Legionnaires’ disease is a serious form of pneumonia, an infection that causes inflammation of the small air sacs in the lungs (alveoli) and their associated tissues.

Why is it called Legionnaires’ disease?

In 1976, there was an outbreak of the disease in Philadelphia amongst people attending a convention of the American Legion and 29 people, mostly men, died. When the disease was identified it was named Legionnaires’ disease. 

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

Medically, the illness is named Legionellosis, but Legionnaires’ Disease is the more common name.

What causes it?

Legionella bacteria are the cause of Legionnaires’ disease. Bacteria are microscopic, single-celled organisms that thrive in various environments, including in the soil, water, and even inside human beings.

Different types of bacteria can be beneficial to us, neutral or harmful. If you've seen yoghurt advertising, you will often hear talk of ‘good' bacteria – bacteria that live in the gut and help us to digest our food. (There is actually more than ten times the number of bacterial cells than human cells in our bodies – we are effectively more bacterial than we are human!) Bacteria are also useful for manufacturing antibiotics and pharmaceuticals, making sourdough bread, pickles and vinegar, manufacturing chemicals, fertilisers and for use in pest control – as just some examples.

On the other hand, bacteria are also one of four types of germs – those germs which many household cleaner ads claim they kill 99.9% of. These types of harmful bacteria cause diseases in humans, plants and animals.

Legionella bacteria live in natural water sources such as lakes, reservoirs, rivers and streams. It can also be found in man-made water systems such as hot and cold water systems, cooling towers and spas. 

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

How do you catch Legionnaires’ disease?

You can catch the disease by inhaling water droplets that contain the Legionella bacterium. These water droplets can then make their way into the lungs via aspiration, which then become infected.

Contaminated water droplets can be created by many and varied methods including hot tubs, cooling towers, showers, taps, and industrial processes - wherever water is warm and a spray is produced.

Where can you get it?

Man-made environments can contain the perfect conditions for the Legionella bacterium to multiply and may produce the water droplets that carry the bacterium.

It is rare to find it in the home where water turnover is generally good.

It is most likely to be found in hotels, hospitals and factories – anywhere with industrial water systems. 

Although the bacteria can be present in ponds, lakes and rivers, these are not likely sources of infection.

In the UK almost half the known cases are associated with a trip abroad.

Is it contagious?

Because you have to inhale water droplets in order to catch the disease, it is generally not contagious, and it is usually safe to be around people with the disease.

What are the symptoms of Legionnaires’ disease?

The initial symptoms of the disease usually show themselves around two to 10 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
  • a 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 persons may experience further symptoms such as coughing up green phlegm and becoming short of breath. They may have chest pain when breathing.

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

If someone has these symptoms it is important for them to see a doctor as soon as possible - they may not be caused by Legionnaires’ disease but a doctor will be able to take steps to determine that.

How serious is Legionnaires’ disease?

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

There are a few risk factors that make you more susceptible to the disease, the most prevalent being smoking, or chronic lung disease. People with organ transplants are also at higher risk as well as anyone who is taking corticosteroid medicines. Men are around three times more likely to contract the disease than women.

At risk groups:

  • People suffering from chronic respiratory or kidney disease
  • People who suffer from diabetes, lung and heart disease
  • Anyone with an impaired immune system
  • Those who are over 45 years of age
  • Anyone who smokes or is a heavy drinker

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

Can it be treated?

If it is 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 which are usually given straight away, without necessarily waiting for test results. However, this may not be effective, and if this is the case, then alternative antibiotics may be prescribed. In severe cases, patients may be admitted to a hospital where antibiotics may be administered 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.

Long term effects

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. In serious cases, there are often general secondary symptoms, 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.

Because water droplets are the means of infection, and they are potentially generated by many industrial processes and commercial equipment in everyday use, it is not possible to eradicate Legionnaires’ outright like some diseases. In fact, in the UK, reported cases are on the increase; from 360 in 2016 to 450 in 2017 to 530 in 2018.

Between the years of 2014 and 2016, there were 77 deaths, and over a thousand confirmed cases of Legionnaires’ disease.

The best way to prevent Legionnaires’ disease is to kill or inhibit the bacteria that cause it.

Stopping the bacteria

As a bacterium, Legionella is a living thing – a microscopic, single-celled organism that can thrive in various environments, including in the soil, water, and even inside human beings.

As a living thing, it needs the right conditions to survive and thrive. So in order to eliminate the threat of disease, various methods can be used to create a hostile environment for the bacteria; killing it or stopping it from multiplying.

One of the key ways of eliminating the Legionella bacteria is to create a hostile environment by controlling the temperature of the water. Like most living things, Legionella will not tolerate extremes of temperature. The optimum temperature for Legionella to multiply is around 37°C – which is also the normal human body temperature.

To kill the bacteria, the water temperature needs to be above 50°C – as at this temperature the bacteria will begin to die off. To ensure a rapid and certain demise, the temperature needs to be above 60°C. As this is too hot for most domestic uses, water is often stored at this temperature, then delivered to the point of use at a lower temperature.

