Legionnaires' disease is a global public health issue. According to CDC's Morbidity and Mortality Weekly Report (August 2011), Legionnaires' disease increased 217% between 2000 and 2009. The disease-causing bacterium, Legionella pneumophila, is a waterborne pathogen found in natural and man-made water systems. Both potable and non-potable (utility) water supplies harbor Legionella pneumophila, and have been linked to outbreaks of both hospital- and community-acquired Legionnaires' disease.  

Ask The Experts

The following FAQs provide general and technical information about the disease. If you can't find the answer you're looking for, please click Ask the Experts tab and submit your question.


What is the upper limit of Legionella for hospital water distribution systems?

What is the upper limit of Legionella for hospital water distribution systems?

In contrast to the situation for cooling towers, there is solid scientific data available for interpretation of culture results from hospital water distribution systems. Risk assessment should not be based on the concentration of Legionella recovered from a given water outlet; quantitation (CFU/mL) does not correlate with incidence of disease (CDC, Infection Control and Hospital Epidemiology 1999, Kohler, Journal of Hospital Infection 1999, Pittsburgh Special Pathogens Laboratory [Best] Lancet 1983).

On the other hand, risk for Legionella infections increase as the extent of colonization with L. pneumophila increases (i.e., a high percentage of water outlets yield Legionella). In two studies, Legionnaires’ disease did not occur unless the percentage of water outlets reached 30% or greater (Best Lancet 83, Stout, in press).

Complete elimination of Legionella from a hospital water system is not necessary to eliminate cases of Legionnaires’ disease in the hospital (Pittsburgh Special Pathogens Lab [Stout], Infection Control and Hospital Epidemiology, 1998).

Monitoring of copper-silver in hospital water systems?

We have had a copper-silver ionization system on our hot water system for more than a year now. No cases of hospital-acquired Legionnaires’ disease have occurred since installation, but the copper and silver ion levels are sometimes less than the recommended range by the manufacturer (copper 0.2-0.8 ppm; silver at 0.02-0.08 ppm (20-80 ppb). Is this cause for alarm or justification to perform additional disinfection like a superheat and flush?

Our experience indicates that ion levels slightly below the manufacturer’s recommended levels are still effective in systems that have been operational for awhile. For example, we have had copper levels in the range of 0.1-0.2 ppm and silver at 0.01-0.02 ppm (10-20 ppb). Our environmental cultures remain well below our threshold of 30% positivity (0-10% most of the time). I would request either cleaning of the electrodes or adjustments in amperage if the level of copper is at or below 0.1 ppm copper or 0.01 (10 ppb) silver.

We would not recommend that a decision to perform a superheat and flush be based on ion levels. This procedure is labor intensive and should be performed in response to either detection of a hospital-acquired case of Legionnaires’ disease or an increase in colonization of the system to 30% or above.

Microbiologically influenced corrosion (MIC) and Legionella?

Part of our job is to analyze fire protection water supplies. One of our nemesis is microbiologically influence corrosion (MIC). When looking for a lab to test for MIC, I received information from one of the testing laboratories who indicated where there is MIC, Legionella bacteria probably will also exist. Is this true? If so, are there documented cases?

We do not know of any documented cases linking MIC to Legionnaires’ disease. The major mode of transmission of  Legionella is through aspiration. This would require a susceptible human to ingest water containing a concentration of  Legionella bacteria. Since consumption of water from fire protection systems is unlikely and assuming your employees are generally in good health, the risk for Legionnaires’ disease is minimal.


Inability of chlorine dioxide to reduce Legionella counts to zero?

I have a few questions regarding a facility using chlorine dioxide (ClO2). Our infection control group has changed the rooms that are cultured for Legionella to reflect more of the high risk areas (ICU, heart transplant wards, HIV, etc.). The first round of testing revealed a few positives in these areas. All the counts were very low, only 0.2 to 1.3 cfu/mL. Swabs taken from these locations were negative. We claimed that these low numbers certainly reduced risk; however, the physicians wanted to reach zero colonization all the time. We believe that it is impossible to reach zero. Are we correct in assuming this?

You are correct. Unfortunately, this is a major flaw for the CDC guidelines for transplant units, which does specify a zero cfu limit. It is nearly impossible to achieve zero colonization all the time. Quantitation of cfu/mL is not an accurate indicator for disease, but proportion of sites yielding Legionella is.


