Understanding Legionella Risk: Why Every Building Needs a Risk Assessment

Legionella bacteria remains one of the most significant yet underestimated biological hazards in the built environment. For building owners, facilities managers, and health and safety professionals, understanding the fundamentals of legionella risk is not optional -- it is a legal obligation.

This guide covers the essentials: what legionella is, how it grows in water systems, who is most vulnerable, and why a suitable and sufficient risk assessment forms the cornerstone of any effective control strategy.

What Is Legionella?

Legionella bacteria acquired its name following an outbreak of pneumonia at a convention in Philadelphia in July 1976. Attendees fell ill, and tragically, several people died -- including a taxi driver who was simply driving past the building. The cause was traced to contaminated water aerosols emitted from the building's cooling towers, which were drawn into the air handling system and distributed throughout the premises.

On 18 January 1977, the causative agent was identified and named Legionella, with the species responsible for the outbreak designated Legionella pneumophila.

Legionellosis -- the disease caused by legionella bacteria -- occurs when fine contaminated water droplets (aerosols) are inhaled. These aerosols can originate from numerous sources, including:

- Cooling towers
- Showers and shower heads
- Air conditioning units
- Spa baths and hot tubs
- Any system that creates a fine mist or spray from stored water

How Legionella Grows

Legionella bacteria is naturally present in water supplies. It does not need to be introduced -- it simply requires the right conditions to multiply to dangerous levels. The key growth requirements are:

- Temperature: The critical growth range is 20 to 45 degrees Celsius. Below 20 degrees, the bacteria becomes dormant. Above 60 degrees, it cannot survive.
- Nutrients: Rust, iron, algae, scale, sludge, and organic particulates all provide food for the bacteria.
- Stagnation: Water that is not regularly circulated or turned over provides an ideal breeding ground.

This is why building water systems -- particularly those with stored water, dead legs, or infrequently used outlets -- are prime environments for legionella proliferation.

Who Is Most at Risk?

While anyone can contract legionnaires' disease, certain groups are significantly more vulnerable:

- People over the age of 45
- Smokers and heavy drinkers
- Those suffering from chronic respiratory or kidney disease
- People with diabetes, lung disease, or heart disease
- Anyone with an impaired immune system

The symptoms closely resemble pneumonia: high fever, chills, headaches, severe muscular aches, cough, breathlessness, diarrhoea, vomiting, and confusion or delirium. However, legionnaires' disease requires different treatment from standard pneumonia, making accurate diagnosis critical.

When completing a risk assessment, the presence of vulnerable individuals in a building must be factored into the overall risk evaluation.

The Legal Framework

Under UK health and safety legislation, employers and those in control of premises have a duty to assess and control the risk of legionella exposure. The key regulatory documents are:

- ACOP L8 (Approved Code of Practice): The legal framework for legionella control, first published in 2001.
- HSG274 (Parts 1-3): Technical guidance on controlling legionella in various water systems, published in 2013.

If you employ five or more people, findings from the risk assessment must be recorded. However, even smaller organisations are not exempt from the duty to assess and control the risk.

Enforcement is active. The Health and Safety Executive (HSE) investigates breaches, and improvement notices are regularly issued to organisations -- including universities and colleges -- that fail to maintain adequate legionella controls, particularly following periods of reduced building usage.

Why the Risk Assessment Matters

A legionella risk assessment is the foundation of your entire control strategy. Without it, you are operating blind. The assessment:

1. Identifies all water systems in the building and how they operate.
2. Evaluates the risk of legionella proliferation in each system.
3. Identifies vulnerable people who may be exposed.
4. Recommends control measures -- both ongoing maintenance tasks and remedial works.
5. Establishes a management structure with clear roles and responsibilities.
6. Provides a baseline against which future reviews can be measured.

The assessment must be carried out by a competent person. For simple systems -- such as a mains-fed office with a combi boiler and basic sanitary facilities -- an in-house assessment may be appropriate, provided the assessor has adequate training and understanding. For more complex systems involving stored water, cooling towers, showers, or any aerosol-generating processes, engaging a specialist water hygiene company is strongly recommended.

The Duty Holder and Responsible Persons

A critical element of legionella compliance is establishing a clear management structure:

- The Duty Holder is the employer, person in control of premises, or a self-employed individual who creates risk to others.
- The Duty Holder must appoint a Nominated Responsible Person (and deputies) in writing.
- The Nominated Responsible Person must accept the role in writing.
- The Responsible Person must understand their roles and responsibilities and be suitably trained.

Having deputies is essential -- if the primary Responsible Person is on leave or absent, someone must be available to maintain the control measures. A clear chain of communication between the Duty Holder, the Responsible Person, and deputies ensures nothing falls through the gaps.

When to Review the Risk Assessment

A legionella risk assessment is not a one-off document. It must be reviewed if:

- A case of legionnaires' disease is associated with the premises.
- There are changes to the water supply (e.g., switching from mains water to a borehole or private supply).
- The use of the building changes (e.g., converting a production facility to warehousing, which reduces water turnover).
- New information about risks or control measures becomes available (e.g., updated guidance or regulations).
- Evidence shows that existing control measures are no longer effective (e.g., persistent temperature failures).

HSG274 removed the previous mandatory two-year review cycle, but many experienced practitioners still recommend reviewing every two years as best practice. Regular reviews keep the assessment current, keep staff engaged with their responsibilities, and ensure evolving risks are captured.

Key Takeaway

Legionella control is not an area where complacency is acceptable. The bacteria is naturally present in water -- it does not need to be introduced. Given the right conditions of temperature, nutrients, and stagnation, it will proliferate. The consequences of exposure range from serious illness to death.

A suitable and sufficient risk assessment is the starting point. It identifies your risks, establishes your control measures, and creates the management framework to keep your building and its occupants safe.

If you do not currently have a legionella risk assessment, or if your existing one has not been reviewed following changes to your building or water systems, now is the time to act.