Further Information

Legionella & Legionnaires' Disease

Legionnaire’s disease is caused by the bacterium Legionella pneumophila, which lives and breeds in natural water sources such as rivers, lakes and reservoirs and also in domestic water systems. It is a pulmonary infection which targets the lungs and is transmitted to humans via inhalation of aerosolized water contaminated with the bacteria. Any domestic water system which creates an aerosol droplet (water suspended in air) can potentially be at risk of harbouring the bacteria and spreading the disease.

Legionnaires' disease acquired its name in July 1976, when an outbreak of pneumonia occurred among the delegates of a convention of the American Legion at the Bellevue-Stratford Hotel in Philadelphia.

In Europe, it is believed 6000 cases of Legionnaires’ disease are diagnosed every year.  While in 2010 more than 350 cases were identified in the UK with approximately 10% of these resulting in fatalities.*

High risk systems

The majority of reported outbreaks of Legionnaire’s Disease are from Domestic Hot Water Systems. The following applications are high risk as they create aerosol droplets.

The disease is particularly associated with hotels, fountains, cruise ships and hospitals with complex potable water and cooling systems.

The main factors affecting bacterial growth are:


  1. Water temperature
    •  Below 20ºC - dormant
    •  Above 70º - instant total kill
    • Bacterial growth most prolific at 37ºC
  1. Stagnation – this allows biofilms & bacteria to attach onto surface of pipework
  2. Nutrients within fresh water – these allow biofilms and legionella bacteria to develop – feeding the bacterial growth
  3. Existence of biofilms - are required by the Legionella bacteria to establish colonies

L8 compliance and legal implications

The Health & Safety at Work Act (HSWA) 1974 and Control of Substances Hazardous to Health Regulations (COSHH) 1999 have identified the risk of legionella. More information can be found here

L8 – HSE ACOP (Approved Code of Practice) gives practical advice on the requirements of HSWA & COSHH L8 has a ‘special’ legal status; following L8 advice demonstrates compliance with the law.

Control options

Legionella can be controlled by either physical (Thermally or by UV/Ozone) or by chemical (Chlorine Dioxide and ionisation) means.

Chemical control methods

Physical control methods

Hattersley's Thermal Circulation Valves

Thermal Circulation Valves are designed to be installed in Domestic Hot Water systems to help to ‘keep hot water hot’ so that the Legionella bacteria cannot survive.

How do Hattersley's Thermal Circulation Valves work?

As the water flows through the Thermal Control Valve the thermal element senses the water temperature and adjusts its stem position in the seat to control the flow rate some valves include an isolation valve.


When reacting to water temperature changes the valve never completely stops the flow.


*Health Protection Report Vol. 6 No. 9, March 2012