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Legionella
in Water Supplies
Legionellas
are commonly found in small numbers in collections
of fresh water, but there they probably do no
harm. They can survive normal drinking water disinfection,
and may enter the mains water supply for homes
and industry. Once in the distribution system,
they may multiply (particularly if the water temperature
is favourable or if the system contains excessive
sediment) and susceptible people may be exposed
to the organism via sprays generated by water
taps or shower heads. This is much less likely
to be a problem in domestic water supplies because
the turnover of the supply does not allow the
water to stagnate and the legionellas to multiply.
Air
conditioning systems may constitute the greatest
hazard. The organisms multiply in the water of
cooling towers in which a cascade of water is
used to cool air in pipes or vanes. The water
does not come into direct contact with the air,
but a great deal of it is lost by evaporation
or as spray, and as cooling towers are usually
situated in the roofs of buildings the clouds
of droplets containing legionellas may be drawn
into the air intakes of the building or fall on
people passing by. Humidification systems have
also been incriminated as a source of infection.
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