LEGIONELLA - World Health Organization

The appropriate design of new water systems is a critical step in controlling Legionella proliferation.
The control measures listed in Table 3.2 should be borne in mind when designing and constructing
the system. Poor design and construction will inevitably compromise attempts to implement
effective control measures, which in turn will have a serious impact on Legionella control.
Monitor control measures
A monitoring programme should be developed for the water system, to ensure that identified
control measures are functioning effectively. Monitoring points should be identified throughout
the system, for each control measure, on the basis of system design, operating parameters and
high-risk areas. Particular attention should be given to areas where control is most difficult to
achieve, and areas where Legionella is most likely to grow.
Monitoring of control measures should be primarily based on tests that are simple and rapid
to apply (ISO 5667 (ISO, 2001) can be used as a guide in developing a sampling method);
and where possible, monitoring equipment should be online and automatic. The equipment
should also be set up in such as way that remedial action is instigated as soon as failures in
control measures are detected, and before levels fall outside predetermined target ranges, such
as those shown in Table 3.3 (each target must be a measurable parameter (e.g. temperature,
biocide doses or heterotroph counts).
Results from system monitoring should be used in assessing the maintenance programme
and in improving the system. All monitoring records should be kept current and accessible,
so that the system can be assessed.
Table 3.3 Examples of microbiological quality monitoring and action level specifications
for cool ng water systems
Aerob c heterotroph c
count CFU/ml
Act on requ red
10 000 or less Acceptable control. No remedial action required.
More than 10 000
and up to 100 000
Review programme operation. The count should be confirmed by
immediate resampling. If a similar count is found again, a review
of the control measures and risk assessment should be carried
out to identify remedial actions.
More than 100 000 Implement corrective action (action to be taken when the
results of monitoring at the control point indicate a loss of control).
The system should immediately be resampled. It should then be
“shot dosed” with an appropriate biocide, as a precaution. The risk
assessment and control measures should be reviewed to identify
remedial actions.
Source: Adapted from HSE (2004)
CFU = colony forming units
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS