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How to minimise Legionella bacteria Cheat Sheet by

10 ways in which to reduce the risk of Legionnaires' disease

Unders­tanding the basics

Legionella bacteria are ubiquitous and can live in water or soil. In order to cause illness they have to enter the lungs, or in rare cases enter a wound in the skin, but breathing in an infected aerosol is the main cause.
This leads to a pneumonia like illness which is confirmed by an x-ray showing pneumonia and a secondary test such as a urinary antigen test. Death occurs in about 10% of cases depending on the suscep­tib­ility of the indivi­dual. Suscep­tib­ility is increased by having other illnesses such as heart disease, lung disease, diabetes and renal problems.
Legionella bacteria enter a building in the mains water in small numbers and then grow and increase to larger numbers within man made water systems such as showers, cooling systems, hot and cold water, and a host of other engineered systems. The system has to create an aerosol for a person to catch Legion­naires' disease from the infected aerosol.
The aerosol needs to be very small, less than 5 microns, to be able to reach the area of your lungs called the alveoli where the Legionella bacteria cause Legion­naires' disease. There are about 60 species and serogroups of Legionella bacteria and only about 20 of these cause illnesses in humans.
In the water systems the Legionella bacteria find niches to live, hide and grow such as biofilms, deadlegs, in some of the materials of constr­uction and in places where they can be undist­urbed. In order to cause illness though they must also be found in the body of the water. So keeping the systems clean in the body of the water as well as system surfaces is important.

Legionella bacteria

Management Structure

In some countries, such as the UK, there is a requir­ement to prevent people catching Legion­naires' disease in the workplace. This workplace can include factories, offices, hotels, hospitals, leisure facilities and a host of other buildings and locations. In the UK the HSE produce the documents that help companies manage safety and with regards to Legion­naires' disease this includes an Approved Code of Practice, L8, and technical guidance, HSG 274, which comes in three parts.
L8 and HSG 274 describe how to manage Legion­naires' disease and start by describing how an organi­sation should be set up to minimise Legionella bacteria. This involves the business owner or manager (the Dutyho­lder) appointing somebody with sufficient authority, knowledge and competence to run the day to day management of Legionella (known as the Respon­sible Person).
The Respon­­sible Person is the key person in managing Legionella as it is that person who should ensure the policies and processes for minimising Legionella are in place and are carried out.
In order to understand the issues on site a Legionella risk assessment should be carried out, though this should not be restricted to just Legionella issues as related problems such as other bacteria, for example Pseudo­monas aerugi­nosa, or even scalding risks are important to consider.

Control Scheme

Once the risk assessment is in place this will identify the risks on site which will lead to a series of works to be carried out. These works are split into on-going and remedial tasks. Knowing what the risks are and what works are required to be carried out leads to the creation of the control scheme. This control scheme is known as the "­Written Scheme­" or it can be called the "­Water Safety Plan"
This scheme or plan details what should be carried out by the company and is managed by the Respon­sible Person. The detailed contents of this scheme for the UK are described in HSG 274.
Other countries may have their own guidance and the World Health Organi­sation also has a detailed document describing the schemes for contro­lling Legion­ella.
The UK written scheme guidance details the recomm­ended actions to be taken on site, the records to be kept, staff competence and inform­ation the site needs to produce that describes the day to day safe operation of the water systems on site as well as what they should do in the event of an upset or cases of Legion­naires' disease.
This written scheme is therefore an important document that an organi­sation needs to create, whether it be a paper based or electronic system, that fully describes the policies, systems and operations carried out on site to minimise Legionella bacteria.


Key to successful management of Legionella bacteria is the training and competence of all staff involved in the process. Relevant training and competence is required based on the role fulfilled. For example toolbox talks may be sufficient if a person is just carrying out temper­ature checks but this is not sufficient for the Respon­sible Person who will require more detailed, indepth training.
Training should be regularly updated, especially if there is a change in guidance and standards.
The Respon­sible Person should also ensure chosen contra­ctors are competent and suitably trained for the tasks they are carrying out. For example water treatment companies should ensure their staff are suitably trained and experi­enced for the tasks they carry out on site. Contra­ctors who carry out plumbing tasks on site should also understand the effect of their work on Legionella manage­ment.
Training and competence of all staff involved in managing Legionella is therefore an important component of managing Legionella on site.

On-going monitoring

The risk assessment carried out will identify the risks on site and using this knowledge the control scheme is created. The control scheme will show the on-going tasks that are required to be carried out. In the UK the details of what is required can be found in HSG 274.
The three parts of HSG 274 offer guidance on the on-going tasks for managing coolimng systems (Part 1), hot and cold water systems (Part 2) and "­other risk" systems (Part 3). Similar advice can be found in the World Health Organi­sation document on managing Legion­ella.
On-going monitoring is a vital part of minimising Legionella as by carrying out various tests and activities you can determine whether the systems you are managing are under control or likely to be allowing Legionella to prolif­erate.
The results from the on-going monitoring should be regularly checked by the Respon­sible Person to look for non-co­nfo­rmances and trends which can identify increased risks and allow for actions to be taken before problems occur.
A well designed and managed set of on-going monitoring tasks is key to managing Legionella and reducing the risks that somebody may contract Legion­naires' disease.

