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Cooling Towers: Potential Risk for Legionella Outbreaks?

Rising Need for Legionella Testing in Cooling Towers

By Dr. Michael Berg, EMLab P&K Senior Molecular Biologist

Legionnaires' disease is considered to be fairly common in the U.S. and the Legionella organism is one of the top three causes of sporadic, community-acquired pneumonia. Approximately 1,000 cases of Legionnaires' disease are reported every year to the CDC, but it is estimated that more than 25,000 cases of the disease occur annually causing over 4,000 deaths.

Legionella pneumophila was first identified by the CDC in 1977 after an outbreak of pneumonia causing 34 deaths at the 1976 American Legion Convention in Philadelphia. The hotels' cooling tower was identified as the likely source of the disease. Disease transmission commonly occurs via inhalation of aerosolized water contaminated with the bacteria. Cooling towers present a special risk for outbreaks of Legionnaires' disease and have been determined as the source of many severe outbreaks.

Cooling Towers

Cooling Towers
Courtesy of Wikimedia Commons.

Cooling towers are designed to remove heat from a building or facility by spraying water down through the tower. As air comes in from the sides of the tower passing through the falling water, heat is exchanged and some of the water evaporates. The heat and evaporated water flow out the top of the tower in the form of a fine, cloudlike mist. Cooler water is collected at the bottom of the tower and pumped back into the system. Cooling towers are often used for heating, ventilation and air conditioning (HVAC) purposes in large office buildings, schools, hotels and hospitals. Because they contain large amounts of warm water, they present an ideal breeding ground for Legionella bacteria if they are not properly disinfected and maintained. A recent study in Greece (Mouchtouri et al, 2010) found that almost half of the tested cooling towers contained Legionella bacteria and 23% of them required remedial action.

The Occupational Safety and Health Administration (OSHA) recommends the following action levels for Legionella bacteria in cooling towers and evaporative condensers:

Action 1 (100 cfu per ml): Cleaning followed by biocide treatment of the system, if appropriate.
Action 2 (1,000 cfu per ml): Cleaning or biocide treatment and immediate steps to prevent employee exposure.

It is important to keep in mind that the probability of infection with Legionella does not only depend on the intensity of exposure or concentration of the organisms. It is also a function of host susceptibility. The likelihood of Legionella bacteria causing disease is significantly higher in smokers, patients with weakened immune systems or chronic lung disease, and in people that are 65 years of age or older. Action may be required at lower thresholds if risk factors for susceptibility are increased, such as cooling towers in hospitals or nursing homes.

1. Mouchtouri VA, Goutziana G, Kremastinou J, Hadjichristodoulou C (2010). Legionella species colonization in cooling towers: risk factors and assessment of control measures. Am J Infect Control. 38(1):50-5.

2. CDC: Cooling Towers

3. HC Info: Recent Outbreaks of Legionnaires' Disease

4. OSHA: Legionnaires' Disease


This article was originally published on April 2011.