Summer Legionella Risk: Why Legionnaire’s Disease Cases Spike Between June and October

As temperatures rise across the UK, so does the risk of Legionnaires’ disease. Data published by the UK Health Security Agency (UKHSA) shows a clear seasonal pattern, with reported cases consistently increasing during the summer and early autumn months. The charts included within the UKHSA’s latest epidemiological report clearly demonstrate that the highest number of Legionnaires’ disease cases typically occur between June and October, a trend that has remained evident across multiple years.

Charts from the UKHSA indicating the spike in Legionnaire’s Disease cases during summer months each year

For dutyholders, facilities managers, landlords, and property owners, this seasonal increase serves as an important reminder that water hygiene controls require particular attention during warmer weather.

Why does Summer Legionella risk increase?

Legionella bacteria occur naturally in aquatic environments such as rivers and ponds, but problems arise when conditions allow the bacteria to multiply within water systems across the built environment.

The bacteria thrive in temperatures between approximately 20°C and 45°C. During the summer months, rising ambient temperatures often create ideal conditions for growth within poorly managed water systems, particularly where water stagnates or temperature control is inadequate.

Common risk factors include:

• Low water turnover in buildings
• Infrequently used outlets
• Dead legs and redundant pipework
• Inadequate hot water temperatures
• Poor cold water storage conditions
• Scale, corrosion, and biofilm accumulation
• Cooling systems exposed to warmer weather

The UKHSA report notes that cases of Legionnaires’ disease follow a distinct seasonal pattern, with symptom onset peaking between June and October. Importantly, these peaks remain visible even when travel-associated cases are removed from the data, suggesting that exposures in our workplaces and places of leisure play a significant role.

Algae growing in a cistern

Warm weather creates ideal conditions

Many building owners assume that Legionella risk only applies to large evaporative cooling towers or complex industrial systems. In reality, a wide range of buildings can provide suitable conditions for bacterial growth.

Examples include:

• Offices
• Schools, colleges, and universities
• Healthcare premises
• Hotels and leisure facilities
• Residential blocks
• Care homes and sheltered housing schemes
• Industrial premises

Summer can affect water systems in several ways: cold water storage tanks may struggle to maintain temperatures below 20°C during prolonged hot weather if they have not been sufficiently insulated; pipework routed through roof voids and plant rooms can experience significant temperature increases as heat is transferred from other sources; and seasonal occupancy fluctuations may also lead to reduced water usage and increased stagnation.

Many organisations experience lower building occupancy during the summer months as staff take annual leave. Reduced demand can leave sections of systems underused for extended periods, creating favourable conditions for bacterial growth.

Who faces the greatest risk?

Legionnaires’ disease can affect anyone, but certain groups face a significantly higher risk of severe illness.

According to the UKHSA report, most reported cases occur in older adults, particularly those aged 60 and over. The data also shows that males are disproportionately affected, accounting for approximately 70% of cases. Smoking remains one of the most common synergistic risk factors identified amongst confirmed cases.

Other vulnerable groups include:

• People with chronic respiratory disease
• Individuals with weakened immune systems
• Those with kidney or liver disease
• People with diabetes
• Elderly residents in care settings

For organisations that cater to these populations, robust water hygiene management becomes even more critical during periods of increased seasonal risk.

What can Dutyholders do to reduce Summer Legionella risk?

The good news is that Legionella risk remains highly manageable when organisations implement effective control measures.

The first step involves understanding the risks present within each property through a suitable Legionella Risk Assessment (LRA) carried out by a competent person. However, risk assessment alone does not control Legionella. Ongoing monitoring, maintenance, and management remain essential, as well as actioning the recommendations made by the LRA.

Taking water temperature measurements as part of a Legionella Risk Assessment

Key control measures include:

Maintaining temperature control

Hot water should reach and maintain temperatures capable of mitigating bacterial growth, while cold water should remain below 20°C wherever practicable.

Preventing stagnation

Regular flushing of infrequently used outlets helps prevent water from remaining static within pipework.

Removing redundant services

Unused pipework, dead legs, and abandoned systems can create ideal environments for bacterial growth and should be removed wherever possible.

Monitoring and inspection

Routine temperature monitoring, inspections of storage tanks, and system reviews help identify issues before they become significant risks.

Cleaning and maintenance

Storage tanks, calorifiers, shower heads, and other system components require periodic inspection and maintenance to prevent the accumulation of scale, sediment, and biofilm.

The importance of proactive water hygiene management

The UKHSA recorded 604 confirmed cases of Legionnaires’ disease in England and Wales during 2023, the highest annual figure reported since before the COVID-19 pandemic. Most exposures occurred within community settings rather than healthcare environments, demonstrating that Legionella risk extends across a wide range of properties and sectors.

While summer weather does not automatically create Legionella problems, it can magnify weaknesses within poorly managed water systems.

The seasonal peaks shown within the UKHSA data provide a valuable reminder that water hygiene and compliance should never become a “set and forget” exercise. Organisations that review their portfolios proactively, maintain robust monitoring programmes, and address deficiencies before problems develop place themselves in the strongest position to protect occupants and demonstrate Legionella compliance. It is also important to remember that Legionnaire’s Disease, if caused by exposure in the workplace, is reportable to the Health and Safety Executive under the Reportable Injuries, Diseases, and Dangerous Occurrences Regulations (RIDDOR).

How Casa Environmental Services can help

Whether you manage a single property or a nationwide portfolio, understanding and controlling Summer Legionella risk requires more than a one-off assessment.

Casa Environmental Services supports clients with:

• Legionella Risk Assessments
• Routine monitoring programmes
• Temperature monitoring
• Water hygiene system audits
• Remedial actions
• Portfolio-wide compliance management

As the summer months bring increased Legionella risk, now is the ideal time to review your water systems and ensure your control measures remain effective. Proactive management today can help prevent costly problems tomorrow. Get in touch with our team today.