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Knowledge Base » Health and Safety » Legionella Risk Assessment

Legionella Risk Assessment

Last updated on 17th April 2023

Legionella is a bacterium found in some water sources that can lead to people becoming unwell. The likelihood of legionella being present in water systems within households, commercial premises and other public places can be assessed using a legionella risk assessment.

What is a risk assessment?

According to the Health and Safety Executive (HSE), a risk assessment is a methodology used to:

  • Identify risks/hazards within an environment.
  • Identify ways to control or eliminate these risks/hazards.
  • Record the findings of the risk assessment.
  • Evaluate the risk assessment’s findings and make improvements.

Risk assessments are done routinely within workplaces, public spaces and commercial and private properties to identify hazards and protect the health and safety of individuals and the general public.

Common types of risk assessment include fire risk assessments and health and safety risk assessments. Some risk assessments are more general and sometimes they are site-specific, for example when building work is being undertaken at a school and it is a ‘live site’ there will be specific considerations to make versus a standard building site where only authorised employees will be present.

Some risk assessments will relate to changes within the workforce, for example if a staff member has become pregnant then they will need to be assessed in case any changes are required to help them to do their job safely.

A legionella risk assessment is performed to assess the likelihood of legionella (which can lead to a type of pneumonia) being present within an area. This is necessary in some circumstances to protect public safety and is sometimes also a legal requirement.

What is legionella?

Legionella is a harmful bacterium that thrives in water under certain conditions.

It can lead to a type of lung infection called Legionnaires’ disease if inhaled through tiny water droplets. Legionnaires’ disease is a serious form of pneumonia that causes inflammation of the lungs and is characterised by a cough and flu-like symptoms.

This bacterium is common in natural water sources such as lakes and reservoirs, however, not usually in large enough numbers to cause a problem or outbreak.

Legionella typically thrives in temperatures between 20 and 45 degrees centigrade, so it is more commonly found in purpose built, man-made water systems.

A woman suffering from legionnaires disease

What are the hazards?

Legionella, which causes Legionnaires’ disease, is potentially very dangerous. It is more likely to be found in areas with a lot of water usage, or where water may stand for some time.

High-risk public places include:

  • Spas.
  • Hotels.
  • Hospitals.
  • Leisure centres.

Although the risk in households is usually relatively low, landlords have a duty of care to their tenants to undertake a legionella risk assessment. This includes landlords who rent commercial properties to businesses as well as those who rent to private tenants.

There is a risk of developing a type of pneumonia known as Legionnaires’ disease after being exposed to legionella.

Symptoms of Legionnaires’ disease include:

  • Cough.
  • Temperature/fever.
  • Shortness of breath.
  • Nausea.
  • Confusion.
  • Muscle aches and headaches.

Medical staff will usually diagnose the presence of the disease by performing tests and conducting a chest X-ray. Most people experience symptoms within 2 to 10 days of being exposed to legionella (although it can take up to two weeks).

If they suspect Legionnaires’ disease, medical staff will want to know if you have spent time in hospital or away from home (particularly abroad) or used a hot tub in the previous two weeks.

On average there are 200-250 confirmed cases of Legionnaires’ disease in England and Wales each year, although only around half of these are thought to be cases of domestic outbreaks and the rest are associated with foreign travel. It is, however, believed that cases are under-reported and that the true number is actually far higher.

A milder infection, known as Pontiac fever, is also possible. This is not as severe as Legionnaires’ disease as it does not lead to pneumonia. Symptoms will often present themselves within a few hours to 3 days of exposure to the bacterium and will normally last for less than a week.

Legionella being examined

What factors should you consider when risk assessing legionella?

Combi boilers, which are common in modern homes, pose a low risk of containing legionella bacteria. These systems keep the water moving which means the bacteria don’t have enough time to grow and multiply. It is more common in open water tanks, often found in old buildings particularly in rural areas.

Places or activities that pose a risk for legionella include:

  • Places where water is left to stagnate (especially if exposed to rust, sludge, organic matters and ‘biofilm’ which helps the bacteria to multiply).
  • Abandoned properties, or those which are left unused for a significant amount of time.
  • Areas where a large amount of water is used, such as in hospitals, hotels or spas.

Particular high-risk areas include:

  • Shower heads.
  • Whirlpool baths.
  • Hot tubs.
  • Birthing pools.
  • Complex, large-scale water systems.
  • Hot water systems.
  • Decorative water features.
  • Old, redundant pipework.

It is far less common to contract the disease from drinking water that contains legionella bacterium or from an infected person.

Some people will be at an increased risk of becoming unwell after being exposed to legionella bacteria and may develop severe complications as a result.

These include:

  • People aged 50 years or over.
  • Those with chronic lung conditions, suppressed immune systems or cancer.
  • Heavy smokers.
  • People with underlying health conditions such as kidney or liver failure.

Most people will make a full recovery from Legionnaires’ disease and it is often treated with antibiotics; however, some people may require oxygen or intubation if they are experiencing severe breathing difficulties, and will be required to spend time in hospital.

A woman being treated for legionnaires disease

Who should carry out their risk assessment?

