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Legionnaires’ disease is a severe form of pneumonia, an infection that causes inflammation of the small air sacs in the lungs (alveoli) and their associated tissues. It is caused by people inhaling the Legionella bacterium. 

Legionella bacterium also causes Pontiac fever, a milder illness resembling the flu. While Pontiac fever usually clears by itself, untreated Legionnaires’ disease can be fatal—even people who have been promptly treated can continue to have problems.

Why is it called Legionnaires’ disease?

On July 21, 1976, more than 4,000 from the American Legion travelled to a hotel in Philadelphia for their annual convention. While it was supposed to be recognised as the 200th anniversary of the Declaration of Independence, history would remember it for being one of the most serious U.S. medical tragedies of the past century.

The first to die after the convention was sixty-one-year-old Ray Brennan, a retired U.S. Air Force Captain who was first thought to have died from a heart attack. It took 33 more deaths and 221 sick Legionnaires before doctors realised the common link. The disease was named Legionnaires’ disease when it was finally identified. 

Later, in 1977, the agent that caused the disease was identified as a previously unknown strain of bacteria, subsequently named Legionella. And the species that caused the outbreak was called Legionella pneumophila.

Medically, the illness is named Legionellosis, but it’s commonly known as Legionnaires’ Disease.

What causes Legionnaires’ disease?

Legionnaires’ disease is caused by Legionella bacteria. Bacteria are microscopic, single-celled organisms that thrive in various environments, including in the soil, water, and even inside human beings. Bacteria can be beneficial, neutral, or harmful. 

Probiotics, for example, are live bacteria and yeasts that are good for you, especially your digestive system. There is more than ten times the number of bacteria cells compared to human cells in our bodies—we are more bacteria than human. Bacteria are also useful for many other processes including manufacturing antibiotics and pharmaceuticals, making sourdough bread, and manufacturing chemicals.

However, bacteria can also be harmful to our health by causing diseases. They are one of four types of germs—the others being viruses, fungi, and protozoa. 

Legionella bacteria live in surface and drinking water and normally pass to humans through aerosols. As they can grow in hostile environments, Legionella species are sometimes ironically referred to as fastidious bacteria. The primary nutrient for the bacteria to grow is called L-cysteine, and ferric iron is also necessary for optimal growth.

Where do Legionella bacteria grow best?

Legionella bacteria live in natural water sources such as lakes, reservoirs, rivers, and streams. However, they become a real cause for human concern when they grow in man-made water systems such as: 

  • Hot and cold water systems

  • Hot tubs

  • Large, complex plumbing systems

  • Cooling towers

  • Spas

  • Showerheads and sink faucets

  • Decorative fountains and water features

Legionella bacteria grow when water temperatures are between 20–45°C, where nutrients, such as rust, sludge, scale, sediment and algae are available, and where water is stagnant in the water system (e.g., if a shower or tap is not regularly used).

How do you catch Legionnaires’ disease?

You can catch Legionnaires' disease from water that is contaminated with Legionella bacteria. If Legionella bacteria grow in water systems, it can spread through droplets small enough for people to inhale. Contaminated water droplets can be created wherever water is warm and a spray is produced. People catch Legionnaires’ disease or Pontiac fever when the water droplets make their way via aspiration into the lungs, which then become infected.

People can also contract the disease when water enters the lungs (aspiration) if they drink water containing Legionella bacteria. Those who have swallowing difficulties are at increased risk of aspiration.

While you can catch Legionnaires’ disease from any body of water (given the right conditions), artificial environments can contain the perfect conditions for the Legionella bacteria to multiply and may produce the water droplets that carry the bacterium. However, it is rare to find the bacteria in homes that are lived in where water turnover is generally frequent. Ponds, lakes, and rivers are also unlikely sources of infection. 

The bacteria is most commonly found in hotels, care homes, hospitals, gyms, and factories. Anywhere that has an industrial water system should frequently test for Legionella bacteria.

Is Legionnaires’ disease contagious?

Because you have to inhale water droplets to catch Legionnaires’ disease, it is generally not contagious, and it is usually safe to be around people with the disease. However, it may be spread via human transmission under rare circumstances.

What are the symptoms of Legionnaires’ disease?

The initial symptoms of the disease usually show themselves around two to ten days after infection. Unfortunately, there are no typical or unique symptoms associated with the disease, and not everyone exposed to the bacteria goes on to develop symptoms. In general, early symptoms are similar to flu and may include:

  • High temperature, feverishness, and chills

  • Dry cough

  • Muscle aches and pains

  • Loss of strength (asthenia)

  • Headaches

If untreated, the infection may become more severe within the first week, and infected people may experience further symptoms such as coughing up green phlegm and becoming short of breath. They may also have chest pain when breathing.

Some people who have Legionnaires’ disease suffer from nausea, vomiting, diarrhoea and abdominal pain. Almost half of the patients suffer from disorders related to the nervous system, such as confusion, delirium, depression, disorientation, and hallucinations.

If you think you might have been exposed to Legionella bacteria or developed symptoms such as fever, cough, chills, or muscle aches, it is important to see a doctor as soon as possible. Telling a medical professional where you have been recently is also important as you might have been exposed to a known outbreak. 

How serious is Legionnaires’ disease?

It is a severe and potentially fatal disease and is especially dangerous to groups at greater risk.

There are several risk factors that make you more susceptible to the disease; the most serious include being a smoker or having chronic lung disease. People with organ transplants are also at higher risk as well as anyone taking corticosteroid medicines. Men are around three times more likely to contract the disease than women.

At-risk groups include:

  • People suffering from chronic respiratory or kidney disease

  • People who suffer from diabetes or heart disease

  • People with chronic lung disease (like chronic obstructive pulmonary disease or emphysema)

  • People with underlying illnesses such as diabetes, kidney failure, or liver failure

  • People with cancer

  • Anyone with a compromised immune system

  • People over 50 

  • Anyone who smokes or is a heavy drinker

It is rare for babies or children to catch the disease.

How is Legionnaires’ disease diagnosed and treated?

People suffering from Legionnaires’ disease have a severe lung infection (pneumonia). Doctors typically test for this condition using a chest x-ray, and they use either a urine test or phlegm sample to discover if the pneumonia is caused by Legionella.

If caught in the early stages and treated, a healthy patient should make a full recovery. Patients with existing health issues that affect the respiratory system or compromise the immune system may suffer prolonged hospitalisation, complications or, in the worst cases, death. 


Like other forms of pneumonia, treatment generally consists of a course of antibiotics that are usually given immediately, without waiting for test results. However, this may not be effective, in which case alternative antibiotics may be prescribed. In severe cases, patients may be admitted to a hospital and given antibiotics through a drip.


Most patients should respond to treatment within three to five days. The majority of people with mild or moderately severe Legionnaires’ disease make a full recovery, although the length of that recovery may vary depending on how severe the condition is, whether the patient has any risk factors, how fit a patient is, how quickly treatment was commenced, and whether any complications develop. Recovery can take as long as six months.

Are there any long term effects of Legionnaires’ disease?

In severe cases, Legionnaires’ disease can be fatal. Serious complications can include respiratory failure, shock, and acute renal and multi-organ failure.

Although most patients will recover without ill effects after treatment, some may suffer from long term issues such as persistent pulmonary scars and restrictive pulmonary disease in patients who experience severe respiratory failure. There are often general secondary symptoms in serious cases, such as weakness, poor memory, and fatigue, which may last for several months.

Other neurological issues that can follow from severe infection include residual cerebellar dysfunction, retrograde amnesia, and other disorders of the nervous system.

According to the Health and Safety Executive, there are approximately 200–250 confirmed cases of Legionnaires' disease each year in England and Wales. Still, this figure is likely to be an underestimation. In the United States, the figure is close to 6000. About 1 out of 10 people who get Legionnaires’ disease die from the infection.

Because water droplets are the means of infection, and they are potentially generated by many industrial processes and commercial equipment used every day, it is impossible to completely eradicate Legionella bacteria. However, many people catch the disease in places with an extensive network of pipes connected to a water system, such as in care homes or hotels. Studies have shown that many cases of the disease in these settings could have been prevented.  

The best way to prevent Legionnaires’ disease is to kill or inhibit Legionella bacteria through good water management—specifically by controlling the temperature in a building’s water systems.

Legionella control and prevention

Effective Legionella control and prevention strategies aim to reduce the chances of Legionella bacteria spreading and infecting individuals. Some key aspects of Legionella control and prevention include:

Controlling legionella bacteria through temperature

Legionella bacteria are alive—microscopic, single-celled organisms that thrive in various environments, including in the soil, water, and even inside human beings.

As living things, they need the right conditions to survive and grow. However, several methods create a hostile environment for the bacteria in hot and cold water systems. Either killing them or stopping them from multiplying will reduce the threat of the disease.

One of the critical ways to eliminate Legionella bacteria is to control the water temperature. Like most living things, Legionella will not tolerate extremes of temperature. The optimum temperature range for Legionella to multiply is between 20 – 45°C (77 – 108°F), the most favourable temperature being 37°C—normal human body temperature.

To kill the bacteria in hot water systems, the water temperature needs to be above 50°C as the bacteria begin to die at this temperature. If the water is maintained at 50°C for two hours, 90% of the bacteria will die; however, if the water temperature is raised to 60°C, the same percentage of bacteria will die in two minutes. Raise that temperature to 70°C, and all the Legionella bacteria present will die straight away.

At the other end of the scale, in cold water systems, keeping the water temperature below 20°C will stop the bacteria from multiplying, but it won’t kill them. Legionella bacteria enter a low metabolic state at these temperatures, which keeps them alive but inhibits growth.

If the water in a system is anywhere between 20 and 45°C, you will have the ideal conditions for Legionella bacteria to multiply.

Although there are several approaches to eliminating the Legionella bacteria, controlling the water temperature is a more reliable and often-used method.

Here’s a quick summary of the effect of temperature on Legionella:

  • 20°C or less (60°F):  Legionella survives but in a dormant state

  • 20°C to 45°C (60°F – 113°F): Legionella will grow rapidly

  • 50°C (122°F): Most Legionella bacteria will die within hours

  • 60°C (140°F): Most Legionella will die within minutes

  • 70°C (158°F): All Legionella bacteria will die instantly

Controlling legionella bacteria using alternative methods to temperature control

Although controlling temperature in water systems is the most common way of reducing Legionella bacteria growth, other methods are also used. Some are best considered when the building is being designed, including pipe sizing and materials used. But others can be applied to existing buildings, including disinfectants and distal devices. Control of Legionella bacteria not only implies limiting their growth, but it can also mean reducing the opportunities for humans to be exposed to aerosols.

Here are some of the most common methods used to reduce Legionella bacteria growth:


Disinfection should be an integral part of a building's water management plan. There are several types of disinfectants to choose from, and they should always be used in combination with a water test to ensure that the system maintains a residual. Your choice of disinfection depends on multiple factors, including cost, water chemistry, corrosion, water use patterns, system configuration, and operator training.

  • Chlorine is a commonly used disinfectant. During treatment, chlorine can be added to water as elemental chlorine, sodium hypochlorite solution, or dry calcium hypochlorite. 

  • Chlorine Dioxide is generally generated on-site to be used by slowly adding a strong acid to a sodium chloride solution.

  • Monochloramine has become popular as the disinfectant is more stable in the distribution system, it minimizes the formation of disinfection byproducts, and it can penetrate biofilms better than free chlorine. 

Ultraviolet Irradiation

Ultraviolet light damages microorganisms, preventing them from replicating and becoming infectious. However, because UV does not provide a residual, it only works at the point being treated and should be combined with a chemical disinfectant for distributed water to control Legionella bacteria.

Copper-Silver Ionization

Using copper-silver ionization in building water systems is a low-cost and low-maintenance way to control Legionella. The copper ions interact with Legionella bacteria’s cell walls, which affects their ability to uptake nutrients and grow. The silver ions then prevent the cell from dividing and growing. 


Flushing of water can help improve water quality and reduce Legionella levels by dislodging loose deposits and biofilm that hold Legionella bacteria. We recommend manual flushing, which is particularly important if buildings are left vacant for more than a few days. Many people began to realise the importance of flushing during the coronavirus pandemic when lockdown buildings were told to flush to prevent water quality issues. There is no consensus on flushing frequency—some guidance recommends weekly flushing and other more frequent.

What can I do at home to avoid Legionnaires’ disease?

Although infections in people’s homes are rare, here is a list of precautions you can take:

  • Domestic water tanks should be inspected regularly and cleaned when necessary.

