Water Treaters


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.  

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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.

Water Treaters

Presence of multiple Legionella species in hospital water?

When water samples are positive for Legionella, is it common or rare to find more than one species present? Does the presence of one species necessarily indicate others are present, but perhaps not detectable by the lab methods in use?

It is not unusual to find multiple Legionella species and serogroups in a water system.  There is no data to indicate that the presence of one species correlates or predicts the presence of another species. Legionella anisa is a common colonizer of water distribution systems. However, we believe that this species is unlikely to be a pathogen.

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

I am an industrial microbiologist currently employed as a consultant for the management and control of Legionellosis to a large number of hospitals in the UK. A question which is now being asked with increasing frequency is the matter of the “infective dose”. Is there an infective dose for Legionella?

For cooling towers, no such dose has been calculable. (One reason it has been well nigh impossible to calculate such a dose is that cooling towers are probably not the source of the legionella in most of the reported outbreaks).

For water distribution system, it has been established that quantitative counts of Legionella from the distal fixtures (faucets) do not correlate with the incidence of the disease in hospitals. On the other hand, 4 studies have found that when distal site positivity is 30% or higher, cases of hospital-acquired Legionnaires’ disease occur (Stout JE. Infect Control Hosp Epidemiol, 2007).

Chlorine and other oxidizing biocides as treatments for cooling towers?

Are chlorine and other oxidizing biocides the only recommended treatments for cooling towers?


There are effective non-oxidizing biocides. The ASHRAE guideline (available at www.baltimoreaircoil.com) does not recommend a specific biocide, but does recommend that you consult with a water treatment specialist.


Maintenance of hot tubs?

What is the best procedure for preventing Legionella becoming a problem in my hot tub? I currently use chlorine as a disinfectant but I am concerned that sometimes this runs out and I understand that the bacteria can build up in the air injector lines. Also if there is chlorine in the water and Legionella builds up in the air lines is the Legionella killed off when the air is turned on when it passes through the chlorinated water?

Legionella must be in contact with the active biocide (chlorine, bromine, etc.) for a sufficient amount of time and at a concentration of the biocide that is active against Legionella. So, control of Legionella would occur in the liquid phase. Maintain the biocide concentration at the recommended level, monitor your biocide levels and keep the system clean. This should keep your system under control.

We also believe some of the outbreaks attributed to hot tubs were incorrect and the drinking water was the actual source.


Legionnaires’ disease in the workplace?

Has Legionnaires’ disease been reported from the workplace? I need information about that.


Many incidents of Legionnaires’ disease in the workplace have been reported. We reported two cases at a plastic injection mold plant. P Muraca, JE Stout, VL Yu, YC Yee. Legionnaires’ disease in the work environment: implications for environmental health. Am Ind Hyg Assoc J 49:584 590, 1988. Another reference is Fry et al, Legionnaires’ disease outbreak in an automobile engine manufacturing plant. J Infect Dis 2003; 187: 1015-1018.

Legionella survival in sea water cooling towers?

A client of ours is installing an evaporative sea water cooling tower. Can Legionella survive in sea water cooling tower conditions? Are there any published papers covering sea water applications?

There is some anecdotal evidence that water samples collected from sea water cooling towers were negative for  Legionella. Survival of Legionella in marine environments is limited by both salinity and warmer temperature.


Legionella quantitations in a floor drain and sink?

We have found Legionella in a sink and floor drain of a building on campus, some of the results have come back as “present, but below quantifiable levels”, and others in the 500 cells/swipe area. I really don’t know what these numbers mean. One of the occupants has been diagnosed with a “Legionella-like disease”, who believes that when the plumbers worked on the system the previous week “caused her disease”. This doesn’t make any sense to me, however. Does the 500 number sound high, normal, etc.? Does this mode of infection make any sense?

Given that the mode of transmission of Legionella bacteria is either via exposure to intense aerosol or aspiration (after drinking contaminated water or after direct instillation into the upper airway), it is not common to culture the drain of a sink or a floor drain. A more appropriate location is the sink faucet or the water in the hot water distribution system. We already have an answer on the web site to your question regarding culture results and quantity vs. extent of colonization on the website. Quantitation is not a useful parameter to gauge risk.

Regarding the individual with the “Legionella-like disease”, our first question is whether a diagnostic test for Legionnaires’ disease was performed (Legionella urine antigen test or respiratory culture) and whether this test was positive. If a diagnosis of Legionnaires’ disease cannot be confirmed, then you need not perform any environmental investigation.

Risk of Legionella from a condensate drain of an air conditioner?

Does the condensate drain on a conventional domestic air conditioner provide a risk of catching the Legionella disease. It has been suggested by my local authority that the reason for connecting the condensate drain to sewer is to reduce the risk of contracting Legionella.

We are not aware of any cases of Legionnaires’ disease acquired from a condensate pan or drain. There would be little opportunity for significant exposure. One benefit of connecting the drain to the sewer is that there might be no need to chemically treat the condensate pan. These pans can often be the source of foul smelling bacteria and molds.

Risk of Legionella from a decorative water wall in a hospital?

As part of a renovation/new building plan, an executive would like to put a water wall in the entrance of the hospital. Five glass panels will be put up and water will be recirculated for continuous flow. Do you see any potential problems with this presentation? In addition, there will be no barriers to prevent people from putting their hands in the water and creating a diversion of the water flow. 

We discourage the installation of decorative fountains in hospitals. However, the water wall in the photo that you provided, aerosolization is unlikely, so the risk would be very low. The water fountains can be treated with disinfectants such as chlorine or bromine.

Our building is colonized with Legionella.

Our site is a Victorian mansion with new buildings attached over the years. The main building is listed, which makes improvements very problematic. A registered water treatment company annually takes samples which return positive readings on Legionella. The company chlorinates the roof header tank and retakes samples. The result will return a lower reading but we just cannot seem to get Legionella out of the system. Recently the water treatment company reported in one part of the building a reading of 825 Legionella pneumophila serogroup (2-14) before chlorination, and a reading of 150 after chlorination. Another part of the building gave a reading of 300 cfu/mL before chlorination and 75 cfu/mL after chlorination. The company advised us to put the hot water on constant to kill the remaining bacteria. Further tests were taken afterwards and the results were still positive. A survey indicated that the run from the boiler is so great that the water loses temperature. We have improved the pumps, used stronger chemicals, chlorinated more frequently – all at a great cost. We have even fitted new boilers, but to no avail. Are we doing enough to satisfy our legal responsibilities/Health & Safety requirements? Or is our only answer to change all the old rusty pipework? This would cause a great problem, as it would put much of the building out of use for some time. The building is also listed, and any works require authorization.

Legionella commonly colonizes large buildings and even homes and residences. However, most people have drunk Legionella without any adverse effects. We advocate culturing of hospital water supplies, because patients  with underlying diseases congregate there. On the other hand, we do not advocate routine culture or disinfecting residential or building water supplies. So, the answer to your question is: We would not recommend routine culture of the water supply of this  mansion. Chlorination longterm will corrode your water system, and the risk to residents or workers within this building is low regardless of the presence of Legionella.

Our building is still colonized with Legionella.

Thank you very much for your reply. It has been very helpful. However, on meeting with the water treatment company yesterday, he has now advised us that our samples after a year returned a reading of nearly 6000 cfu/mL, and not 825 cfu/mL as we had originally stated. If we chlorinate more regularly we could keep the level to below 1000, cfu/mL, but this would also rot the pipes. Does this new reading alter the advice that you very kindly offered in your previous email?


Chlorination is ineffective long-term against Legionella. Note that there is no clear-cut need to disinfect your  mansion. Keep in mind in your neighborhood all large buildings and 10% of homes are colonized  with Legionella. None of them are undergoing disinfection, nor should they. If you must disinfect, copper-silver ionization is effective but maintenance is necessary.

Risk of Legionella from a bubble fountain?

I am involved with a volunteer group who wish to place a small bubble fountain in an external patient “sensory garden” at a local hospital. The fountain will not be connected directly to any water supply and will recirculate the water it uses. Some members of hospital maintenance staff have cited risk from Legionella as a reason to oppose the fountain. Would you consider the risk significant given that the fountain is outside and not connected directly to the potable water supply?

We believe that the risk for Legionella infection in your situation is small. And, many of the outbreaks linked to fountains may have been incorrect; nevertheless, given the undue publicity of Legionnaires’ disease and fountains, we believe it would be prudent to avoid this issue altogether. So, for hospitals, we discourage the placement of fountains. The fact that the water is recirculating rather than coming from the potable water supply does not mean that Legionella will be absent or even have a lower likelihood of being colonized with Legionella.


Can ozone be used as a disinfectant for Legionella water systems?

Is ozone used at all to disinfect contaminated (or suspected) areas.


Ozone can be used to disinfect for Legionella, however ozone leaves no residual for long-term control of Legionella in distribution systems. Ozone has been evaluated in a water distribution system, but results were inconclusive. To see an overall evaluation, please see Lin JAWWA 1998 in Publications.


Risk from sucking obstructed condensation drains of air conditioners?

I am a heating and air conditioning service technician. Often the condensation drain lines on air conditioning equipment become clogged with a reddish sludge. It is necessary to clear the drain with suction to prevent blowing the material back into the drain pan. In the past I have connected clean pipe to the drain line and sucked the obstruction loose careful not to get a mouthful of dirty water. After reading the information on your site and another site, this method is obviously wrong. What kind of risk have I put myself in. and if there is a risk why don’t I know more about it.

The risk to you is obviously small because you never have gotten sick. But the sludge may contain germs or toxins that may enter your mouth accidentally. If they got into your lung (which is possible if you are a smoker) you might contract pneumonia. And, if they got into your stomach, you could get diarrhea. Except for convenience, there seems no advantage to using your mouth for suctioning. A vacuum apparatus would be safer (but probably less convenient). By the way, what method do you use for unclogging sewage lines?


Legionella and spas?

How is Legionella contracted from spas/hot tubs and what actions ensure that this never happens?  We deal in portable spas and there has been an article in the national paper recently stating that a person has died through infection from a spa.

Problems occur when biocides are not used or they are used at ineffective concentrations.  The American Society of Heating, Refrigeration Air-conditioning Engineers (ASHRAE) has a section on spas and hot tubs in their publication “Minimizing the Risk of Legionellosis in Building Water Systems”.  You can get a  copy from ASHRAE or from the Baltimore Aircoil web site www.baltimoreaircoil.com.

Can Legionella survive in chlorinated water?

Can Legionella survive in chlorinated water?

Legionella has been isolated from chlorine treated water.  The levels of Legionella that elude the water treatment process are essentially undetectable.  When this water travels through the system and loses much of the chlorine residual, either from retention time or warm temperatures in water distribution systems, Legionella can then be cultured.  The colonization rate for large buildings is 20-70%, depending on the geographic region. Residential water systems can also harbor Legionella.

Does the presence of Legionella anisa in hospital water require disinfection?

Should a water distribution system harboring Legionella anisa be disinfected?

The majority (>90%) of cases of Legionnaires’ disease reported in the U.S. are caused by Legionella pneumophila. L. anisa is frequently isolated from environmental specimens but almost never causes disease. L. anisa is generally nonpathogenic and disinfection is not necessarily indicated for this particular species (Yu VL, J Infect Chemother 2004).


What is the recommended storage temperature for building hot water tanks?

I work for a building management company which involves controlling domestic hot water tanks, I would like to know what the official minimum storage temperatures are and the length of time they have to be at that temperature. I am under the impression that it is a minimum temperature of 60oC for at least 1 hour in any 24 hours (therefore the temperature can be dropped for the remaining 23 hours) can you tell me if this is correct. 

The temperature should remain stable at about 140oF at the tank. It would not be wise to only hold this temperature for only one hour and then reduce it.

Please obtain a copy of the American Society of Heating, Air Conditioning and Refrigeration Engineers (ASHRAE) guide to “Minimizing the Risk of Legionella in Building Water Systems”. This document has a section regarding hot water storage temperature. You can also download a copy from the Baltimore Aircoil web site (www.baltimoreaircoil.com).

The “official” minimum storage temperatures and durations may vary from country to country and even state to state depending on local guidelines. As a general source you may want to request guidelines from the following organizations:

Your suggestion of maintaining a water temp greater than 60oC in the tank for 1 hour may kill Legionella in the tank; however, regrowth of Legionella is likely to occur in the hot water distribution system where temperatures could drop to less than 50oC depending on use. Ideally for Legionella disinfection you would want to maintain a high temperature throughout the distribution system (hence flushing).

Whatever heating regime you try, monitoring of distal sites and the hot water tank is necessary to document the effectiveness of disinfection specific to your system and temperature set points.

What is the acceptable upper limit of Legionella in cooling towers?

What is the upper limit of Legionella for 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).

Legionella in decorative fountains?

If Legionella is found in decorative fountains, is remediation needed?

We would discourage the installation of decorative fountains in hospitals.

For buildings that do not have persons at risk for pneumonia, the water fountains can be treated with disinfectants such as chlorine or bromide. Ultraviolet light may also be effective. Matt Freije (www.hcinfo.com) recommends draining and cleaning decorative fountains at least twice a year – or more often if slime or dirt is visible. Surfaces can be scrubbed with a disinfecting solution (1 part chlorine to 20 parts water may be appropriate). Consult a water treatment service for advice.

What sites should I test for Legionella?

I have been tasked with collecting water samples for a hospital with three campuses. Two of the campuses have large chiller systems with cooling towers. I am wondering what are the best samples to take to test for Legionella.

For hot and cold potable water, should I try to test the furthest point from the source, directly from a tap?


Sample one or two locations per floor and a minimum of ten outlets (sinks and showers) is recommended. Along with that, sample the hot water tanks and/or the hot water recirculating line (return line).


Do the ice machines need testing?


As a healthcare facility with high risk occupants, we would say yes – test ice and water (if there is a dispenser) from ice machines. You may have to fill a couple of bottles with ice to provide enough water for testing.


Should I test the condensate drains from air handlers that distribute air throughout a building?


Generally speaking, these pans should not accumulate water and are not a generator of significant aerosols. Pans are more often a source of odor problems due to mold growth rather than a source of transmission for Legionella. We do not recommend testing these.


Cooling towers are located outside and on the roofs. Are these potential sources of Legionella?


Yes, cooling towers should be tested for Legionella when operating. There are outbreaks that have been linked to cooling towers. I’d recommend quarterly testing. For seasonally operated towers, test during the start up, during peak demand, and in the early fall when they may be used intermittently.

Filters plus copper-silver ionization?

Can risk for Legionella be reduced with chlorine while maintaining safe drinking water?

If the water is heavily chlorinated but still within the legal 0.5-4.0 PPM of chlorine, can the risk for Legionella be greatly reduced while the water being still safe to drink?

Chlorine is not very effective at controlling Legionella. Here is a review of chlorine from the EPA Technologies for Legionella Control. When asking what PPM of chlorine would be considered adequate to kill Legionella, you need to look carefully at the studies. Lower concentrations can kill a pure culture of Legionella in a jar in a laboratory, however in the environment, especially a plumbing system, much higher concentrations are required to reduce Legionella growth. For a shock disinfection to address high level of positivity or for building commissioning often chlorine concentrations in the 30-50 ppm ranges are used. This can kill Legionella, but re-growth will typically occur within a couple of weeks after the shock. This is because 1) the chlorine will not reach all parts of the plumbing system due to biofilm, sediments, and system construction 2) Legionella can live inside other organisms and studies have found resistance up to 50 ppm of chlorine because of this relationship, and 3) Legionella can re-enter the plumbing system from the public water supply.

Some facilities have tried to operate with an elevated continuous level of chlorine in the 2-4 ppm range (just under the EPA drinking water level). This is typically not successful and leads to early corrosion and failure of plumbing systems because of the high chlorine levels. These chlorine levels can also have other unintended consequences such as elevated disinfection by-products. Here is a 2011 review paper by Lin which is a good reference and you will see there under chlorine that the hospitals which used chlorine had switched to another method (bottom page 4/top of page 5).

So for chlorine, we would not recommend it be used as a continuous disinfectant because there are other more effective and less harmful options. Chlorine can be used as a shock disinfection at high levels, but again understand re-growth will occur quickly after the shock is completed.