Keeping things clean

Legionella likes to live in biofilms, scale and sludges on surfaces as it is an ideal enviro­nment for the bacteria to seek protection and food. Conseq­uently a system that has these deposits can encourage Legionella bacteria to prolif­erate.
In order to minimise the presence of Legionella it is therefore important to keep surfaces clean and free from deposits. In some cases it may be possible to achieve this by the use of chemicals, for example in the case of a cooling system or a spa pool. In other systems, such as hot and cold water, such chemicals should not be used and ensuring water flows and is not stagnant is one means of keeping surfaces clean.
Biocides can be used in all systems to minimise biofilms and so reduce Legion­ella's ability to survive inside a biofilm.
In all cases regular off-line cleaning techniques can be used to ensure systems are kept clean which will minimise Legionella bacteria.


Hot is hot, cold is cold

Temper­ature is a key tool in managing Legionella bacteria in hot and cold water systems. Legionella bacteria thrive in temper­atures between 20oC and 45oC and so avoidance of these temper­atures is important.
In hot water systems this means mainta­ining a minimum of 60oC in a calorifier (known as an Immersion heater in a home) and ensuring hot water comes out of the hot tap at a minimum of 50oC.
Note that 50oC is hot enough to scald some people and so in some circum­stances a Thermo­static Mixing Valve or Thermo­static Mixing Tap is used to reduce the hot water to a safer temper­ature. If these are present then extra works will be required as part of the on-going monitoring to ensure they are kept clean and free from bacteria.
Cold water should be kept below 20oC in the UK (the WHO suggest below 25oC in hot countries) to reduce the growth of Legionella bacteria. Below 20oC Legionella bacteria will still live however they enter a state known as "­Viable but non cultur­abl­e" (VBNC) which means they don't grow into dangerous levels of bacteria.
Ways should therefore be found on site to ensure hot water is kept hot and cold water is kept cold to reduce risks.


In some systems, such as cooling systems, spa pools and process waters, it is permis­sible to use industrial biocides to kill or control bacteria, including Legion­ella. In hot and cold water, which is fit for human consum­ption, the choices of allowable biocides is much more restri­cted.
If biocides are to be used then they must be controlled carefully to ensure they are are always present in the right quantity for the right amount of time to ensure biological control.
The biocides most readily approved for use are chlorine and chlorine dioxide, which are known as "­oxi­dising bioxid­es" though there are other oxidising biocides which may be used. There are also "­non­-ox­idi­sin­g" biocides that can be used in some circum­sta­nces.
Biocides are hazardous as they are there to kill bacteria and other organisms so they must always be used with care with all relevant health and safety consid­era­tions being closely followed.

Choose your support

It is unlikely that a Respon­sible Person has all of the skills and compet­ences required to manage a site by themselves and so contra­ctors are often used to help.
Using contra­ctors does not absolve the Respon­sible Person of respon­sib­ility for managing Legionella on site so it is important contra­ctors are chosen with care.
Contra­ctors should be able to show experi­ence, skill and competence relating to the task required of them. In the case of water treatment companies this means their staff should know how to carry out suitable and sufficient risk assess­ments, manage water systems, advise on the use of chemicals and how to compet­ently clean water systems.
Seeking references for similar works carried out elsewhere, using an industry expert to audit the work of the contractor or seeking relevant accred­ita­tions are all suitable ways of managing contra­ctors
Bearing in mind the seriou­sness of Legion­naires' disease it is worth taking time to choose the contractor you trust and will be happy working with to manage Legionella on site.

Keep your eye on the ball

You now have a suitable and sufficient risk assessment identi­fying the risks on your site, from this you have identified the remedial tasks required to bring your systems up to a suitable standard, and have a list of on-going monitoring tasks.
Your control scheme is in place with management struct­ures, training, a well chosen contractor all of which will generate large quantities of inform­ation.
Systems will change over time, bacteria, being living organisms, will grow some times faster than others, standards and rules will change from time to time.
All of this means it is very important to keep on top of what you have in place. You can do this by having regular audits of the systems and processes in place whether carried out by the Respon­sible Person or by an indepe­ndent external audit.
Whatever means you choose you should learn from the inform­ation you have as this means you can reduce the risks on site. Look at trends and at how you remediated problems as these same issues may well reoccur.
Your risk assessment will need to be reviewed regularly so you can use this learning to inform your new risk assessment which should result in reduced risks on site.
All of this inform­ation, your systems, processes and learning will prove to someone like the HSE that you have Legionella under control. It may not be possible to eradicate Legionella but what is possible is to have a well managed system in place to control Legion­ella.

External support

If after reading this you find that you need external help and support then take a look at the following websites:


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