Legionella is a type of bacterium found naturally in freshwater environments, like lakes and streams. It can become a health concern when it grows and spreads in human-made building water systems like: Showerheads and sink faucets; Cooling towers (structures that contain water and a fan as part of centralized air cooling systems for building or industrial processes); Hot tubs that are not drained after each use; Decorative fountains and water features; Hot water tanks and heaters and Large plumbing systems. Home and car air-conditioning units do not use water to cool the air, so they are not a risk for Legionella growth.
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