Risk of nosocomial Legionnaires’ disease is better predicted by the proportion of water system sites testing positive for Legionella than by the concentration of Legionella bacteria. (Kool JL, et al. Infect Control Hosp Epid 1999; 20:797-05.)

No correlation between quantitative counts and Legionella cases. There was a correlation between site positivity (>30%) and cases (Best et al. Lancet 1983;307-310.)

No correlation between quantitative counts and Legionella cases (Kohler JR et al. J Hosp Infect 1999; 41:301-311).

In our recent experience, no hospital-acquired cases of Legionnaires’ disease were diagnosed at two hospitals during our evaluation of Cl02 despite the persistence of Legionella pneumophila in the water system. However, it took many months to significantly reduce the level of Legionella in the system; the percent positivity rate was <30%. Moreover, it took many months of chlorine dioxide disinfection to significantly reduce the level of Legionella in the system of both hospitals (Sidari JAWWA 2004).

Can Legionella resistance to chlorine dioxide occur?

The question was raised about L. pneumophila building up immunity to low concentrations of ClO₂ around 0.8 ppm in hot water.

I am not aware of any literature demonstrating resistance to 0.8 ppm chlorine dioxide – either Legionella or other bacteria. You state that the level of chlorine dioxide in the hot water is around 0.8 ppm. Is that correct? We have never seen that level in hot water.


Should sink aerators be removed?

Aerators on sinks, should they keep them or get rid of them?

We would remove them from patient care areas. We have found aerators and laminar flow devices harbor higher concentrations of bacteria in general. Removal of aerators or laminar flow devices may necessitate adjustment of water flow to avoid splashing.

Respirator or dust mask?

What type of breathing protection should you use if you have to work near a source of water that has tested positive for Legionella? Will a dust mask, type 3M 8210, be ok or do you need to wear a respirator?

If your work involves aerosols of water or mist, then a P95 respirator is ideal. If it does not, then a dust mask is ok. Breathing aerosols of water vapor or drinking the water can lead to Legionnaires’ disease. Touching the water is not a risk.

Personal protective equipment for cooling tower workers?

I have a contractor working in a cooling tower known to be infected with Legionella. What special personal protective equipment do they need to work in that area? They are wearing tyvex suits, rubber boots, rubber gloves, and a half mask respirator. Is there any chance they might need a full face respirator due to the mucous membranes in the eyes?

A P95 half face mask respirator is sufficient to prevent inhalation of  Legionella that may be in an aerosol from the tower water. Legionnaires’ disease is not transmitted via mucous membrane exposure. The rest of the equipment that you mention will protect them from the chemicals commonly used to treat cooling towers.


Ozone as disinfectant for Legionella in drinking water?

In the recent article by Lin, Stout and Yu (Feb 2011), they tested a number of water disinfection methods for hospitals, but did not include ozone in the tested methods. Can you tell me why? Thank you.

Ozone has been studied by our group in vitro and an in vivo plumbing model (Muraca P. Appl Environ Microbiol 1987); it was effective against Legionella. It was also installed in a Los Angeles hospital (Edelstein P. Appl Environ Microbiol 1982); evaluation was equivocal. Ozone was included in a review of disinfection methods by our group in 1988 (Muraca PW. JAWWA 1988). Ozone was also briefly discussed in our 1998 review (Lin YSE. JAWWA 1998).

Lower risk of Legionella from instantaneous steam (tank-less) heaters?

 I am a drinking water engineer at a Department of Health. The Health Department by statute is a member of the state Plumbing Board. Recently, the Plumbing Code was updated and due to the Health Department’s efforts to reduce the risk of Legionella, all water tanks must now be maintained 140 degrees F. To prevent scalding, mixing valves are required, so that at the tap, consumers don’t receive water hotter than 120 degrees F. at any tap/shower.

We have been approached by the manufacturers and lawyers of tankless heaters for a waiver. Their argument is that tankless heaters don’t have a reservoir where Legionella could amplify, their product is simply a flash heater of the cold water line. As we don’t treat cold water per se for Legionella, I think their argument holds merit, but I deferred to our nurse epidemiologist. She provided me with the ASHRAE standards which don’t exactly address this situation, but do quote 140 degrees for health care facilities hot water storage and quoted the Legionella growth temperature range of 77-108 degrees F.

Would you be able to comment on tankless heaters and the efficacy of holding them to a standard promulgated specifically to reduce Legionella risk?

We once thought, as you suggested, that elimination of the large volume hot water heater would significantly reduce  Legionella colonization in the hot water system. In a survey of 15 hospitals in western Pennsylvania, in 2 hospitals with instantaneous steam heating systems (tankless heaters) no Legionella was isolated from these hospitals. We then replaced the 1000+ gallon hot water tanks in one of our VA hospitals with instantaneous heaters in an effort to decrease Legionella colonization. These heaters flash heated the cold water to 190o and then blended the water back down to the pre-set temperature (which was 140oF). Disappointingly, this installation had virtually no effect on the downstream colonization of fixtures. We now understand that the entire network of pipes is coated with a slime layer (biofilm) within which  Legionella resided. The bulk of the  Legionella colonization of the water system was downstream of the hot water tanks! This is why active disinfection throughout the system is required to control Legionella bacteria in warm water systems. So, if the temperature of the blended water as it exits from the tankless heater is 140oF, then that should satisfy the Health Department’s objective.

Is plastic pipe more conductive to Legionella growth therapy than copper pipe?

The water piping in my house is plastic instead of the commonly used copper piping. Does that increase the risk of contracting the Legionella bacteria?

Various plumbing materials have been evaluated for their ability to support the growth of Legionella (and other bacteria). Copper pipe is inhibitory, whereas plastic (PVC) pipe will support the attachment and subsequent growth of bacteria- including Legionella. If the hot water system temperature is maintained at or above 130oF, growth will be inhibited regardless of pipe materials.

Hydrogen peroxide and ozone disinfection?

I’m a medical student at Karolinska Institute in Sweden currently involved in a project about prevention of Legionnaires’ disease, and I have found your website to be a valuable resource.


I’m writing about the use of free radicals in the elimination of Legionella in water systems. Is this something you are planning on evaluating? The technique is very interesting and has been proven very efficient on the elimination of Legionella species – among others – in water systems.



We are assuming you are speaking of the free radicals of oxygen obtained by disinfecting with ozone or hydrogen peroxide. We have not investigated hydrogen peroxide in our laboratory.

The Yamagiwa et al article presents a view of hydrogen peroxide (Yamagiwa K, et al. (2001) Disinfection Kinetics of Legionlla pneumophila by Hydrogen Peroxide. Journal of Chemical Engineering of  Japan. 34(8),1074-1077). Hydrogen peroxide is slightly more stable than ozone, but maintenance of a sufficient residual in large drinking water systems may still be difficult. Additionally, we are not sure in the  U.S. if hydrogen peroxide could be legally used as a drinking water disinfectant. If hydrogen peroxide is effective it may be most useful in applications such as spas and whirlpools.

There have been numerous other studies of ozone which compares the recommended concentration with other oxidizing biocides. Ozone was effective in our model plumbing system ( Muraca Appl Environ Microbiol 1987). One note on ozone, it is very reactive and as such does not produce a residual to carry disinfection effects into a distribution system. This can be a problem for piping systems which have an established biofilm harboring Legionella far from the point of ozone application.

For copper-silver systems, converting ionization cell current to ppm?

For copper-silver ionization systems, what are the calculations to convert ionization cell current (assume 100mA) to ppm for a given water flow rate please? Assume pH of 7 and potable water.

The generating capacity is flow dependent. At 6 gallons per minute, the system will generate 0.5ppm (mg/L) per amp. If using 70:30 electrodes: 7 amps at 6 gpm = 3.5 ppm copper or 0.5 ppm per amp. At 100mA output and a 6 gpm flow rate. 0.05 ppm of copper would be generated.


Does scaling of pipes predispose to Legionella contamination?

I am a Senior Director of Engineering for retirement communities. We had a problem in the first two buildings there. Scaling had built up on the copper pipes as the water softening system was not put into operations until 6 months after opening. We suspected that the scaling encouraged the harboring of Legionella bacteria.


1) Is it a reasonable assumption that scaling on pipe assists the bacteria in harboring and subsequently colonizing?

2) As galvanized pipe naturally encourages scale build-up, is galvanized pipe a poor selection for domestic water systems?

3) Does the size of the pipe make a difference?

1) Scale does provide a surface area for bioflim accumulation, but it is not neccessarily a critical factor. We have demonstrated rapid colonization of a newly constructed building. (Legionnaires’ disease in a newly constructed long-term care facility. J. Am. Geriatric Society. 48:1589-1592, 2000)

2) We believe that galvanized pope is not the best choice for water distribution systems. Once of our never facilities was constructed with extensive use of galvanized pop in both the cold and hot water systems. They are now having to replace large sections of pipe due to corrosion. New copper piping is inhibitory to Legionella. We learned this since our early model water systems were constructed of copper; it was difficult to keep Legionella alive in this system.

3) We do not think pipe size is a factor, but we have never tested this hypothesis. We have found that higher flow causes more Legionella to be deposited on the pipe surface due to mass transfer.

Disinfection technology for ice machines?

Is there a specific technology (e.g., in-line UV light unit) that might be appropriate to use with a hospital ice machine to decrease the risk of Legionella contamination of the unit/ice? We are contemplating purchase of new machines for selected units in the hospital. 

There are numerous point-of-use (POU) treatment technologies that are commercially available. These technologies typically do not provide a disinfecting residual and include techniques such as UV and filtration. We have not tested any of these technologies in our VA lab so we cannot provide a recommendation on the “best” one.

There have been recommendations to use simple shock chlorination of the ice- machine to disinfect. Go to and see MMWR, Vol. 52 (RR10):1-42 Guidelines for Environmental Infection in Health-Care Facilities. You may also wish to talk to your ice-machine vendor about the pre-packaged disinfecting systems, although we have no experience of the validity of the claims of these vendors.

Remember POU technologies only disinfect where they are installed, so without proper maintenance you can still have contamination downstream from the point of disinfection (i.e. in the ice-machine). You may want to shock-disinfect the machine immediately after installation to help insure the parts down-stream from any POU system are Legionella free, the POU system will help prevent Legionella from reaching the ice-machine and colonizing it.


Can hydronic radiant heating systems transmit Legionella?

What are the chances of getting Legionella from open system of hydronic radiant heat where you get your house heating and drinking water from the same source?

The risk from hydronic heating is unknown, but we know of no cases of Legionnaires’ disease from such sources. However, one can contract Legionella from home water distribution systems and large building water systems. But the risk is extremely low and close to zero if you are not a cigarette smoker. And, if you were a smoker, I would quit smoking and not worry about the hydronic heat.

Legionella in sea water?

Has Legionella ever been found in sea water? I work as an engineer at a desalination plant in the Canaries and our plant provides the water to the community. The query is: can the bacteria be sourced from the sea water used? The water is chlorinated prior to distribution.

We have no direct experience with the presence of Legionella in seawater. However, we surmise that seawater would not be a likely source for Legionella. Legionella can be isolated from rivers and lakes, but we know of no survey of Legionella in sea water.

An indirect piece of evidence: Legionella can be killed by a high concentration of sodium ions, a major component of seawater.


Multiple questions on disinfection

I have multiple questions regarding disinfection.

1) The majority (>90%) of cases of Legionnaires’ disease reported in the US are caused by L. pnuemophila. Is it correct that L. anisa is frequently isolated from the environment but it almost never causes disease?


2) Are the normal rules and regulations (>55°C, no dead legs/stagnation, circulation loop maintenance, monitoring, etc.) sufficient to control the presence of Legionella in hot water systems?


3) What is the “infective dose” for contracting Legionnaires’ disease?


4) Is there a Legionella risk from a condensate drain of an air conditioner?


5) Does the scaling of pipes predispose to Legionella contamination?


6) Air conditioners are not a source of Legionnaires’ disease?


7) Can ozone be used as a disinfection for Legionella in water systems? Is ozone used at all to disinfect contaminated (or suspect) areas?


8) Should cooling tower workers use personal protective equipment?


9) When water samples are positive for Legionella, is it common or rare to find more than one species present?


10) Should we use aerators on sinks?

1) Correct. L. anisa is generally nonpathogenic and disinfection is not needed when this species is present.

2) These factors are insufficient and while often recommended, may not affect Legionella growth. The only way to be sure is to culture your water system. Download the Stout ASHRAE 2007 article for more information.

