In the summer of 1976, Legionella was thrust into the public spotlight following an outbreak at an American Legion Convention in Philadelphia's Bellevue Stratford Hotel. A total of 221 cases of pneumonia, and 34 deaths, occurred among the more than 4,000 convention attendees as a result of exposure to this previously unidentified bacteria. It wasn't until 1977 that the offending agent, Legionella pneumophila, was finally identified by the Centers for Disease Control (CDC).

However, this was not the first outbreak. Retrospective studies have shown a number of pneumonia outbreaks which we now know were caused by Legionella The earliest documented outbreak of Legionnaires' disease occurred in 1957 in Austin, MN. A total of 78 persons, 46 of whom were employees at a local meat packing plant, were hospitalized with acute respiratory disease of unknown cause. Eventually two of the patients died.1

In July and August of 1964, at least 81 patients at the St. Elizabeth Medical Center, a large psychiatric hospital located in Washington DC, developed pneumonia after being exposed to Legionella bacteria. Seventeen of the patients died as a result of the disease. Evidence suggests that the bacteria may have originated from extensive soil excavations that were being carried out on the campus of the hospital during the summer months.2

The Ecology of Legionella

Legionella are gram-negative rod-shaped bacteria that are ubiquitous in freshwater environments, occurring naturally in surface waters including lakes, rivers, streams, and mud. Legionella apparently survive in low numbers in routine water treatment used to produce potable water. They can be carried in the treated drinking water into buildings, where they colonize in the plumbing fixtures, especially in hot water systems. Cooling towers and other systems, therefore, may become contaminated through the makeup water.

Legionella is known to grow in water temperatures between 68°F and 122°F, with optimal growth occurring between 95°F and 115°F. It will not multiply at temperatures below 68°F, and cannot survive in water above 140°F. Other factors affecting growth include a pH range of 2.0 to 9.5, the presence of L-cysteine-HCL and iron salts, sediment in water which supports the growth of supporting microbiota, and free-living aquatic amoebae and other protozoa. Algal slime provides a stable habitat for the multiplication and survival of Legionella.

In addition, Legionella is an intracellular parasite on other bacteria and protozoa. Once it insinuates itself into a larger bacteria, it resists the host defenses and then multiplies.3 Legionella ,due to its susceptibility to desiccation and ultraviolet radiation, tends to be shortlived in the air.4

Currently, at least 35 species and 54 serogroups of Legionella have been identified, with more than 20 of these species linked to human diseases. Some serogroups are even further separated into subtypes. Over 80% of all legionellosis is caused by L. pneumophila, with serogroup 1 being the most frequently identified form of the bacterium isolated from patients with Legionnaires' disease.5 However, other serogroups and subtypes of the bacterium are also frequently isolated from water sources and often are linked with disease.

Although many other species have not yet been associated with human diseases, this may be because of their extremely rare occurrence in nature. As such, until proven otherwise, all strains of Legionella should be considered potentially pathogenic.6

Health Risks and Legionella Bacteria

Legionella is rather unusual in that exposure to it can cause two different types of disease: Legionnaires' disease, a severe form of pneumonia, and Pontiac fever, a nonfatal flu-like illness. The major mechanism or route of infection is inhalation of aerosolized water droplets or particles containing Legionella bacteria. No person-to-person spread of the disease has ever been documented, and ingestion of contaminated water does not appear to cause disease, either.7

Pontiac fever is a self-limiting, influenza-like illness which does not cause pneumonia. It has a very high attack rate, affecting over 95% of exposed individuals, and usually appears 36 to 48 hrs after exposure. Symptoms typically disappear within 2 to 5 days of onset.8, 9

This variant first appeared in 1968 in Pontiac, MI, hence the disease's name. Ironically, 95 out of 100 employees in the Oakland County (MI) Health Department became sick. The CDC sent three investigators into the building who subsequently became sick. They were replaced by three other investigators who also became sick. The problem was eventually traced to an evaporative condenser in the basement, which was vented within 6 ft of the hvac fresh air intake.10

Symptoms of Legionnaires' disease can vary from a mild cough and low fever, to a rapidly progressive pneumonia and coma. Early symptoms, which commonly appear three to nine days after exposure, often include malaise, muscle aches, and a slight headache. Subsequent symptoms can include a high fever of up to 105°F, a dry cough, and shortness of breath.

