Introduction To Legionellosis

Legionnaires Disease is caused by breathing in water infected with bacteria known as Legionella. The bacteria are common in natural water sources such as rivers, lakes and reservoirs, but usually in low numbers. They may also be found in purpose-built water systems such as cooling towers, evaporative condensers, hot and cold water systems and spa pools.


The bacterium enters the water systems in the incoming mains water supply at very low levels.  Domestic water systems then provide favourable conditions for the bacteria to grow and pose a risk to the occupant’s health.


Legionella bacteria requires the following to multiply:


  • A water temperature between 20-45 °C.

     The optimum temperature for growth is at 37°C, Below 20°C the bacteria will remain dormant and at sustained temperatures         of 60°C the bacteria will be neutralised within 2 minutes.

  • A nutrient source that can support bacterial growth.

      This can be in the form of rust, sludge, scale, organic matter and bio films within water systems.

  • Dead legs and stagnant water.

      This can be redundant lengths of pipe work within the domestic water system which have been capped off leaving the water         within the pipe work nowhere to go. Cold water storage tanks which are over capacity for the buildings usage or sinks, taps         and toilets within a building which are seldom used.


The often fatal illness of Legionnaires’ disease is contracted through the inhalation of water droplets contaminated by Legionella bacteria. Water droplets known as aerosol can be created by opening a water tap or flushing a toilet; however systems such as showers, cooling towers and jet washes create an increased amount of aerosol which enhances the potential for people to become exposed.


Legionnaires’ disease is a potentially fatal form of pneumonia and everyone is susceptible to infection. The risk increases with age but some people are at higher risk including:


  • people over 45 years of age.

  • smokers and heavy drinkers.

  • people suffering from chronic respiratory or kidney disease, diabetes, lung and heart disease.

  • anyone with an impaired immune system.


If you are an employer, or someone in control of premises, including landlords, you are responsible for health and safety risks associated with legionella and need to take the right precautions to reduce the risks of exposure to legionella bacteria. You must understand your Duties under the Health and Safety at Work etc Act 1974 (HSWA) extend to risks from legionella bacteria, which may arise from work activities.

The Management of Health and Safety at Work Regulations (MHSWR) provide a broad framework for controlling health and safety at work.  More specifically, the Control of Substances Hazardous to Health Regulations 2002 (COSHH) provide a framework of actions designed to assess, prevent or control the risk from bacteria like Legionella and take suitable precautions.  The Approved Code of Practice: Legionnaires’ disease: The control of Legionella bacteria in water systems (L8)[1] contains practical guidance on how to manage and control the risks in your system. 

All commercial, public buildings including rented and social residential buildings with a domestic hot or cold water system will require a legionella risk assessment to be conducted on them to determine the level of risk associated with legionella bacteria.

From the risk assessment a control plan needs to be implemented for the control and prevention of legionella bacteria (written scheme). The risk assessment will determine what procedures need to be done and when.