LEGIONELLA - World Health Organization

Prevention of low flow and stagnation
Preventing low flow rates and stagnation of water is an essential and important control measure,
and the system should be designed to minimize areas of stagnation and low flow. Care should
be taken to ensure that any modifications to the system do not introduce areas of stagnation
and low flow. Where such areas are unavoidable, design and operation should aim to at least
reduce stagnation and low flow.
For example, around mixing valves, the outlet should be as close to the valve as possible.
Commercially available thermostatic mixers can be fitted into the outlet to minimize the
zone at risk of being colonized by bacteria.
Water systems at risk from stagnation should be periodically flushed or disinfected, and
temperatures that are optimal for growth of Legionella should be avoided. However, where flushing
is used, the likely exposure of people to aerosols generated during flushing must be considered.
Control of temperature
Keeping water temperature outside the ideal range for legionellae is an effective control
measure for both hot and cold-water systems (Figure 3.3 shows the effects of temperature on
survival and growth of Legionella).
Water systems should:
• avoid water temperatures between 25 °C and 45 °C to prevent Legionella colonization
• ideally, maintain cold water below 20 °C
• ideally, maintain hot water above 50 °C.
In many systems (e.g. cooling towers and some cold and hot-water systems), maintaining these
temperatures is not possible because of the nature of the system. Within such systems, temperatures
should be maintained at the upper or lower limits of the Legionella multiplication range.
In domestic and public hot-water systems, control measures for reducing the proliferation of
Legionella must not increase the risk of scalding, particularly for children, the elderly or
people with disabilities.
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS