Legionella pneumophilia normally grows inside free living amoebae in the environment- in fact the bacterium can't grow on its own as it needs an essential amino acid (cysteine) and iron for growth.  When Legionella are accidentally inhaled into the lungs, the bacterium infect white blood cells – and can cause pneumonia (see Health Effects). Legionella can be found naturally in freshwater environments like lakes and streams. It can become a health concern when it grows to high levels and is released in airborne water droplets in human-made water systems like:

  • Hot tubs that aren’t disinfected or drained after each use
  • Hot water tanks
  • Large plumbing systems
  • Cooling towers (air-conditioning units for large buildings)
  • Decorative fountains
  • Emergency Showers
  • Humidifiers

Warm, stagnant water provides ideal conditions for growth. At temperatures between 68°-122°F (20°C-50°C) the organism can multiply. Temperatures of 90°-105°F (32°C-40°C) are ideal for growth. Rust (iron), scale, and the presence of other microorganisms can promote the growth of Legionella – primarily by providing an environment for amoebae growth. Protected within an amoeba cyst (the environmentally resistant resting stage), Legionella pneumophila may survive for months and can be resistant to high levels of disinfectants.

Routine system maintenance is critical to controlling Legionella growth, from maintaining a minimal disinfectant residual, to flushing and cleaning of deposits, and ensuring that hot water stays hot and cold water stays cold. Legionella bacteria is described in World Health Organization (WHO) and CDC documents.