Controlling Legionella


The Legionella Control Association (LCA) discusses the control of the disease in housing, as well as its risks, and emerging trends.

Legionnaires’ disease (LD) is a potentially fatal type of pneumonia, a bacterial lung infection. LD is contracted from bacteria of the genus Legionella, within a water-based aerosol, entering the lungs.  Exposure is generally preventable if water systems are properly maintained and there are legal implications where there are failures to control this risk.

Legionella is naturally occurring and a potentially fatal pathogen with over sixty species. Guidance from the HSE does not make any distinction based on species and any Legionella species is considered a risk.

The risk associated with Legionella and human infection was first established in 1976, following an outbreak of disease at an American Legion convention. This led to over 150 cases of which over 15% were fatal.

Data suggests there are over 250 confirmed cases of LD in the UK annually.

Fundamental principles that reduce the risk of Legionella growth

  • Avoid growth temperatures (20-45oC)
  • Avoid stagnation so there is no time to grow
  • Avoid materials that support growth
  • Control spray to minimise transmission risk
  • Keep the system and the water clean, to minimise nutrients
  • Use water treatment techniques, particularly where the above cannot be controlled
  • Maintain the system properly

These principles form the basis of the requirements of the HSEs Approved Code of Practice for the control of Legionella in water systems and underpin all the UK guidance for controlling Legionella.

In practice, some of these can be a challenge. When the conditions for growth are right, then Legionella can grow within pipework. Once present, legionella can be a challenge to remove from a system.

Legionella & the Law

The Law applicable to Legionella is the Health and Safety at Work Act (1974).

Legionella bacteria is a hazardous substance under COSHH Regulations. Risks must be assessed, identified risks must be eliminated, substituted for lower risks or where this is not possible control measures must be used.

There is an Approved Code of Practice for Legionella (ACoP L8) and technical guidance in HSG274 published by HSE ACoP L8 and the HSE guidance outline how to comply with the law and keep your activities safe. They highlight the legal requirement for risk assessment; to assess the risk posed by your undertaking that may allow Legionella to proliferate. Where risks are identified, you are also required to have a written scheme of control to manage the identified risk. This is commonly achieved through maintaining control of water temperature, use of water treatment, regular cleaning of commonly fouled components, etc. together with regular monitoring and inspection to ensure the regime is effective.

In Healthcare settings, such as hospitals, doctors’ surgeries, dentists’ premises and nursing homes, the Health and Social Care Act and associated regulations will apply.  There is also specific guidance from the Department of Health.

Legionella Control Association (LCA)

The LCA Code of Conduct is highlighted in L8 as guidance to work standards an end user should expect. A voluntary membership organisation with c.400 Legionella specialist Member companies.

The LCA Code has 27 requirements with Service Delivery Standards for Legionella control services. LCA Members must have procedures in place to deliver a consistent and compliant service; these, and evidence, are audited by the LCA. Where a member’s performance falls short, we will suspend and eventually terminate the member.

Legionella Transmission Pathway

Legionella can be present at low level in mains water. You cannot contract LD from direct contact with the water or by drinking water containing Legionella.  LD is not transmissible from person to person – the route of infection is through inhalation of an aerosol containing sufficient Legionella bacteria to cause disease.

The risk of acquiring LD arises when contaminated water droplets become aerosolised.  This could be from showers, aerosol created as flow from taps hits the sink, garden sprinklers, kitchen sprays, etc.

While Legionella can infect anyone, some are more susceptible than others. Anyone who has existing lung disease is especially vulnerable and high-risk individuals following COVID-19 with c. 20% of secondary infections are identified as LD in one study.

Risk Assessment

The law is quite clear here. Where the Health and Safety at Work Act applies there must be a Legionella Risk Assessment.

Control Measures

Any risk needs to be controlled to an acceptable level. The HSE uses the term as low as reasonably practicable (ALARP), and this is the target level of risk for any system.   Legionella control measures usually include maintaining temperature, use of water treatment, flushing and physical control.

Sampling & Analysis

Sampling should be driven by risk assessment; where high-risk areas are identified sampling might be recommended.  Where control measures are lost, reactive sampling for legionella weekly should be carried out until control is re-established.

Positive Legionella Results

Around 13% of all samples in the UK return a positive so it’s not uncommon to have positive Legionella results.

Act on a result to address the immediate risk (the symptom) and the underlying issues (the cause).  Cleaning & disinfection is commonly used as an immediate risk reduction measure, but a review of the risk assessment and identification root cause analysis is critical.


LD is potentially fatal but preventable.  Employers and those in control of premises have a legal duty to ensure risks are identified and controlled.  Legionella prosecutions result in large fines and custodial sentences, and it is not necessary for there to be a death or case of disease for a successful prosecution.  If your water system exposes stakeholders to risk of harm, you are committing an offence.

The LCA is a voluntary organisation whose membership comprises providers of services and products concerned with the control of legionella bacteria in water systems