Facts about Legionellosis

Water treatment consultant and author Dr. Joseph Cotruvo recently called Legionaries’ Disease, or Legionellosis, “the most important waterborne disease in the United States.”

According to Dr. Cotruvo, Legionellosis has been a reportable disease only since 2001. The disease is not caused by ingestion of the water, but rather by inhalation of aerosols such as during showering or from inhaling blow down from cooling system heat exchangers, or probably even humidifiers. Those at particular risk are the elderly and especially people with impaired immune systems such as those who are hospitalized and in extended care facilities.  

It is well known that legionella are detectable in a high percentage of plumbing systems, including in homes, hotels and other buildings, and a substantial number of people in the general population are susceptible because of their ages or health or immune status. Several hospital-related outbreaks of legionella-related diseases are reported annually around the world, but undoubtedly most are not identified or attributed to the water system. CDC has estimated that between 8,000 and 18,000 people are hospitalized with Legionnaires’-related disease each year in the U.S. It is not known how many of those are attributable to exposure from the plumbing or cooling systems.

Legionella microorganisms are common environmental and soil bacteria. Water treatment at the central plant is capable of removing them from influent water, but if even a few enter distribution/plumbing systems, through the plant or during main breaks or from being present on pipe interior surfaces at installation, the water distribution environment is conducive to colonization. They can proliferate at temperatures in the range of about 25oC  to 55oC (77oF to 130oF) such as can be found in hot water systems, shower heads and in cold water systems in warm climates, or during warm times of the year. Maintaininghot water temperatures below 120 F for energy efficiency and to reduce water heating costs can actually create an ideal condition for legionella proliferation.


Treatment is a real challenge and a cookie cutter approach is not likely to be widely successful. The problem of controlling microorganisms colonizing plumbing and distribution systems is not trivial because many of them are associated with biofilms or protozoa such as amoebas and can be protected by them. Disinfectants that have been employed have had limited success. They include chlorine, chloramine, chlorine dioxide, ozone, UV light, copper/silver ionization and shock thermal and steam treatments. Each of them has its benefits and weaknesses, and often a combination of treatments must be applied on a regular basis supported by monitoring to indicate the conditions of the system and the time to re-treat.

Chlorine is a powerful disinfectant, but even when applied in a temporary hyperchlorination mode (e.g., 50 ppm for several hours) total eradication may not be achieved. Chloramines, which are far less potent than free chlorine, have demonstrated considerable success in reducing legionella counts in some water plumbing systems. This could be due to the lower chemical reactivity of chloramine and greater hydrophobicity that allows greater penetration into biofilms.  Ozone and UV might have some efficacy in recirculating systems, but they will be primarily effective against organisms in the water column. Copper/silver in combination and individually have shown successes when they are properly managed and maintained. Shock thermal treatment for several hours at temperatures above 70oC have shown temporary success, but a complete strategy would require a combination of initial biofilm cleanout with a disinfectant system that will retard regeneration of the biofilm.

The concept of final barrier protection has value in situations where sufficient risk exists. For example, instant hot water delivery systems leave a smaller volume of water to stagnate and provide a growth environment.

Dr. Cotruvo concludes that pathogenic microorganisms such as legionella frequently colonize water plumbing systems, and they now are the most significant public health risk associated with drinking water. Many illnesses and deaths are attributable to that problem and drinking water standards are not designed to deal with them. Indeed, the existing standards may actually result in increased public health risks because they provide a disincentive for health care facilities to take corrective actions. The risk benefit balance is clearly in favor of eliminating the burdens that are imposed upon those facilities, so that they will be more likely to take actions to reduce risks to their patients.