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

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

There are a number of approaches to eliminating the Legionella bacteria, but controlling the temperature of water is one of the more reliable and most often used.

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

20° or less – Legionella survives, but in a dormant state

20° to 45° – Legionella will grow rapidly

50° – Most Legionella bacteria will die within hours

60° – Most Legionella will die within minutes

70° – All Legionella bacteria will die instantly

What can I do as an individual to avoid Legionnaires’ disease?

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

  • Make sure your boiler or hot water tank is working properly and keep it well maintained.
  • Keep the temperature settings on your boiler or hot water system set to heat up to 60 degrees Celsius. 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 showerheads 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.

In the United Kingdom, there is a legal requirement for any company with five or more employees to follow the Health and Safety at Work etc Act 1974 and carry out a risk assessment to assess the risk of Legionnaires’ disease on their site and then to record of any significant findings – retaining the results of any inspections, checks or tests for a minimum of five years.

A written scheme of control should be produced for every building and process where there is a foreseeable risk of Legionella growing and being aerosolised.


In the UK, the legal requirement for a risk assessment is underpinned by the following health and safety legislation:

  • The Health and Safety at Work Act 1974
  • The Management of Health and Safety at Work Regulations 1999
  • The Control of Substances Hazardous to Health (COSHH) regulations 2002
  • The HSE's Health and Safety Guidance HSG 274 "Legionnaires Disease Technical Guidance"
  • The Health and Safety Executive (HSE) Approved Code of Practice ACOP L8 "Legionnaires Disease: The Control of Legionella Bacteria in Water Systems"

The last document in that list has been developed to help business owners and landlords conform to the legislation and carry out its requirements. It is an Approved Code of Practice (ACoP) with the current edition being 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.

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 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

Why is it important to take precautions?

Apart from the moral and ethical obligation of taking all precautions to prevent employees and the public contracting a serious, potentially fatal disease, the Code 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 Code, 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, or board of directors, should be aware of the Code 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 code will normally be enough to comply with the law.

If you are an employer, or someone in control of premises (e.g. landlord), 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 that your illness is as a consequence of your work then you should report this to your manager, as well as your health and safety representative and occupational health department, if you have one.
  • There is a legal requirement for employers to report cases of Legionnaires' disease that may be acquired 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’ may be off work for some considerable time.

Financial penalties

If the potential loss of human life were not enough, there can be severe financial consequences if you fail to control the risks.

In December of 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 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 elderly man died at one of its nursing homes in Essex, although the fine was reduced to £1.5 million after judges ruled the parent company profits were wrongly taken into account. The crown court heard that Mr Ibbetson died after contracting Legionnaires’ disease from the tap of his bathroom. It was concluded that the most likely cause of the infection was the failure to flush and disinfect pipes following refurbishment work. 

Needless to say, any outbreak of Legionnaires’ is very bad for business.

Given that you can take steps to prevent the conditions that allow Legionella to multiply and become dangerous, it is up to business owners to ensure the appropriate equipment is checked and inspected regularly to prevent serious illness or fatalities.

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

In February 2019, the BSI published an updated British Standard: “…providing further guidelines for assessing water quality and the risk of Legionnaires disease.” And as of the time of writing (February 2019), this is the current British Standard covering the issue.

There is also a UKAS accreditation (ISO/IEC 17020) published for inspectors that covers Legionella Risk Assessment.

As a general guide, it is important to carry out an LRA whenever there is any form of 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

What are the key elements of an LRA?

A formal independent Legionella Risk Assessment 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 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, the compromised immune systems of some patients can increase the risks posed by Legionella bacteria.
  2. Descriptions of both cold water and hot water systems - for example are they mains fed, from water storage, or 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, and for hot water, a temperature of above 50°C at the outlets. At temperatures below 20°C, Legionella is dormant, while at temperatures of 60°C and above it cannot survive.
  4. An assessment of the cold and hot water system/s - if there is a cold water tank present, is it accessible, insulated and completely enclosed to prevent ingress of vermin or foreign matter which can act as a nutrient source for bacteria? 
  5. In the case of a hot water system, what type of system is it (unvented cylinder, combi-boiler, cylinder fed etc) and are the distribution pipes adequately insulated?
  6. Identification of additional areas of risk - are showers and mixing valves correctly installed and well maintained? Are there any dead legs, 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 flush water systems on a regular basis?

While LRA's tend to follow a fairly prescribed format, there are some factors that can help you differentiate one water hygiene provider's approach from another.

The aim of an LRA is to help you identify real risks and to provide you with the steps to take in order to remedy any problems. Clarity is key - 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 of the highest priority and which will need to be attended to as soon as possible.

Tasks with an amber light might indicate that prompt action is recommended, perhaps within an advised time-frame; while a green light could refer to non-urgent actions that, whilst not essential, would assist in improving or streamlining your processes. 

It is vital that the risk assessment is 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.