  • Make sure your boiler or hot water tank is working correctly, and keep it well maintained.

  • Keep the temperature settings on your boiler or hot water system set to heat up to 60°C. Be sure to take adequate precautions against scalding. 

  • Turn on showers and taps for two minutes, preferably at a high temperature, if they have not been used for more than a week, and flush toilets with the lid down.

  • Keep all your shower-heads and taps clean and free from a build-up of limescale, mould, or algae growth. 

  • Drain any hose pipes after use and keep them out of direct sunlight.

  • Maintain any spas or hot tubs you have. Keep them clean, free from debris, and check and clean the filters according to the manufacturer’s instructions. If you haven’t used the spa or hot tub for a while, change the water before using it.

If you are a building manager, a landlord, or an employer, it is imperative you understand the health risks associated with Legionella and understand when to test for Legionella bacteria in water systems.

There is a legal requirement for any company in the U.K. with five or more employees to follow the Health and Safety at Work etc Act 1974 and carry out a Legionella risk assessment to assess the risk of Legionnaires’ disease on their site. They must then record of any significant findings and retain the results of any inspections, checks, or tests for a minimum of five years. Also, landlords have legal responsibilities to ensure the health and safety of tenants by keeping the property free from health hazards. They must comply with the Health and Safety at Work Act and the L8 Approved Code of Practice.

The person responsible should produce a written scheme of control should for every building and process where there is a foreseeable risk of Legionella growing and being aerosolised.

Which regulations govern Legionella control and prevention?

Understanding how Legionella can affect your business is vital. In the UK, the two most important pieces of legislation that help understand the rules and regulations governing Legionella control and prevention are: 

Additionally, the legal requirement for a risk assessment is underpinned by these additional pieces of health and safety legislation:

ACOP L8 was developed to help business owners and landlords conform to the legislation and carry out its requirements. It is an Approved Code of Practice (ACoP), and the current edition is L8 (Fourth Edition) published in 2013. It is commonly referred to by the shortened acronym, ACoP L8.

ACoPs are approved by the Health & Safety Executive Board with the consent of the Secretary of State. ACOP L8 compliance is crucial.

The Fourth Edition simplified the text and removed the technical guidance, which is now published separately as HSG274 – and available to buy as a book or download as a PDF for free from the Health and Safety Executive website.

The Approved Code of Practice: Legionnaires Disease (ACoP L8) provides practical advice on the steps to take to manage and control Legionella risk in your environment, which include:

  • Ensuring the proper control of the release of water spray

  • Avoiding water temperatures or conditions that support the growth of Legionella

  • Eliminating the use of any materials that may encourage the growth of Legionella

  • Treating water to limit the growth of Legionella

  • Keeping pipe lengths short to avoid stagnation of water 

  • Maintaining records of control measures, repairs and maintenance


What is Legionella training and do I need it?

Legionella training is an essential component of any Legionella control management programme. Training courses are designed to educate individuals and organisations about the risks of Legionella bacteria and how to prevent outbreaks.

Whether you are a duty holder or responsible person, the onus falls on you to effectively manage and mitigate Legionella risks. Legionella training equips you with the knowledge and skills required for this role. Through training, you gain a comprehensive understanding of Legionella bacteria, its modes of transmission, and the associated health hazards.

Though not legally required to undergo training, the Health & Safety Executive Approved Code of Practice L8 (ACoP L8) states:

“Those specifically appointed to implement the Legionella control measures and strategies should be suitably informed, instructed and trained and their suitability assessed. They must be properly trained to a level that ensures tasks are carried out in a safe, technically competent manner, and receive regular refresher training.”

By undergoing training, you become adept at identifying and assessing Legionella risk factors specific to your environment. You also become well-versed in regulatory requirements and guidelines, enabling you to ensure compliance and implement effective preventive measures. Training empowers you to employ control strategies, manage water systems, and conduct regular maintenance practices to curtail Legionella proliferation.

What's more, Legionella training emphasises the importance of clear communication and well-defined emergency response protocols. This equips you to educate stakeholders about Legionella risks, disseminate preventive measures, and respond swiftly and efficiently in case of an outbreak. Armed with the necessary training, you can confidently carry out your responsibilities as the responsible person, fostering a safer environment and safeguarding the well-being of those under your care.