3) Quantitative cultures are not predictive, but percent site positivity is predicitive for water systems.

4) In our opinion, there is no risk.

5) Yes, due to the presence of biofilm.

6) No. This belief is a myth.

7) There is one published attempt with ozone in a Los Angeles hospital, but the results were inconclusive.

(Muraca, P. Stout, J. Yu, VL. Comparative assessment of chlorine, ozone, and ultraviolet light for killing of Legionella pneumophila within a model plumbing system. Appl Environ Microbiol. 1987; 53:447-453.)

8) No comment. Ask the CDC.

9) Presence of multiple species of Legionella in water is common.

10) We discourage their use because of their predilection for biofilms.

Characteristics of a polypropylene pipe?

If I were to use polypropylene pipe that does not corrode, would this have the same negative characteristics as PVC piping?

There have been several studies done looking at the effects of different plumping materials on microbial and Legionella growth in water systems. Any internal surface within a water system will eventually establish a biofilm growth. The speed and extent of the biofilm growth does vary with the material, however there is no conclusive data correlating biofilm quantity to Legionella (i.e. water system will have biofilm but there may or may not be Legionella growth).

There is evidence indicating increased biofilm and Legionella growth on various types of piping material with elastomeric materials, appearing to best support biofouling in short term studies. Long term studies (over 200 days) have shown that piping material may not factor in influencing biofilm or Legionella growth.

Regarding your specific question, a 1994 study by Rogers et al. found the same order of magnitude of microorganism growth and L. pneumophila colonization on polypropylene and PVC during the 28 day model system experiment, suggesting no difference between these specific piping materials.

Based on the current data, there would not be a significant microbial difference between polypropylene and PVC piping when used in water system construction.

Chlorine for disinfection of a hospital?

We are trying to put together a quick estimate on what it will cost to upgrade our water system to combat Legionella growth. One of the methods being investigated is the use of a disinfection systemfor the treatment of water coming into the hospital.I found a publication on your site from 2011 that provided some estimated costs for different systems, but there was no cost listed for a system using chlorine. We are just looking for a ballpark figure, but I have no idea if this would be a few thousands or a few hundred thousands. Could you give me a rough estimate on what it would cost to get a system installed at a 250 bed hospital and a 190 bed nursing home?

Chlorination should not be used as long term disinfection for two reasons:

1) Lack of efficacy and corrosion of the water distribution system longterm.

2) Moreover, carcinogens are being added to drinking water.

The Pittsburgh VA installed a chlorination system in November 2012 with disastrous results – failure to eradicate Legionella and corrosion within three months.

The 2011 publication (Lin 2011) you reviewed on our site is the a great source reviewing disinfection options. Before deciding on a disinfection modality, you should survey the water supply of the hospital and nursing home to ascertain whether Legionella is present in >30% of the water sites. If it is not, we would not recommend installation of a disinfection system. Other less expensive options are available. Our lab (Special Pathogens Laboratory) performs environmental water cultures for numerous VA facilities and could do so for yours as well.

Galvanized water tanks and Legionella?

I work for an education board. We are refurbishing two toilet blocks (six sinks in each) and have discovered that the toilets are supplied off a large galvanized water tank in the roof space. The tank seems well sized and has a good through-put of water, with feed and supply to the tank on opposite ends. Does an old galvanised water tank hold any inherent dangers of Legionella?

Legionella can colonize or contaminate any water tank, including new tanks. The location of the tank (on the roof) could result in heat gain during warmer months. So despite having a cold water supply (low Legionella risk) the tank could acheive a temperature conducive to Legionella growth.

Galvanized pipes/tanks will result in higher iron in the water. Higher iron has been associated with a greater probability for Legionella.

The situation where you should be concerned about Legionella is in buildings housing individuals at risk for Legionnaires’ disease (nursing homes, hospitals, etc.). Legionella may not be a risk in your situation, but you should test the water for Legionella to provide evidence for future steps.

Resistance of Legionella to chlorine?

There are claims that Legionella can survive at a chlorine level of 5 ppm at pH 7.0-7.5 in tap water. What is your opinion of this?

There are multiple reasons for the relative resistance of Legionella to chlorine. These include an outer membrane with ubiquinones and other fatty acids, intracellular location within cysts of amoebae, and genetic responses to oxidative stress from exposure to chlorine.