LEGIONELLA - World Health Organization

4.3.1 Document and describe the system
In documenting and describing the system, all relevant information and documentation should
be compiled. Box 4.2 lists the particular components of a potable water distribution system
that should be assessed.
Box . Components of potable water d str but on system to be assessed
Particular components of a potable water distribution system that should be assessed include:
• the quality of water entering the system
• the design and construction of equipment (including operational information about
temperature regime and water circulation)
• treatments (e.g. anticorrosion, antiscaling and disinfection) and timing of treatments
• systems, system components and equipment that have the potential to generate aerosols
• the temperature of storage tanks and the environment in which the system is located
(both in buildings and outside), including the location of the system network (e.g. pipes
in conduits, ceilings, walls and floors)
• the periods of water use; for example, on a daily or weekly basis (e.g. sports facilities
may use water on a weekly basis)
• the turnover of water in areas such as storage tanks
• the population using the system, including any particularly susceptible people
• the management structure
• the competence of personnel responsible for the system.
Potential exposure pathways and the proliferation of Legionella should be taken into account
at the design stage, because modifying existing facilities can be complicated and expensive.
Once a desktop review of the system has been completed, a sanitary survey or “onsite” survey
should be carried out to verify the system (see Chapter 4 of WHO, 2004).
4.3.2 Assess hazards and prioritize risks
This step involves collecting and evaluating information on hazards and conditions leading to
their presence, to decide which are significant for safety and therefore should be addressed in
a safety plan.
In assessing hazards, it is reasonable to assume that all water supply systems have the potential
to become seeded with microorganisms, including legionellae, during construction, repair and
maintenance, even if the water is treated. Legionellae are widespread in surface water, and
numbers of L. pneumophila ranging from 10 4 to more than 10 7 cells/litre have been observed
by direct immunofluorescence assay (Chapter 11).
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS