The UKHSA’s official statistics for Legionellosis in residents of England and Wales for 2024 state the following:
“Legionellosis is a spectrum of diseases caused by Legionella bacteria. Illness can range from mild (non-pneumonic legionellosis or Pontiac fever) to LD, which is a form of atypical pneumonia that can be severe and potentially fatal.
Anyone can become infected by the bacteria, but certain population groups, such as those aged 50 years and over, smokers, and individuals with weakened immune systems, are more vulnerable.
LD is a seasonal disease, with activity in England and Wales increasing during the summer months, usually reaching a peak between July and September.”
In total, there were 472 LD cases reported in 2024, with 2.8% resulting in fatalities.
Of these cases, 52% were community-acquired, while 44% were related to travel outside the UK.
Reported case rates ranged from 0.5 to 1.3 persons per 100,000 of the population, depending on location.
The five-year average for confirmed LD cases (2020–2024) in England and Wales was 437.
By most measures, it is reasonable to suggest that LD is an occasional and relatively uncommon disease.
Is Legionella a risk in cold weather?
The incoming cold water supply is not typically conducive to bacterial growth (or at least very little).
During winter we also spend more time indoors, using hot and cold water systems frequently.
At first glance, the conditions may appear less favourable for bacterial growth and proliferation than during warmer periods.
For many domestic hot and cold water systems, Legionella risk management may also seem relatively straightforward.
Guidance has remained broadly consistent for several years, and many organisations will have a written scheme in place and established procedures for monitoring and maintenance.
Legionella risks can change faster than you think
It is easy to become complacent, and on the surface many aspects of Legionella risk management are not overly complicated.
However, domestic water systems, particularly older systems, can experience rapid changes in operating conditions.
In some cases, organisations may not be as vigilant as they believe, or may rely on staff who are not as competent as assumed.
A closer look at the statistics also reveals that the seasonal peak in LD cases has become noticeably flatter in recent years, including in 2024.
When overseas travel-related cases are removed, LD cases, and therefore overall risks, are distributed far more evenly throughout the year.
Legionella risk doesn’t hibernate in winter
During colder periods of the year, several factors can still create favourable conditions for Legionella growth:
- Cold outdoor temperatures can influence domestic water system operating conditions. While systems should avoid temperatures between 20–45°C, significant areas of circulation systems may still fall within this range.
- Certain sectors experience seasonal building closures. Industries such as education, hospitality and leisure may see periods of reduced occupancy during winter holidays. Reduced water usage and stagnation in stored water systems can create conditions that allow risks to develop, particularly when temperatures later rise.
- Poorly managed water systems create opportunities for bacterial growth at any time of year. Even well-maintained planned preventative maintenance (PPM) records are only indicators of potential conditions in which bacteria may grow.
- A history of clear Legionella test results should not create complacency. Previous compliance should never result in a relaxation of risk management measures.
- Older properties remain common. Many buildings still contain ageing water infrastructure where system vulnerabilities may only be partially visible.
- Post-pandemic occupancy patterns may still fluctuate. Although COVID-19 restrictions have eased significantly since 2021, some buildings may not yet have returned to consistent occupancy levels, reinforcing the need for ongoing vigilance in all seasons.
- Guidance consistently recommends routine monitoring and maintenance. Winter should never be treated as a reason to reduce statutory obligations.
- Human factors remain a risk. Even well-trained and competent staff can make mistakes.
Guidance exists for a reason
Maintaining interest and engagement in Legionella compliance can sometimes be challenging when the core guidance has remained largely unchanged for more than 25 years (aside from technological developments).
However, organisations must still be able to demonstrate that building occupants are as safe as possible. Key guidance from the HSE and Department of Health, including HSG274, ACoP L8 and HTM 04-01, should be followed rigorously.
Much of the existing guidance represents the minimum requirements necessary to ensure effective water safety management.
Winter is no excuse to relax Legionella controls
Overarching risk-minimisation measures during winter periods may include:
- Full and continued completion of relevant planned preventative maintenance tasks in line with guidance and the site’s Legionella written scheme of control or water safety plan.
- Reviewing and implementing flushing regimes for infrequently used outlets, ensuring they remain flexible and comprehensive enough to reflect day-to-day and seasonal risks. Pre- and post-closure measures may also be required.
- Continuing microbiological sampling where necessary, based on a documented sampling plan.
- Maintaining effective communication between staff and departments to minimise unforeseen occupancy issues.
- Avoiding reductions in hot water temperatures as a method of offsetting increased winter energy costs.
- Ensuring hot water temperatures are not compromised by competing heating demands within the building.
- Reviewing PPM schedules to ensure planned tasks can still be completed during periods of reduced staffing over winter holidays.
- Educating building occupants about simple actions that help keep domestic water systems clean and safe.
- Using periods of potentially lower occupancy during winter to complete outstanding Legionella risk assessments or implement additional risk-reduction measures.
Although there is no need to second-guess a robust risk management strategy, the value of ongoing review, water safety audits and lessons learned should never be underestimated.
Year-round Legionella management remains essential. Breaking the “chain of causation” helps minimise the risk of legal consequences, financial loss and reputational damage, making sustained vigilance a worthwhile and necessary pursuit.