LEGIONELLA - World Health Organization

Validate effectiveness of water safety plan
This step involves developing procedures to verify that the WSP is working effectively, and
will meet the predetermined target; that is, it involves monitoring individual components of
the water system to determine whether the WSP has effectively controlled Legionella in the
system. Validation and verification are defined in Box 3.2.
Box 3.2 Verification and validation
Val dat on is the process of obtaining accurate and reliable evidence that a water safety
plan is effective.
Ver f cat on is defined as the use of methods, procedures or tests, in addition to those
used in operational monitoring, to determine whether the performance of the supply complies
with the stated objectives outlined by the health-based targets. Verification might be undertaken
through independent surveillance; it provides an indication of the overall performance
of the system.
Source: WHO (2004).
If control of Legionella is found to be inadequate, the operational procedures should be reviewed
and control measures re-evaluated as a matter of urgency. A health risk assessment of the
system may be necessary, to determine whether the management contingency plan should be
used (e.g. shot dosing the system with biocide).
There appears to be little correlation between Legionella culture test results and human health
risk (Kool et al., 1999; Bentham, 2002). Legionella testing cannot be considered a control
measure, because of:
• uncertainties about the reliability of culture
• time delays
• differences between culture requirements for different Legionella species
• dynamics of the population.
Chapter 11 provides more details on laboratory diagnosis of Legionella.
Although Legionella testing cannot be considered a control measure, it can be used in validation
to provide some evidence that the WSP is effective and that control measures are operating
properly. Validation normally includes more extensive and intensive monitoring than routine
operational monitoring, and its aim is to determine whether system units are performing as
assumed in the system assessment (see Section 3.3.1). Operational monitoring of control
measures should be by measures that provide real-time results (e.g. monitoring of biocide
concentrations, temperature and pH); sampling for Legionella cannot provide results sufficiently
quickly to be useful in operational monitoring.
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS