Following An Outbreak

What happens when there is an outbreak?

Legionnaires disease

Local authorities have special plans for dealing with major outbreaks of infectious disease including legionellosis. These are usually investigated by an Outbreak Control Team whose purpose is to protect public health and prevent further infection. The Health and Safety Executive and/or the local authority environmental health department may also be involved in investigating compliance with health and safety legislation.

Breaking the Chain!

There is a four step chain of events which need to take place for someone to catch Legionnaires’ disease and which provide the basis of assessing and minimising the risk. If this chain is broken at any point you will prevent legionella disease.

Step 1 – A water system becomes contaminated with legionella bacteria

Certain water sources, such as untreated river or canal water, are more likely than others to contain legionella bacteria and will therefore increase the risk. However, it is not safe to assume that a treated incoming water supply is free from legionella and indeed you should assume that all water systems will become contaminated with low levels of legionella at some point.

Step 2 – Legionella multiply within the water system


Whilst it may be almost impossible to prevent a water system being contaminated with low levels of legionella it is entirely possible to prevent them multiplying to dangerous levels and it is important therefore to devise control measures to control their growth. The nature of those control measures will vary from system to system. In the case of domestic water systems this will involve keeping the hot water hot and the cold water cold and avoiding warm water conditions. In the case of a cooling tower this will involve an effective chemical water treatment programme. In all cases it will involve trying to avoid stagnant water conditions and keeping the water system clean and free from a build-up of deposits.
Whilst addressing all of the links in the chain is important, preventing legionella multiplying is undoubtedly the most important.

Step 3 – Contaminated aerosols need to be released to the atmosphere

It doesn’t matter how many legionella a water system contains if they are never released as an aerosol that someone can breath-in. A legionella risk assessment should therefore consider what the potential is for aerosols to be produced during both normal and abnormal usage (such as maintenance). Aerosols are generated by water sprays, air bubbles bursting through the water surface and by water splashing against a hard surface. In certain instances it may be possible to eliminate the risk by eliminating the process that produces the aerosol, or by reducing the quantity of aerosols that are released, but in many instances such as a domestic shower that isn’t possible and therefore the risk has to be minimised by focussing on preventing the multiplication of legionella within the water system.

Step 4 –Susceptible people inhale contaminated aerosolsbluewater2560x1600

The final step in the chain is for a susceptible person to breath-in enough contaminated aerosols to cause legionellosis. Individuals’ susceptibility varies and there are a number of factors which increase it:

  • Men are 2 to 3 times more susceptible than women
  • Increasing age – (people over 45 are generally more susceptible)
  • Existing poor health – Smoking, alcoholism, diabetes, cancer, respiratory conditions etc.

A risk assessment needs to consider whether the population who potentially might be exposed are particularly susceptible, such as a hospital or elderly persons home, and whether the location of the system means there is greater potential for more people to be affected such as a city centre.

Water hygiene should be of paramount importance to every company in the UK. Consumption of impure and unfit water may lead to many diseases such as Legionnaires, Eczema and other health disorders. However, some companies do not pay the required attention to water hygiene and therefore remain vulnerable to many diseases.

There are many bacteria that are born in tanks and pipes.

To maintain water hygiene it is not only necessary to clean and filter before use but is equally important to clean the places where it is stored and the sources through which it flows to drinking fountains, bathrooms and kitchens. There are many bacteria that are born in tanks and pipes and therefore need to be properly maintained. This requires the deployment of a comprehensive treatment procedure to maintain water hygiene. The HSC approved code of practice for legionella (L8) applies to ‘any undertaking involving a work activity and to premises controlled in connection with a trade, business or other undertaking where water is used or stored and where there is a means of creating and transmitting water droplets which may be inhaled, thereby causing a reasonably foreseeable risk of exposure to legionella bacteria’.

In short – if you have water on site that people can be exposed to, you need to assess and manage the risk from legionella bacteria. If you require any further information regarding your responsibilities as an employer to control the risk of legionella in your premises, please visit the Health and Safety Executive website at