Legionella and Where It Grows

The organism and the illness

Legionella bacteria occur naturally in water at low concentrations and become dangerous when they multiply in engineered water systems. Inhalation of contaminated aerosol can cause Legionnaires’ disease, a severe pneumonia with a significant fatality rate, and milder illnesses. It is not transmitted from person to person.

People most at risk include older people, smokers, those with chronic respiratory or kidney disease, diabetes, and anyone whose immune system is impaired. A building serving such a population — healthcare, care homes, sheltered accommodation — carries substantially higher consequence and correspondingly stricter requirements.

Conditions that allow proliferation

• Temperature — the bacteria multiply between roughly twenty and forty-five degrees Celsius, with the most rapid growth around the middle of that range. Below about twenty they remain dormant; above about sixty they are killed rapidly. This single fact drives the entire control regime: keep cold water cold and hot water hot.
• Stagnation — water that does not move. Infrequently used outlets, dead legs, redundant pipework and oversized storage all allow water to sit.
• Nutrients — scale, sediment, rust, organic matter and biofilm on internal surfaces provide food and shelter. Biofilm protects bacteria from disinfection, which is why physical cleaning matters as much as chemical treatment.
• Aerosol generation — the route of infection. Showers, spray taps, hoses, humidifiers, fountains, vehicle wash, misting systems and cooling towers all produce breathable droplets.

Systems that present the risk

• Hot and cold water services in any building — the most common systems requiring control.
• Cooling towers and evaporative condensers — the highest risk, capable of dispersing aerosol over considerable distances and responsible for the largest outbreaks. In many jurisdictions they must be notified to the local authority.
• Spa pools and hot tubs — very high risk because of warm water and intense aerosol generation.
• Humidifiers and air washers that generate spray.
• Emergency showers and eyewashes — rarely used, and therefore stagnant by definition.
• Fire sprinkler and hose reel systems, vehicle wash systems, water features, irrigation and misting.
• Dental and medical equipment with water lines.

Why it is a facilities priority

Outbreaks have caused multiple deaths, prosecutions, imprisonment and very large fines. Investigations almost invariably find the same pattern: no adequate risk assessment, or an assessment whose recommendations were not implemented; monitoring not carried out or not recorded; temperatures known to be out of range and not corrected; and nobody clearly accountable. None of these is a technical failure. They are management failures, and they are entirely preventable.