Courses range from Legionella awareness training to Legionella responsible person training. Discover all Legionella training courses here.

Why is it important to take precautions against Legionella?

Apart from the moral and ethical obligation of taking every precaution to prevent employees, tenants, and the public from being exposed to Legionella bacteria in commercial water systems, the ACoP has a special status in law.

Should you be prosecuted for breach of health and safety law, and you did not follow the advice provided in the ACoP, you will need to demonstrate that you have complied with the law in some other way, or a court will find you at fault.

This means every business owner, board of directors, landlord, or building manager should have a good understanding of the ACoP and have procedures and policies in place that can demonstrate compliance with the code of practice.

Whilst it is not compulsory, and you could take other action to ensure you have Legionella under control, following the guidance in the ACoP will normally be enough to comply with the law.

If you are an employer, landlord, or building manager, you have a legal duty to understand and manage Legionella risks.

You must understand how to:

  • Identify and assess sources of risk

  • Manage any risks

  • Prevent or control any risks

  • Keep and maintain the correct records

  • Carry out any other duties you may have

For further information, visit the Health and Safety Executive website.

It is important to note that under UK Health and Safety regulations, any outbreak or suspected case of Legionnaires’ must be reported.

  • If you suspect your illness is a consequence of your work, you should report this to your manager, as well as your health and safety representative and occupational health department, if appropriate.
  • Employers are legally obliged to report cases of Legionnaires' disease that may have been caught at their premises to the Health and Safety Executive.

Because of the nature of the symptoms and the potential length of the recovery period, employees with Legionnaires’ disease may be off work for some considerable time.

Are there financial penalties for not complying with Legionella rules?

If the potential loss of human life were not enough, there could be severe financial consequences if you fail to control the risks. For example, a South Tyneside car parts manufacturer was fined £800,000 after a Legionnaires’ disease outbreak.

In December 2018, a UK District Council was fined £27,000 after an outbreak of Legionnaires’ disease left a man close to death. The judge stated that the fine would have been ten times higher for a commercial business. The court was told that after bringing Legionella control in-house, those responsible were not properly trained.

Other companies have been fined millions of pounds when Legionnaires’ disease has resulted in a fatality. In the summer of 2018, a care home provider was fined £3 million after an older man died at one of its nursing homes in Essex. The crown court heard that Mr Ibbetson died after contracting Legionnaires’ disease from his bathroom tap—the most likely cause of the infection was the failure to flush and disinfect pipes following refurbishment work. 

There have even been cases where Legionella bacteria has not been detected, but companies have been found guilty of not mitigating risks. This was the case for a firm in Newport, which was fined £75,000 for safety failings.

Any outbreak of Legionnaires’ disease—or failure to comply with the regulations—is very bad for business.

We can’t banish Legionella bacteria, but you can take steps to prevent the conditions that allow it to multiply and become dangerous. Therefore, it is up to business owners to ensure the appropriate equipment is checked and inspected regularly to prevent serious illness or fatalities and to ensure industrial water safety.

By far the most important first step is a risk assessment. A full risk assessment should be carried out of all hot and cold water systems by a qualified individual. Following this, adequate measures should be put in place to control the risks.

Is Legionella testing a legal requirement? 

Conducting a Legionella Risk Assessment (LRA) is a legal requirement under the Health and Safety at Work Act 1974, and building managers have a duty of care to the people in their buildings. An LRA should be conducted by a person who has the competence and authority to undertake physical and administrative checks. 

The main British Standard dealing with legionella risk assessments was updated in 2019 and is called BS 8580-1:2019. It is a code of practice and applies to risk assessments undertaken on-premises, plants, and systems and where control measures may have been implemented. It also now applies to food and beverage manufacturers.

As carrying out an LRA is a legal requirement, this standard is invaluable for anyone responsible for the safe management of water systems, especially within hospitals, care homes, leisure centres, and schools.

For inspectors, there is a UKAS accreditation (ISO/IEC 17020) that covers Legionella Risk Assessment.

When should you carry out a Legionella Risk Assessment?

The old “industry standard” was for a Legionella risk assessment to be recommended at least every two years. However, the Approved Code of Practice L8 advises that arrangements should be made to review the LRA, “when there is reason to suspect it is no longer valid”, so the assessment should always be up to date.

This is particularly relevant when building occupants are categorised as high-risk (the elderly, for example)—an assessment may need to be carried out even more regularly.

As a general guide, it is important to carry out an LRA whenever there is a change.

This could include:

  • A change in the use of the building where the specific water system is located

  • A change in the water system itself

  • An update in relevant legislation

  • A change of key personnel within the company

  • When there has been a reported case of Legionnaires’ disease

Who can carry out a Legionella Risk Assessment?

A Legionella risk assessment should be conducted by a competent person or a team with the necessary knowledge, skills, and experience to assess and manage Legionella risks effectively.

The specific individuals or roles responsible for carrying out a Legionella risk Assessment can vary depending on the nature of the organisation and the complexity of the water systems involved.

The Legionella Risk Assessment should be instructed by the Legionella responsible person who should have adequate knowledge and understanding of the water management systems in their building. The responsible person should also have sufficient authority to be able to act upon the findings of the audit. This person must ensure compliance and, ultimately, prevent Legionella bacteria from growing.

What are the key elements of an LRA?

A formal independent Legionella Risk Assessment carried out by a specialist company should ideally comprise:

  • An evaluation of your on-site assets

  • An appraisal of the condition of water storage systems

  • Temperature checks

  • A photographic register of assets

  • A schematic to show the inter-relationship of equipment and associated risks

  • A list of recommended actions to ensure compliance and remain safe

In particular, the LRA will focus on the following six key elements:

  1. An assessment of risk to tenants/employees/visitors: is there anyone on site who is particularly susceptible due to age or health considerations? In the healthcare sector, for example, some patients’ compromised immune systems can increase the risks posed by Legionella bacteria.

  2. Descriptions of both cold water and hot water systems: are they mains fed, do they come from water storage, or are they a combination of the two?

  3. A record of water outlet temperature: water services must be operated at temperatures that inhibit Legionella growth. For cold water systems, this means a water temperature of below 20°C at outlets and where water is stored. For hot water, this means a temperature of above 50°C at the outlets.

  4. An assessment of the cold and hot water system/s: if there is a cold water tank, is it accessible, insulated, and completely enclosed to stop vermin or foreign matter entering that can act as a nutrient source for bacteria? 

  5. For hot water system/s, what is the system (unvented cylinder, combi-boiler, cylinder fed, etc.), and are the distribution pipes adequately insulated?

  6. Additional areas of risk: are showers and mixing valves correctly installed and well maintained? Are there any dead legs or redundant or rarely used pipework on the property that will need to be removed or altered? If areas of the property are left unattended for periods of time, is there a schedule in place to regularly flush water systems?

While LRAs tend to follow a fairly prescribed format, there are some factors that can help you differentiate one water hygiene provider's approach from another. Here is a free downloadable Legionella risk assessment checklist to see what you should demand from a water specialist.

What is the purpose of a Legionella risk assessment?

The main feature of an LRA is to help you identify real risks and provide you with the steps to take to remedy any problems. Clarity is critical, so it makes sense to look for an LRA that simply and succinctly explains the issues and that offers a clear set of actionable solutions.

To this end, an LRA that incorporates a traffic-light system can also be a useful way to help you prioritise your tasks. A red light, for example, might indicate actions that are top priorities and that you will need to remedy as soon as possible.

Tasks with an amber light might indicate that prompt action is recommended, perhaps within an advised time frame. A green light could refer to non-urgent actions that, whilst not essential, would help improve or streamline your processes. 

The risk assessment must be kept up to date to safeguard the well-being of occupants and visitors of any workplace where the presence of Legionella poses a potential risk.

While the law doesn't stipulate that duty holders employ the services of a qualified risk assessment assessor, the ACoP L8 regulations do specify that any person who is appointed to implement LRA control measures and strategies should be "suitably informed, instructed and trained and their suitability assessed" and that they are "properly trained to a level that ensures tasks are carried out in a safe, technically competent manner.”

It is therefore highly advisable to choose a reputable company, for example, one that is a member of the UK Legionella Control Association.

Selecting a water hygiene company that operates as part of a larger group can also offer additional reassurance and a greater breadth of experience, to ensure that your company's risk assessment and Legionella control procedures are conducted in an efficient and compliant manner.