Legionella is in most natural water systems like rivers, lakes and streams.
However, there, the levels are low and do not pose a health hazard. The situation is different in man-made water systems.
Biofilm build-up in pipework and other parts of man-made water systems can cause the spread of legionella and other pathogens.
Water temperature is another factor. Where the temperature range of a system is 20–45°C, this offers an ideal condition for legionella to thrive.
When Does Legionella Pose a Health Risk?
The combination of certain conditions in a water system, and the exposure of people to these conditions, can cause outbreaks of legionnaires’ disease.
Legionnaire’s disease is caused by the legionella pneumophila bacteria. It is a potentially fatal form of pneumonia.
Certain groups are more vulnerable, including people with weak immune systems, people with chronic respiratory disease and people aged 45 years or over.
Where can legionella be found in water systems? Legionella can grow and spread in a broad variety of building water systems, including:
- Large and complex plumbing systems
- Hot and cold water storage tanks
- Taps and shower heads
- Pipes, valves and fittings
- Cooling towers
- Spas and hot tubs.
Water systems can emit tiny droplets and these cause the airborne transmission of legionella.
The incubation period for legionnaires’ disease is 2–10 days, with the onset of the disease 3–6 days.
How Long Does It Take for Legionella to Develop in Water?
Within optimum water conditions, at 20–45°C, legionella can develop quickly.
In a laboratory, if you incubate legionella on an agar plate within this temperature range, in 7 to 10 days, it will form a colony of bacteria.
Given the right conditions, legionella will grow and spread rapidly. This is why it is such a risk in man-made water systems, and why legionella risk management is essential.
What Contributes to Legionella Growth?
The key factors that contribute to legionella growth are:
- Conditions that allow legionella into the water system
- Conditions that encourage the growth and spread of legionella within the system
- Transmission of legionella via inhalation of aerosol or small water droplets.
To control legionella, you need to identify the conditions that can contribute to its growth and act to reduce and control legionella risks.
What do these risks look like? There are several key things to look out for in water systems that can indicate the risk or presence of legionella:
- Stagnant or slow-moving water in pipes and systems
- Biofilm in water
- Water temperatures between 20°C and 45°C
- Areas of pipe in a network that receive very little or no water flow
Taking Steps to Control Legionella
Effective water system management can control the risk of legionella and prevent its growth and development.
An important element of this management is water treatment using an approved disinfectant.