your Legionella Risk Assessment Up to Date?
If your written scheme of control has not been reviewed recently, or if you are experiencing temperature control challenges during warmer weather, our specialist team can help.
Get a Legionella Risk AssessmentHow Swiftclean Can Help
At Swiftclean, our water hygiene specialists work with duty holders across a wide range of sectors to ensure that Legionella management programmes are fit for purpose in current conditions. We are accredited members of the Legionella Control Association, and all of our risk assessments, written schemes of control, and ongoing monitoring programmes are carried out in line with ACOP L8 and HSG274.
As climate conditions continue to change, we are increasingly supporting clients with seasonal risk reviews, updated temperature monitoring programmes, and the targeted use of water sampling as a diagnostic and due diligence tool where temperature control parameters are being challenged by ambient conditions. Our engineers understand both the regulatory framework and the practical realities of managing water systems in buildings where the environment is changing around them.
Swiftclean water hygiene services include:
- Legionella risk assessments and written schemes of control
- Temperature monitoring and ongoing water hygiene management
- Water sampling as a targeted diagnostic and due diligence tool
- Cold water storage tank cleaning and disinfection
- Hot water system temperature profiling and remediation
- Nationwide coverage with locally based teams
Speak to Our Water Hygiene Team
Whether you need a new Legionella risk assessment, a review of your existing written scheme, or support with temperature control challenges, we are here to help.
Frequently Asked Questions About Legionella and Climate Change
How does warm weather increase Legionella risk?
Legionella bacteria multiply most rapidly in water temperatures between 20 and 45 degrees Celsius. As UK ambient temperatures rise, cold water systems in particular can struggle to stay below the 20 degree threshold, especially in tanks located in roof spaces or plant rooms exposed to outdoor heat. Warmer conditions also accelerate bacterial growth in any stagnant water present within the system.
What should I do if my cold water temperature exceeds 20 degrees Celsius?
The immediate response is to increase flushing to draw cooler mains water through the system. However, during periods of warm weather, incoming mains water may itself be at elevated temperatures, so flushing alone may not resolve the issue. Where temperatures remain above the safe threshold despite flushing, targeted water sampling can help assess whether bacterial growth is occurring and inform decisions about further remedial action such as disinfection or system modifications.
How often should a Legionella risk assessment be reviewed?
ACOP L8 requires that a Legionella risk assessment is reviewed regularly and whenever there is reason to believe it may no longer be valid. This includes significant changes to the building or its water systems, changes in occupancy patterns, and changes in environmental conditions that affect the risk profile of the building. A programme that was reviewed before recent warm summers may need updating to reflect the temperature conditions now being experienced.
Is Legionella water sampling a legal requirement?
Water sampling is not explicitly required by law in all circumstances, but it may be specified in your Legionella risk assessment or written scheme of control depending on your system type and risk level. Under ACOP L8 and HSG274, sampling can form part of a wider monitoring and management programme where appropriate. A qualified water hygiene specialist can advise whether sampling is necessary for your specific site and circumstances.
Which buildings are most at risk from climate-related Legionella challenges?
Any building with cold water storage tanks in exposed or unconditioned spaces is at increased risk as ambient temperatures rise. Healthcare facilities, care homes, hotels, schools, and larger commercial buildings with complex water systems are particularly vulnerable. Buildings with seasonal occupancy fluctuations, such as holiday accommodation or schools, face additional risk from stagnation during low-use periods, which is compounded by warmer temperatures during summer closures.
