Evaluating Sources of Exposure

Key points

  • Some water systems and devices are commonly implicated sources of Legionnaires' disease outbreaks.
  • Evaluating these sources helps to determine how and why Legionella got into the building and spread to make people sick.

Potable water

Potable water outbreaks are typically less "explosive" than outbreaks associated with cooling towers. However, without remediation, building water systems colonized with Legionella may cause disease over years before a problem is recognized.

Potable water can come from either a public utility or from a private source. The U.S. Environmental Protection Agency (EPA) regulates public water quality and requires a minimum amount of disinfectant to be present (i.e., disinfectant residual).

Building owners are responsible for maintaining the quality of the water once it enters the building water system(s). However, buildings that have their own disinfection systems may also be subject to EPA and/or state regulations.

Water quality, including disinfectant level, can drop as water travels through building water systems. Residual disinfectant supplied by the public utility is lost as organic matter consumes it, it naturally off-gases, and as water ages. Heating the water speeds up loss of residual disinfectant in pipes or fixtures.

Next steps if potable water is implicated

Consider implementing immediate control measures.

Cooling towers

A cooling tower is a structure that contains water and a fan as a part of a centralized air-cooling system for a building or industrial processes. Cooling towers remove unwanted heat from the system by exposing heated water to cooler air.

In contrast, home AC units don't use water to cool. This means they don't aerosolize water and aren't a risk for Legionella growth and spread.

Identifying cooling towers

In areas with many buildings, identifying all cooling towers can be challenging. However, some areas are starting to require registration of cooling towers to track their locations.

Using Geospatial Information Systems (GIS) technology may be useful for aerial cooling tower identification. Some cooling towers are located on the ground or on sides of buildings and may be difficult to see from aerial photography.

A physical assessment, street view imagery, or checking with building owners and managers is necessary in some circumstances.

Learn strategies for identifying cooling towers.

Next steps for implicated devices

Ensure any implicated cooling towers have been shut down, but not drained or hyperchlorinated, before sampling.

Hot tubs and pools

Being in or near a hot tub or hydrotherapy tub while it's turned on is a possible exposure risk. These devices can aerosolize water containing Legionella.

Legionella are unlikely to grow in typical swimming pools because water temperatures are usually too cold.

Next steps for implicated hot tubs

Ensure any implicated hot tubs are turned off, but not drained, before sampling.

Next steps for implicated pools

Sample pools if they're associated with a possible exposure or temperatures are within the permissive range (i.e., 77–113°F).

Decorative fountains

Decorative fountains are a possible exposure source for Legionella, particularly in enclosed spaces.

Submerged lighting and warm ambient temperatures in fountains can contribute to Legionella growth.

Next steps for implicated devices

Ensure any implicated fountains are turned off, but not drained, before sampling.