Legionella risk assessments should always be carried out by a person who has relevant knowledge of the hazards that legionella bacteria can pose. In hotels, hospitals or leisure facilities there will often be on-site maintenance personnel who have been on courses and gained relevant experience in performing different kinds of risk assessments.

Trained professionals may also be called in who can offer expert advice on the risk of legionella and they will know exactly what to look for. This is especially useful in buildings which have complex water systems or those that have had building work done where bits may have been ‘added on’, as well as in large commercial premises.

Landlords are legally required to perform a risk assessment for legionella, but they do not have to present their tenant with a certificate relating to this or obtain the services of a professional.

Although some landlords might feel that they are competent to assess the risk of legionella in their properties, it is often advisable to request expert advice and there might be technicalities involved that are beyond your own experience. Simply put, although the risk posed in most houses or flats is small, there is a legal obligation to protect your tenant’s health.

By requesting the help of an expert who is trained in inspecting water systems and identifying pathogens, as a landlord you will have an official record of any risk assessment that has been done and the peace of mind that you have paid for a professional job.

Any plumbing work should always be undertaken by a trained professional. It is unwise to cut corners, particularly when you are renting out commercial property, so it is important to use a reputable company.

Because legionella will thrive in areas where water can stagnate, if additional pipework is added or parts of a bathroom suite moved, the old pipework should be removed rather than just being capped off and left there as a ‘dead leg’. This removes the risk of old water being left to languish, as well as rust and bacteria growing in the redundant pipes.

With most systems that are being considered during a legionella risk assessment, there will be some kind of supply of mains water, which is covered under legislation including the Water Supply (Water Fitting) Regulations 1999 and the Water Supply (Water Quality) Regulations 2001.

When performing a risk assessment, in addition to understanding the relevant legislation, it is necessary to consider:

  • The source of the water and any associated risks of legionella.
  • Is there any potential for bacteria or microorganisms to grow?
  • Could aerosols be released? (i.e. droplets small enough to be inhaled).
  • Are there any members of the public or tenants that are likely to be adversely affected and do they have underlying reasons that they could be more at risk?

If a potential threat of legionella is found then it will be necessary to undertake work that controls or removes the risks immediately.

What happens after the risk assessment?

In most residential properties the risk of legionella will be low.

Once a landlord has conducted their risk assessment and not found any incidents of legionella risk, no further action is immediately required. If a risk is identified, remedial work will be required.

An open line of communication should always be encouraged between landlords and tenants and landlords should always ensure they have provided up-to-date contact details. This means that tenants can report any problems or concerns that they may have and gives the landlord the opportunity to investigate and fix any issues that may arise before they pose any kind of risk.

It is also important that tenants know how they can reduce risks of legionella themselves, for example by regularly cleaning and disinfecting showerheads and making sure that units are used regularly, or at least the water is run through them regularly.

In commercial properties such as hotels or spas, regular inspections are necessary to ensure compliance and so as not to cause a risk to public health.

It is possible to contact an expert who can conduct a legionella risk assessment and draw up a plan of action to mitigate any risks. It is also important to keep up to date with minimising the risks associated with legionella, even after you have completed a risk assessment.

If a property is to be left empty for a period of time, you can mitigate the risks by entering it every few weeks and switching on the shower at full heat and allowing the water to run for a period of time. It is also wise to employ a competent contractor to ‘flush out’ the system prior to a new tenant moving in. This will help to remove any build-up of sludge, debris or old water from within a heating system and it also helps the system to work more economically.

If a new boiler is installed, for example, or other changes made to the hot and cold water systems in a property, a new legionella risk assessment should be performed. Electric showers and combi boilers pose a low risk as they do not work by storing water, so where possible, it is wise to install these in your properties.

Risk assessment for legionella


Legionella is a harmful bacterium that can lead to people becoming extremely unwell and even requiring hospital treatment. It is found in water systems, particularly ones in which water is left to stagnate or where tiny aerosols are released that can be breathed in.

Landlords are required to undertake a risk assessment for legionella in their properties, either themselves or by employing the services of an expert. They must assess the risk of legionella being present and the likelihood of it posing a threat to health.

Other high-risk places such as hospitals, hotels and spas have their own duty of care to ensure they are not putting public health at risk and are minimising the chance of an outbreak of Legionnaires’ disease caused by legionella bacterium.

This might include testing the water systems for the presence of bacteria as well as conducting temperature checks which assess that the hot water systems are under control.

A record should be kept of each risk assessment that is done and any risks that are identified or remedial work that is done as a result of the original risk assessment for legionella.

Wherever possible, the risks of creating a breeding ground for legionella should be mitigated, or removed entirely, in order to comply with HSE legislation and to protect public health, in particular the health of vulnerable people.

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About the author

Vicky Miller

Vicky Miller

Vicky has a BA Hons Degree in Professional Writing. She has spent several years creating B2B content and writing informative articles and online guides for clients within the fields of sustainability, corporate social responsibility, recruitment, education and training. Outside of work she enjoys yoga, world cinema and listening to fiction podcasts.

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