Legionella bacteria cause Legionnaires' disease and **Pontiac
fever**. They become a health concern when they grow and spread in **human-made
building water systems**:
- Showerheads and sink taps
- Hot tubs
- Decorative fountains and water features
- Large, complex plumbing systems
- Cooling towers — structures containing water and a fan, part of
centralised air-cooling for buildings or industrial processes
The one nobody thinks of
**Legionella can also grow in the windscreen washer fluid tank of a
vehicle** — and **it is more likely to grow if the tank has been filled with
water rather than genuine screenwash.**
Screenwash contains alcohols and detergents that suppress bacterial growth.
Water does not. A tank topped up with water, sitting warm in an engine bay,
then sprayed as a fine mist directly in front of an open window — that is the
transmission route in miniature.
It matters most for people who spend their working day in a vehicle, which is
why the CDC publishes specific guidance for commercial vehicle operators.
What is not a risk
**Home and car air-conditioning units do not use water to cool the air, so
they are not a risk for Legionella growth.** That includes window units, of
the kind found at some hotels.
Worth knowing, because "the air conditioning gave me Legionnaires'" is a
common belief. The risk from air conditioning is in the cooling tower of a
large centralised system — water evaporating on a rooftop — not the unit in a
room.
How it reaches people
Breathing in mist is the most common way. Water containing Legionella
spreads in droplets small enough to inhale, and inhaling them causes
Legionnaires' disease or Pontiac fever.
The other route is swallowing water into the lungs — aspiration, where
water goes down the wrong way.
**In general, people do not spread Legionnaires' disease or Pontiac fever to
other people.** It is a building's disease, not a contagious one, which is why
the response to a case is an investigation of plumbing rather than of
contacts.
Source: Centers for Disease Control and Prevention.
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