What is legionella?

What is legionella?

Legionella pneumophila is a type of bacteria 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.

How does legionella grow?

Biofilm in water systems (such as a spa pool) encourages the rapid colonisation of legionella bacteria. Biofilm development goes through the following stages:


Bacteria in the water begin to adhere to the surface. In this case, the surface is a balance tank that has not been cleaned regularly.


At this stage, the biofilm is beginning to form and will be difficult to remove.


The biofilm is well established now and the surface will feel slimy to the touch.


Bacteria break away from the biofilm, contaminating the water and forming new biofilms. This presents a serious risk of infection.

Other Risk Factors

Other conditions that help legionella spread:

  • water temperature between 20-45 degrees Celsius
  • deposits such as scale, rust and sludge
  • aerosols (very small liquid droplets, suspended in the atmosphere)

How does legionella spread and infect people?

The legionella bacteria is made airborne and dispersed via aerosol droplets. For example, shower spray releases tiny droplets of water (aerosol) into the air. If the shower water system has not been maintained properly, there could be legionella bacteria in the water (and therefore also in the aerosol in the air).

Aerosol is inhaled by people in the area.

The aerosol droplets are so tiny that they travel deep into the lungs. The legionella bacteria can then multiply and cause an infection of the lungs.

The diseases (Legionellosis)

Legionellosis is a collective term for diseases caused by legionella bacteria, including the most serious: Legionnaires’ disease and the similar but less severe conditions of Pontiac fever and Lochgoilhead fever.

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, e.g.

  • people over 45
  • smokers and heavy drinkers
  • people suffering from chronic respiratory or kidney disease
  • diabetes, lung and heart disease
  • anyone with an impaired immune system

Reducing the Risk

Outbreaks of the illness occur from exposure to legionella growing in purpose-built systems where water is maintained at a temperature high enough to encourage growth, e.g. cooling towers, evaporative condensers, hot and cold water systems and spa pools used in all sorts of premises (work and domestic).

It is essential to control the risk by introducing measures which do not allow the proliferation of the organism in the water systems.