Legionnaires' disease has a low attack rate (around 5%), but it boasts a fairly high fatality rate (10% to 15%).11 Among elderly or immunosuppressed patients, the fatality rate is even higher - up to 80% if left untreated.12 Approximately 1,000 cases of Legionnaires' disease are reported each year to the CDC. However, it is estimated that upwards of 25,000 cases of Legionnaires' disease (range 10,000 to >100,000), and 3,000 to 7,000 subsequent deaths occur in the U.S. annually.13, 14

In order to cause disease, the bacteria must be virulent, present in sufficient quantities to cause disease, aerosolized from its water source, and inhaled deeply into the lungs. Once there, it can evade the host defenses by parasitizing phagocytes, in much the same way as salmonella and tuberculosis bacteria infect people. Legionella is characterized as an opportunistic disease which most frequently attacks elderly or immunocompromised/immunosuppressed individuals.

Additional host-specific risk factors include being a male over 50 years of age, cigarette smoking, diabetes, kidney failure requiring dialysis, organ transplants, corticosteroid therapy, AIDS, or other underlying diseases such as cancer. However, even healthy individuals can acquire the disease if they are exposed to a high enough airborne concentration of the bacteria.15

The mere presence of Legionella either in heat rejection systems or water services will not in itself cause disease. High numbers of Legionella have been noted in cooling towers and other sources with no associated disease. Most outbreaks from cooling towers and evaporative condensers have been associated with high numbers of Legionella, 1,000 colony forming units per ml (cfu/ml) or more, in the implicated water source. In a potable water system associated with an outbreak, the reported numbers of Legionella averaged 160 cfu/ml (range <1 to 1,500 cfu/ml), while as few as 10 cfu/ml in a fogger reservoir may have caused disease.16 Using the various available data regarding outbreaks, a quantitative Legionella risk assessment criteria was subsequently developed by one laboratory and subsequently referenced by OSHA.17, 18

Legionnaires' disease can occur as either epidemic clusters (11% of all reported cases) or sporadic (89%), and can be hospital-acquired (23%) or community-acquired (77%).19 The disease has been documented nearly worldwide, including in England, Australia, and New Zealand. Interestingly enough, to date all reported outbreaks/epidemic clusters have been caused by L. pneumophila, while sporadic cases have been caused by a variety of species including L. pneumophia, L. micdadei, L. bozemanii, L. dumoffii, L. longbeachae, L. anisa, and other Legionella spp. 20

Hospitalization with appropriate antibiotic therapy is essential in order to reduce the chance of fatality. The most commonly used antibiotic for Legionnaires' disease, erythromycin, does not actually kill Legionella but prevents it from multiplying intracellularly, thereby giving the immune system a chance to work. A second antibiotic, rifampin, may be prescribed for more severe cases. Even with treatment, though, 7% to 9% of previously healthy individuals, and 24% of individuals with seriously impaired immune systems, will still die.21

Amplifiers and Disseminators

A variety of building systems have been linked to Legionellosis, including cooling towers, evaporative condensers, domestic hot water systems, spas, whirlpools, humidifiers, decorative fountains, faucets, showerheads, and even vegetable misters in supermarkets.22 Plumbing systems have been implicated in numerous outbreaks, including cases in nursing homes, workplaces, and private residences.

Hot water systems tend to be ideal breeding habitats for Legionella bacteria. Legionella can flourish in a hot water system, particularly if the temperatures are maintained below 140°F. Scale and biofilm buildup within the hot water tank and the piping not only furnish the bacteria with food, but also provide protection from high temperatures and any chemical disinfectants. Legionella are also often found in dead legs, or unused sections of piping, where the stagnant water can provide an ideal environment for amplification. Sampling data from 107 New York City commercial buildings, collected between January 1995 and October 1998 by our laboratory, indicated that 18.4% of the hot water system samples contained Legionella bacteria at detectable levels. Studies in residential hot water systems have shown Legionella contamination in 30% and 32% of the heaters tested.23, 24 Another separate study, conducted at a Canadian hospital over a four-year period, showed 24.7% of the samples were positive for L. pneumophila.25

Both cooling towers and evaporative condensers have been verified as potential disease transmitters, 26, 27 although evaporative coolers have not.28 Cooling towers provide all of the elements needed for Legionella to grow - warm water, algae, fungi, protozoa, dirt, and biofilm. Contrary to what some water treatment companies may claim, studies indicate that there is no significant correlation between total bacteria counts and Legionella concentrations in cooling towers.29 The only way to determine if Legionella is present, is to sample and analyze for it specifically.

Sampling data from 51 New York City commercial buildings, collected between January 1995 and October 1998 by our laboratory, indicated that 31% of the cooling tower system samples contained Legionella bacteria at detectable levels. In a comparable study, where 304 samples were collected from commercial building cooling towers in an urban area, 16.4% of those were found to have Legionella present.30

Control of Legionella

In order to reduce or eliminate the risk of Legionellosis, it is necessary to minimize the concentrations of Legionella in the affected system, and/or prevent the transmission of Legionella bacteria from the affected system to susceptible individuals.

Decontamination of contaminated hot water systems usually consists of thermal pasteurization, or "superheating" as it is commonly referred to. Superheating simply consists of raising the water heater temperature to at least 158°F (70°C) for 24 hrs, and flushing each outlet for 20 min. The temperature at the outlet should reach at least 140°F (60°C). Prior to performing superheating, all dead legs should be removed because these stagnant areas can "re-seed" the system following treatment.

For older buildings, where there may be significant scale buildup inside the piping, additional flushing time may be necessary. This is because the scale acts as an insulator and helps protect the Legionella bacteria buried within it from temperatures necessary to kill them. The insulating properties of scale can be quite significant, as shown in Table 3. One possible treatment method in older buildings is to leave each outlet open just a fraction, enough to allow a slow stream of water flow passing through the piping over a period of 8 hrs or so. This should allow enough time so that the entire scale layer becomes heated to over 140°F.

Periodic chlorination is another method of controlling Legionella in hot water systems. Scale and sediment are first removed from the tank. Sodium hypochlorite is then added until approximately 10 ppm of free residual chlorine is produced. Since the biocidal activity of chlorine is very sensitive to pH levels, the pH of the water should be maintained between 7.0 and 7.6. All taps are then flushed until the distinct odor of chlorine is evident, and the water should be allowed to stand for 1 hr. The tank is then drained and refilled, and each outlet flushed to remove the chlorine from the system.31

There are two disadvantages to chlorine shock treatment vs. superheating in hot water systems. The first is fairly obvious: Chlorine is very corrosive and will shorten the service life of metal plumbing. The second is that chlorine will not penetrate any scale layers, and therefore, any Legionella buried within these layers can recontaminate the system after a short period of time following treatment.

Continuous chlorination of water distribution systems with 2 ppm free chlorine following superheating, has been shown to be effective in controlling Legionella.32 However, a silicate-based solution also needs to be added in order to help control the corrosion of the piping.

Alternative methods to control Legionella growth include the use of metal ions (see sidebar above) such as copper or silver in solution. Findings have shown that when these ions are absorbed by bacteria, they affect the organism's enzyme balance which then inhibits its reproduction and respiration capabilities.33 Commercial, in-line ultraviolet (UV) systems have proven effective in controlling Legionella and can be installed on incoming water lines or on recirculating systems. However, stagnant zones, such as dead legs, can diminish the effectiveness of this treatment, as can scale buildup on the UV lamp surface. Ozone injected into the water is yet another method to control Legionella.34

Biological treatment of cooling towers usually consists of the addition of algaecides and bactericides, such as gluteraldehyde. These chemicals help to control multiplication of bacteria, protozoa, and algae. This, in turn, reduces the amount of microbiota available to Legionella as a nutrient source. However, the control of other microorganisms does not necessarily indicate the control of Legionella, so any testing performed to determine the effectiveness of treatment must include sampling for viable Legionella bacteria. Biocides should be administered via a continuous feed rather than slug dosing, in order to maintain consistent concentrations in the system.

Cleaning of the system is very important in the overall control of Legionella. Without any cleaning, a buildup of algae, fungi, protozoa, dirt, and/or biofilm can occur in the system. Such buildup can place a greater demand on a biocide, and may prevent it from making contact with microorganisms. Studies have indicated that bacteria sequestered in amoebae are afforded a 30- to 120-fold increase in protection from water treatment regimens.35 At a minimum, cooling tower systems should be bled (blowdown) and flushed twice a year. All surfaces should be thoroughly cleaned and allowed to air dry.

For cooling towers that have been implicated in outbreak or cluster of Legionnaires' disease, or where high levels have been found, emergency treatment is necessary. One such procedure calls for the slug chlorination of the system with 50 ppm of free residual chlorine, along with a dispersant. Then maintain 10 ppm of free residual chlorine for 24 hrs. As noted earlier, the pH must be within a range of 7.5 to 8.0 for the chlorine to be effective. The entire system is then drained and the chlorination process repeated.36 Bromine, which is less affected by pH levels, can be used in place of chlorine.

Since even well-maintained towers can still be colonized by Legionella, it is important to eliminate any possible transmission of drift from the tower to people. The first step, obviously, is the use of high-efficiency drift eliminators. Such eliminators, however, may actually aid in Legionella growth by increasing the system temperature.37

Next, if possible, locate the cooling towers as far away as possible from operable windows, outdoor air intakes, and outdoor areas frequented by people. At a minimum, outdoor air intakes should be located at least 25 ft from any cooling towers. In addition, prevailing wind directions must be taken into account with regards to air intakes and cooling towers.


Legionella bacteria tend to be an unwanted occupant of the building environment. Their ability to contaminate domestic water systems, coupled with their potential to cause severe health complications, presents a very real concern to building owners and managers. Fortunately, there are methods readily available for both the detection of Legionella and its control. Establishing a proactive approach of periodic testing, along with proper water treatment in both cooling towers and hot water systems, is the best way to avoid disease outbreaks and potential lawsuits. ES

End Notes

1 Osterholm, M.T., Chin, T.D., Osborne, D.O., Dull, H.B., et al: "A 1957 Outbreak of Legionnaires' Disease Associated with a Meat Packing Plant," Am. J. Epidemiol. 117 (1):63-67 (1983).

2 Korvick, Joyce A., Yu, Victor L., Fang, Guo-dong: "Legionella Species as Hospital-Acquired Respiratory Pathogens," Seminars in Respiratory Infections, vol. 2, no. 1 (March), 1987 pp. 34-47.

3 Fliermans, Carl B.: "Legionella Ecology" in Burge, Harriet A., ed. Bioaerosols, CRC Press (Boca Raton, FL 1995), pp. 49-76.

4 Dillon, H.K., Heinsohn, P.A., Miller, J.D., eds.: Field Guide for the Determination of Biological Contaminants in Environmental Samples, American Industrial Hygiene Association (Fairfax, VA 1996) pp. 97-117.

5 Occupational Safety and Health Administration (OSHA): OSHA Technical Manual, Section II: Chapter 7, Legionnaires' Disease, (Washington, DC 1996).

6 Morris, George K., Shelton, Brian G.:"Legionella in Environmental Samples: Hazard Analysis and Suggested Remedial Actions," PathCon Laboratories, Technical Bulletin 1.4 (1994).

7 Zwadyk, Peter: "Legionellaceae" in Joklik, W.K., Willett, H.P., Amos, D.B., Wilfert, C.M., eds., Zinsser Microbiology, 20th ed., Appleton & Lange (East Norwalk, CT 1992) pp. 694-699.

8 Benenson, Abram S.: Control of Communicable Diseases in Man, 15th ed., American Public Health Association (Washington, DC 1990) pp. 235-238.

9 Hung, Ling-Ling L., Yang, Chin S.: "Legionella Bacteria in Office Buildings," Enviros: The Health Building Newsletter, vol. 2 no. 3 (March 1992).

10 Kaufmann, A.K., McDade, J.E., Patton, C.M., et. al.: "Pontiac Fever: Demonstration of its Mode of Transmission," Am. J. Epidmiol. 114:337-374 (1981).

11 American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE): "Legionellosis Position Statement, Legionellosis Position Paper," ASHRAE (February, 1989).

12 Edelstein, Paul H.: "Control of Legionella in Hospitals", J. Hosp. Infection 8:109-115 (1986).

13 Millar, J.D., Morris, G.K., Shelton, B.G.: "Legionnaires' Disease: Seeking Effective Prevention," ASHRAE Journal, January, 1997, pp. 22-29.

14 Centers for Disease Control (CDC): Questions and Answers on Legionnaires' Disease, No. 28L0343779, DHHS, CDC (Atlanta).

15 American Industrial Hygiene Association (AIHA): "Draft - Taskforce on Legionnaires' Disease," AIHA Indoor Environmental Quality Committee (unpublished).

16 Morris, George K., Shelton, Brian G.: "Legionellå Environmental Samples: Hazard Analysis and Suggested Remedial Actions." PathCon Technical Bulletin 1.4 (Norcross, GA 1994).

17 Ibid., p. 2

18 Occupational Safety and Health Administration (OSHA): OSHA Technical Manual, Section II - Chapter 7, Legionnaires' Disease (Washington, 1996), p. 32.

19 Yang, Chin S.: "Evaluation and Management of Legionella Bacteria in Buildings," P&K Microbiology Services, Inc. (Cherry Hill, NJ 1997).

20 Burge, Harriet A, Feeley, James C.: "Indoor Air Pollution and Infectious Diseases" in Samet, J.M., Spengler, J.D., eds., Indoor Air Pollution, Johns Hopkins University Press (Baltimore, MD 1991) pp. 273-284.

21 Fliermans, Carl B.: "Legionella Ecology" in Burge, Harriet A., ed. Bioaerosols, CRC Press (Boca Raton, Fl 1995) pp. 49-76.

22 Hoge, Charles W., Breiman, Robert F.: "Advances in the Epidemiology and Control of Legionella Infections," Epidemiologic Reviews, vol. 13, Johns Hopkins University School of Hygiene and Public Health (1991).

23 Arnow, Paul M., Weil, Diane, Para, Michael F.: "Prevalence and Significance of Legionella pneumophila Contamination of Residential Hot-Tap Water Systems," J. Infect. Dis. 152:145-151 (1985).

24 "Legionella and Domestic Water Heaters in the Quebec City Area", Can. Med. Assoc. J. 132:160 (1985).

25 Marrie, T.J., Haldane, D., Bezanson, G., Peppard, R.: "Each Water Outlet is a Unique Ecological Niche for Legionella pneumophila," Epidemiol. Infect. 108:261-270 (1992).

26 Dondero, T.J., Rendtorff, R.C., Mallison, G.F., et. al.: "An Outbreak of Legionella Disease Associated with a Contaminated Air Conditioning Cooling Tower," N. Engl. J. Med. 302:365-370 (1980).

27 Centers for Disease Control (CDC): "Legionnaires' Disease Associated with Cooling Towers - Massachusetts, Michigan and Rhode Island, 1993," MMWR 43:491-499, July 15, 1994.

28 Puckorius, P.R., Thomas, P.T., Augspurger, R.L.: "Why Evaporative Coolers Have Not Caused Legionnaires' Disease," ASHRAE Journal, January, 1995, pp. 29-33. 29 Cappabianca, R.M., Jurinski, N.B., Jurinski, J.B.: "A Comparison of Legionella and Other Bacteria Concentrations in Cooling Tower Water," Appl. Occup. Environ. Hyg. 9(5):358-361 (1994).

30 Ibid., p, 359

31 American Society for Testing and Materials (ASTM): Standard Guide for Inspecting Water Systems for Legionellae and Investigating Possible Outbreaks of Legionellosis (Legionnaires' Disease or Pontiac Fever), ASTM D 5952-96.

32 Snyder, M.B., Siwicki, M., Wireman, J., Pohlod, M., et. al.: "Reduction in Legionella pneumophila through Heat Flushing Followed by Continuous Supplemental Chlorination of Hospital Hot Water," J. Infect. Dis. 162:127-132 (1990).

33 Wilsey, Charles A.: "Alternative Water Treatment for Cooling Towers," ASHRAE Journal, April 1997, pp. 43-46.

34 Muraca, Paul W., Yu, Victor L., Goetz, Angella: "Disinfection of Water Distribution Systems for Legionella: A Review of Application Procedures and Methodologies," Infect. Control Hosp. Epidemiol. 11(2):79-88 (1990).

35 Bowman, E.K., Vass, A.A., Mackowski, R., Owen, B.A., Tyndall, F.L.: "Quantitation of Free-Living Amoebae and Bacterial Populations in Eyewash Stations Relative to Flushing Frequency," Am. Ind. Hyg. Assoc. J. 57:626-633 (1998).

36 Wisconsin Department of Health and Social Services: Control of Legionella in Cooling Towers: Summary Guidelines, Wisconsin Division of Health, August, 1987. 37 Freije, Matthew R.: Legionella in Cooling Towers, Engineered Systems, July 1998, pp. 64-70.