LEGIONELLA - World Health Organization

Even when a source reaches a state at which it is infective, the proportion of people who acquire
Legionnaires’ disease is small (usually less than 5% of those exposed). Conversely, in outbreaks
of Pontiac fever, a high percentage (about 95%) of those who are exposed become affected.
A preliminary risk assessment by Ambroise & Hartemann (in press) compared exposure linked
to aerosols produced by cooling towers and by showering. The study considered expected numbers
of cases of infection, clinical sickness and death, for similar concentrations of L. pneumophila
serotype 1 in the air (ranging from 0.02 to 200 CFU/m 3 ). The authors found that exposure
through cooling towers led to more cases (by a factor of 100–130) than exposure during
showering.
3.3 Water safety plans
Developing a WSP is the preferred approach to managing specific health risks of exposure to
Legionella from water systems (WHO, 2004; Davison et al., 2005). In some jurisdictions, other
terms are used; for example, the term “risk management plan” is used by the Department of
Human Services, Victoria, Australia. Such plans are similar to a WSP, but are less clearly defined.
For the purposes of this document, the term WSP is used.
Authorities responsible for water system safety or building safety should develop systemspecific
WSPs. Major benefits of developing and implementing such a plan are the systematic
and detailed assessment and prioritization of hazards (biological, chemical or physical agents,
or water conditions, with the potential to cause adverse health effects), and the operational
monitoring of barriers and control measures.
The steps involved in developing a WSP are shown in Figure 3.2. A plan consists of the following
key components:
• system assessment (Section 3.3.1) — determination of whether the water quality at the point(s)
of potential exposure or use meets the health-based target, based on a risk assessment for
the population likely to be exposed
• monitoring (Section 3.3.2) — identification and monitoring of control measures used to
ensure water safety (e.g. biocide levels, temperature, pH)
• management and communication (Section 3.3.3) — to document the system assessment
and monitoring, and describe actions to be taken during normal operation and after
incidents, including documentation and communication (e.g. a plan for remedial actions
after adverse monitoring results, such as low residual biocide levels, and listing those to be
informed of an event).
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS