Strategies for Identifying Cooling Towers

Key points

  • Use our stepwise procedure and strategies to locate cooling towers.
  • In an outbreak scenario, identifying cooling towers quickly and accurately is essential to limiting the number of people exposed.
Photo of a cooling tower with a visible water pipe in front of it


Cooling towers can cause outbreaks of Legionnaires' disease when not adequately maintained. In an outbreak scenario, identifying cooling towers quickly and accurately is essential to limiting the number of people exposed. However, searching for cooling towers during an outbreak can be difficult due to

  • Building density
  • Poor visibility
  • Issues with accessibility

Use our steps and strategies -- alone or in combination -- to locate cooling towers. Additional research and development of new cooling tower identification technologies may improve speed and accuracy by reducing opportunities for human error.


All cooling towers have three telltale characteristics:

  • A fan
  • An open basin of water
  • Water pipes leading in and out of the basin


See images of cooling towers in a variety of shapes and sizes.

Cooling towersA are often confused with refrigerant-based air conditioner (AC) units. These general characteristics and features of cooling towers help distinguish them from AC units:

  • Most buildings have some type of air cooling system, either cooling towers or AC units.
    • Buildings without a readily visible air cooling system warrant further investigation.
  • Cooling towers are typically located on the roof or near the building.
  • Fans are usually located on the top or side of cooling towers.
    • Side mounted fans are difficult, if not impossible, to see from aerial photographs.
    • Oblique (tilted) aerial imagery or street view may aid in identifying side-mounted fans.
  • Compared to AC units, fans in cooling towers tend to
    • Be larger
    • Have more spacing between the fan blades
    • Spin slower (This helps distinguish cooling towers in aerial photographs. When in motion, fan blades in AC units appear as a "blur" while those in cooling towers do not.)
  • Water lines run between the cooling tower and building, while AC units have air ducts or small refrigerant lines.
    • Water lines are typically made of cast metal or plastic, while air ducts are made of sheet metal.
    • Usually, water lines are smaller and rounded. Air ducts tend to be larger and angular.
  • Investigators with experience identifying cooling towers may notice cooling towers make a distinct "hum" or a "cascading water" sound.

Cooling towers are heat rejection devices that transfer heat to the atmosphere through evaporation. Most common uses include air conditioning and removing heat from commercial and industrial processes. Cooling towers are inherently more efficient than air conditioners. As such, certain types of buildings are more likely to have cooling towers.

Look for larger buildings with significant air conditioning requirements, such as

  • Hospitals
  • Hotels
  • Schools
  • Places of worship
  • Multi-story office buildings

Look for buildings housing commercial or industrial processes, such as

  • Tool and die manufacturers
  • Cold storage facilities
  • Plastics injection molding and chemical manufacturers
  • Data centers

Keep in mind that cooling towers may be in operation during the winter because industrial processes sometimes use them.

Home and car AC units do not use water to cool the air. For this reason, they are not a risk for Legionella growth or spread.

When investigating a community outbreak of Legionnaires' disease, use epidemiologic data to define and prioritize the cooling tower search area(s). Collect a detailed history of each patient's whereabouts during the 14 days before illness onset. Consider using a tool such as a hypothesis generating questionnaire. In addition, the following patient (or proxy) interview tips may help collect and capture possible exposure locations:

  • Refer to a calendar for dates, days of the week, and holidays.
  • Ask about activities for each of the 14 days before illness onset. Options include:
    • Begin 14 days before onset and work forward day-by-day.
    • Begin with the day before onset and work backwards day-by-day for 14 days.
  • Ask patients to review or have items available during the interview to enhance recall, including:
    • Their calendar or daily planner
    • Receipts or credit card statements
    • Text messages or emails
  • Ask about holiday activities if any holidays fell in the 14 days before onset.
    • For example, "Last Tuesday was the Fourth of July. Do you remember if you did anything for the holiday?"
  • Ask about specific potential destinations such as:
    • Work
    • Grocery stores and shopping centers
    • Restaurants
    • Homes of friends or relatives
    • Community events
    • Medical appointments
    • Religious services
    • Bank visits
  • Ask about the specific route and mode of transportation taken from place to place.

Obtain a suitably large map of the local area. Indicate patients' residences, destinations, and transit routes to look for any spatial clustering. Most cooling towers implicated in outbreaks were within a 0.5-to-1 mile radius of the geographic center of possible exposure locations1234567.

To investigate temporal aspects of potential exposures, record patients' whereabouts on a spreadsheet. Each row should represent one patient, and each column should represent one calendar day.

Search strategies

Aerial and satellite imagery

Visually scanning aerial or satellite images is the most common cooling tower identification technique. It is often the first search technique employed in an investigation. The key advantages of aerial and satellite imagery include:

  • Satellite images are easily obtained for almost any area
  • Large areas can be visually investigated relatively quickly
  • Investigators do not have to be physically present at the site

The primary disadvantages are:

  • The technique is subject to human error
  • Many cooling towers may be difficult to see if they are:
    • Small
    • Hidden
    • Underground
    • Obscured by shadows
  • Cooling tower fans on the side of the unit may be difficult to see
  • Maps may be out of date
    • This is especially true for rural areas or areas of rapid growth

Consider making a search grid and marking suspected devices as "possible" or "probable." Consider the potential for investigators to experience eye strain and fatigue when searching for extended periods of time.

On-the-ground scouting

On-the-ground scouting or searching is an essential search technique. Investigators can either drive or walk the area of interest. Ground searching is accurate and may generate opportunities to gather additional information from locals. Disadvantages include:

  • Investigators must be on-site
  • Slow and time consuming
  • Parking can be problematic
  • Cooling towers can be hidden from public view
  • Safety concerns when accessing cooling towers

Industry Sources

Industry professionals may know the locations of cooling towers in their area. Consider contacting cooling tower manufacturers, service representatives, chemical sales representatives, and Legionella control and remediation consultants.

This technique requires minimal investment of time and resources. In addition, investigators do not have to be physically present within the area of investigation. However, investigators must consider investigation implications when contacting industry professionals during ongoing investigations. Industry professionals may feel uncomfortable providing information due to privacy or liability concerns.

Utility water use records

Cooling towers use or discharge large volumes of water. For this reason, officials at the local water and wastewater utilities may know about cooling towers in their domain. Upon request, utility managers may provide a list of their customers with the largest volume water use. Investigators can use this list to look for buildings likely to have cooling towers.

Permitting or registration records

Some jurisdictions require permits or registrations for cooling towers. Investigators should contact local officials, either directly or via local contacts, for a list of permitted or registered cooling towers. It can be difficult and time consuming to determine which cooling towers are located within the search zone. In addition, permitting and registration records may be outdated or incomplete.

Future technologies

Technology is likely to have an impact on cooling tower identification. Computer-aided scanning of satellite or aerial photographs, direct-image satellite scans, and artificial intelligence have immense potential to advance cooling tower identification.

  1. Cooling towers are often part of the centralized air cooling system for buildings or industrial processes. Importantly, these structures contain water and fans to remove heat from the air. In this process, inadequately maintained cooling towers can create aerosols (droplets of water in the air) that contain Legionella bacteria. The heat-rejection fans in cooling towers then spread these bacteria.
  1. Benowitz I, Fitzhenry R, Boyd C, et al. Rapid identification of a cooling tower-associated Legionnaires' disease outbreak supported by polymerase chain reaction testing of environmental samples, New York City, 2014–2015. J Environ Health. 2018;80(8):8–12.
  2. Weiss D, Boyd C, Rakeman JL, et al. A large community outbreak of Legionnaires' disease associated with a cooling tower in New York City, 2015. Public Health Rep. 2017;132(2):241–50.
  3. Burckhardt F, Brion A, Lahm J, et al. Confirming Legionnaires' disease outbreak by genome-based method, Germany, 2012. Emerg Infect Dis. 2016;22(7):1303–4.
  4. Sala Ferré MR, Arias C, Oliva JM, et al. A community outbreak of Legionnaires' disease associated with a cooling tower in Vic and Gurb, Catalonia (Spain) in 2005. Eur J Clin Microbiol Infect Dis. 2009;28(2):153–9.
  5. Sabria M, Alvarez J, Dominguez A, et al. A community outbreak of Legionnaires' disease: Evidence of a cooling tower as the source. Clin Microbiol Infect. 2006;12(7):642–7.
  6. Nhu Nguyen TM, Ilef D, Jarraud S, et al. A community-wide outbreak of Legionnaires disease linked to industrial cooling towers — How far can contaminated aerosols spread? J Infect Dis. 2006;193(1):102–11.
  7. Rota MC, Pontrelli G, Scaturro M, et al. Legionnaires' disease outbreak in Rome, Italy. Epidemiol Infect. 2005;133(5):853–9.