Legionella – A reminder of the key issues

Legionnaires’ disease is one of a group of diseases collectively known as legionellosis. It is also known as Pontiac fever. Legionnaires’ disease is the most serious form of the disease.


Those most at risk include the elderly and in particular those who are smokers, alcoholics. Also effected are people suffering from cancer, diabetes, chronic respiratory or kidney disease. But healthy people can also be infected. Most reported cases have been in people aged between 40 and 70; men are more likely to be affected than women.



Legionella bacteria are widespread in natural sources of water including rivers, streams and ponds and may even be found in soil.

The bacteria are also found in many recirculating and hot and cold water systems. Outbreaks of legionnaires’ disease have occurred in or near large building complexes such as hotels, hospitals, offices and factories. There is no evidence that water systems in domestic homes present any risk.

Infection is caused by people breathing in water droplets contaminated with legionella bacteria.

The bacteria will multiply if

  • water temperature in the range 20°C to 45°C
  • the presence of sludge, scale, rust, algae and organic matter to provide nutrients.

Cooling towers, such as those which form part of an air conditioning system, can represent a particular hazard because they readily generate fine water droplets and there is an air current to carry them away. Because they are often located on roof-tops there is a potential for infecting large numbers of people. As the small particles mix in the air.

The bacteria may also colonise hot and cold water systems – showers and spa baths have been associated with infection outbreaks.


Since legionella is widespread in the environment, it cannot be prevented from entering water systems. However, the risk of an outbreak developing can be reduced by taking the following precautions:-

  • Hot and cold water cisterns and pipe work should be designed so that water is not allowed to stand undisturbed for long periods;
  • cisterns should be well covered to prevent the entry of dirt, debris and vermin.
  • water temperatures between 20°C and 45°C should be avoided by insulation of cold water tanks and pipes in warm spaces, and by storing hot water at 60°C and circulating at 50°C. Where there is a risk of scalding for the very old, and young children, thermostatically controlled taps may be needed;
  • only water system fittings and materials which comply with water authority by-laws should be used (certain materials, e.g. leather, some rubbers and plastics, support the growth of bacteria and should not be used). 

Cooling tower

In some cases organisations have redesigned their water service to remove the need for a water cooling tower, thus eliminating the risk. However in some industries and situations where large amounts of water must be cooled they are still in use. The risks can be controlled by;-

  • well designed, maintained and operated; the fitting of efficient drift eliminators which reduce the escape of spray is especially important;
  • systems should be cleaned and disinfected at least every six months;
  • water should be treated to prevent scale, corrosion and microbiological growth;
  • where reasonably practicable, cooling towers should be replaced with dry cooling systems.


The Health and Safety at Work etc Act 1974 and the Control of Substances Hazardous to Health Regulations 2002 both apply.

The HSE publication L8 provides an Approved Code of Practice which provides a basic framework of risk management for preventing further outbreaks of the disease.