Last month, the Centers for Disease Control and Prevention notified Miami-Dade health officials that three tourists, all of whom stayed at the luxury EPIC Hotel, had contracted Legionnaires’ disease, a severe form of pneumonia. Investigators initially attributed the outbreak to low levels of chlorine in the hotel’s drinking water, which presumably allowed Legionella to contaminate the water supply; however, the chlorine levels are not likely to have been the primary factor in the infection of the Miami tourists because Legionella are resistant to conventional chlorine practices.

Legionella are bacteria that are found universally in natural, uncontaminated waters and soils. They flourish in temperatures between 40 and 50^oC and can multiply if suitable nutrients are present. There are 35 recognized species of Legionella.

Initially, air conditioning systems contaminated with Legionella were thought to be the main cause of Legionnaires’ disease. Epidemiological studies indicate, however, that Legionella contamination of water piping systems, particularly hot water systems in hotels, hospitals, and other institutions housed in large buildings, are more likely the cause. Temperature ranges in water heaters and water pipes in such institutional systems sometimes favor the organism’s proliferation.

Humans are rarely infected by drinking water contaminated with Legionella. The main threat of Legionella to human health is inhaling mists or droplets containing this pathogen. Shower heads can become a reservoir for these bacteria. Other favored environments for Legionella include spray nozzles, spas, hot tubs, and whirlpool baths. Legionella is also frequently associated with biofilms (thin surface films of microorganisms), scaling, and sediment in water systems. In such environments, bacteria can find necessary nutrients as well as protection from disinfectants.

Legionella are resistant to conventional chlorination practices. The following disinfection methods have been used to control this microorganism in institutional water systems:

1. Hyperchlorination
2. Chlorine dioxide
3. Copper-silver ions
4. Superheat and flush
5. Ultraviolet light
6. Tankless flash heaters

Hyperchlorination is usually carried out in either of two ways, shock hyperchlorination or continuous hyperchlorination. Shock hyperchlorination involves elevating the chlorine dosage to obtain a specified high level throughout the water system and holding it there for a sufficient time to kill the Legionella. Water is then drained and the chlorine level reduced. Continuous hyperchlorination entails elevating the chlorine levels on a sustained basis well above the levels normally maintained in a water system, but not as high as in shock chlorination. This method is frequently combined with the superheat and flush method (see below).

Chlorine dioxide (ClO₂) has been used successfully in Europe since the mid 1990s to control Legionella in institutional water systems. Chlorine dioxide is effective against many microorganisms and is more potent than many other disinfection methods over a short contact time.

Copper-silver ion disinfection for Legionella control in institutional water supplies continues to gain acceptance. It is generally installed only on the hot water system prior to heating. The cold water piping depends on the chlorine residual in the service line. There appears to be a synergistic effect when copper-silver ion and conventional chlorination are used together.

Superheat and flush was the first method used for Legionella control in hot water systems. It can be used on an emergency basis during an outbreak or utilized intermittently to control or suppress widespread contamination of the system. Unfortunately L. pneumophilia, the bacteria that cause Legionnaires’ disease, can re-establish itself in a hot water system within a month or two, so disinfection is only temporary. This method is sometimes used in conjunction with continuous hyperchlorination.

UV light has been used for Legionella control, particularly in re-circulating hot water systems so as to provide continuing exposure. UV treatment for Legionella is usually carried out in combination with filtration to assure water clarity so that UV exposure will not be reduced. Because there can be re-growth of Legionella in a pipe following exposure to UV, it is most effective when installed near faucet or shower outlets. UV is sometimes used in combination with routine chlorination to suppress or prevent re-growth.

Finally, tankless flash hot water heaters have also been employed to reduce Legionella. These devices eliminate the problem of thermal stratification in conventional hot water heaters, but they do not eliminate Legionella in biofilms on the pipe walls, in shower heads, etc. The closer they are to the outlet, the more effective they are.

The actual cause of the outbreak of Legionnaires’ disease in Miami has not yet been conclusively determined. Lack of a chlorine residual could have been a contributing factor because, while this level of chlorine helps suppress the proliferation of Legionella, it is insufficient to inactivate the bacteria. Nevertheless, a residual of chlorine-based disinfectant is indispensable because it is effective against the majority of bacteria and viruses and other germs that commonly cause waterborne disease, and it provides continuing protection as water journeys from the treatment plant to your glass.

For more information about the safety benefits of chlorinated water, please visit our website.

(Fred Reiff, P.E., is a former official of the Pan American Health Organization/ World Health Organization and a member of the Water Quality & Health Council).

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