The risk assessment is the responsibility of the employer or person in control of the premises. You may be able to carry out the assessment yourself but, if not, you should call on help and advice from within your own organisation or if this is not available, from outside sources, e.g. consultancies. You need to find out if your water systems (including the equipment associated with the system such as pumps, heat exchangers, showers etc) are likely to create a risk. Ask yourself the following:

• Are conditions present which will encourage bacteria to multiply? For example - is the water temperature between 20-45°C?
• Is it possible that water droplets will be produced and, if so, could they be dispersed over a wide area? For example, consider showers and aerosols from cooling towers
• Is it likely that anyone particularly susceptible will come into contact with the contaminated water droplets?

If you decide that the risks are insignificant, your assessment is complete. You need take no further action other than to review the assessment periodically in case anything changes in your system.
If a risk is identified which cannot be prevented, you must introduce proper controls. Risks from legionella in water systems can be controlled but careful planning, a successful management policy, competent staff and attention to proper control strategies are all essential.

You should consider whether you can prevent the risk of legionella in the first place by looking at the type of waters system you need. For example, is it possible to replace a wet cooling tower with a dry air cooled system? You need to prepare a written scheme which sets out how you intend to control the risk from Legionella. You should describe:

• your system - an up-to-date plan or schematic diagrams are sufficient
• who is responsible for carrying out the assessment and managing its implementation
• the safe and correct operation of your system
• what control methods and other precautions you will be using
• what checks that will be carried out on the control scheme and how often these checks will be carried out.

The key point is to design, maintain and operate your water services under conditions which prevent or control the growth and multiplication of Legionella.
You should:

• ensure that the release of water spray is properly controlled
• avoid water temperatures and conditions that favour the growth of legionella and other micro-organisms
• ensure water cannot stagnate anywhere in the system by keeping pipe lengths as short as possible or by removing redundant pipe work
• avoid materials that encourage the growth of Legionella
• keep the system and the water in it clean
• treat water to either kill Legionella (and other micro-organisms) or limit their ability to grow.

Keeping the water in a cooling tower system clean will not only control legionella, but also lead to other advantages. By reducing scale and fouling, you are also ensuring that the cooling process is operating efficiently – scaling reduces the effectiveness of biocide treatment and fouling can lead to loss of plant performance.

Cooling towers/systems are often treated using a wide range of biocides. But there are other treatment strategies available such as ultra-violet (UV) irradiation, copper/silver ionisation and ozone. In hot and cold water systems legionella has traditionally been controlled by storing hot water above 60°C and distributing it at above 50°C -and keeping cold water below 20°C if at all possible. Other methods which are used include chlorinisation and chlorine dioxide.