Legionella: Minimizing Risks

Dec. 28, 2000

Legionellosis, the disease caused by Legionella spc., is common, though most people would guess it is extremely rare. Outbreaks of Legionellosis, defined as a cluster of three or more cases in a single locale, occur regularly in the United States and much of the developed world. Outbreaks have been reported in Australia, Holland, Thailand, Japan, England and many other countries. In the U.S., the Center for Disease Control (CDC) receives reports of 1,000 cases of Legionellosis annually. They believe 10,000 to 15,000 cases go unreported each year. Many of these people are infected by Legionella living in HVAC cooling towers, making Legionella a significant threat to indoor air quality.

A building with indoor air contaminated by chemicals or bacteria is said to be suffering from Sick Building Syndrome (SBS). An article published in the Journal of the American Medical Association specifically lists Legionella infection as one of the effects of sick building syndrome. Representative Waxman of California introduced a bill (HR2919) in Congress to authorize a national program to reduce the threat of disease posed by exposure to contaminants in indoor air.

Legal Exposure

Recently, personal injury attorneys have been alerted to the health effects of indoor air pollution, especially in regards to microbial infection. In several instances this has already led to tort action. In one case, everyone from the architects to the suppliers of a building were sued when it had to be abandoned due to the effects of indoor air pollution. According to this precedent, the building owners, their maintenance team and the entire construction team, including architects, engineers, construction managers, contractors, subcontractors and building material suppliers, can be held responsible should anyone contract Legionellosis from a contaminated air conditioning system. Already there have been suits for very large amounts including one claim filed in Texas for $4.6 billion.

The law holds owners and members of the building construction and maintenance teams to a high standard of care. They have a duty to avoid or abate SBS conditions that create an unreasonable risk of physical harm and are required to take a proactive role in identifying and abating SBS conditions or face expensive and protracted litigation.

Unfortunately, in our litigious society, should the owner, construction or management teams fail to meet these high standards of care, attorneys for the plaintiff would certainly include all parties in any suits that are filed. Owners are especially vulnerable here, since they are usually removed from the construction and daily maintenance operations and may assume that all team members have adequately met their expected standards. The owners are the ones targeted for a suit should any other building team member fail in their responsibilities. Therefore, it is in the best interest of building owners to insist on the best possible disease preventing design and technology.

The duties of the building team include keeping informed of advances in the science of building design and maintenance. Since this technology advances faster than industry standards change, it will be difficult to argue that adherence to traditional methods is adequate to meet required standard of care. One legal source said, "It may become increasingly difficult to convince a jury that [building owners] contractors and design professionals have met their respective standards of care simply because they rigorously adhered to traditional industry standards."

Possible Legal Action

In SBS cases, plaintiffs can consider several causes of action, including breach of contract and express warranty, breach of implied warranty, strict liability, negligence, breach of covenant of quiet enjoyment by constructive action, fraudulent concealment and misrepresentation, nuisance, assault and battery and emotional stress. Of these possible actions, most attorneys would choose negligence. The others are more difficult to prove or do not allow for significant damage claims. According to the precedent set in Ward vs. Hobart Manufacturing Co., to recover damages for negligence four elements must be demonstrated.

Duty–the plaintiff must be protected under some rule of law against the defendant’s conduct.

Breach–the defendant’s conduct must have violated this duty.

Casual relationship–the plaintiff’s injury must be the result of the defendant’s conduct.

Damage–the plaintiff must have suffered a loss.16

Cooling Towers and Legionella

Numerous studies in several countries have found Legionella to be common in cooling towers everywhere. In one study, more than half of the 80 samples collected from cooling towers contained Legionella. Most authorities agree that cooling towers are a principal source of Legionella infection.

Legionella are hardy bacteria that are difficult to kill with normal chemical biocides. They thrive in the backwaters of the system, on submerged equipment and particle surfaces where they become part of the bacterial slime coating. Embedded in this slime, Legionella are protected from exposure to biocides dissolved in the water. Further protection is afforded by the fact that Legionella are protozoonotic; they live and reproduce within the body of other microbes, especially Amoebae and Paramecia. These protozoa are not very sensitive to the biocides used to control bacteria and therefore represent a sanctuary for populations of Legionella. Even after cleaning, Legionella from surviving protozoa can within weeks reestablish a population in a cooling tower. For this reason, the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) and other groups, recommend regular cleaning, water treatment and monitoring of the tower as the best way to minimize the potential for disease transmission.

Many common biocides are ineffective at eliminating Legionella. There is some concern that through constant exposure Legionella may become resistant to even these effective agents. Consequently, some authors recommend rotating biocides.

Oxidizing biocides often are reported to be effective in controlling Legionella. Chlorine-based oxidants seem to work best if used as periodic shock treatment in large slugs. Protozoa are not as susceptible to chlorine as are bacteria, and it must be used at high levels to ensure these Legionella-protecting organisms are eliminated. Ozone also proves effective and can be used continually.9 Ozone will break down the organic constituents of slime and, if concentrations are correct, will deny this protective habitat to Legionella. Though a regular biocidal treatment program cannot guarantee that Legionella are not present in your system, it will ensure the minimum population. This will reduce the potential for transmission of the disease.

Legionella growth is stimulated by contaminants in cooling water. Dissolved organic materials serve as nutrients for bacteria and protozoa. Particulates, especially in the finer size ranges, provide surfaces for bacterial colonization. If these particulates are organic, they can supply an additional source of nutrients. A side-stream filtration system will reduce the level of suspended particulates. Many side-stream technologies exist, including cartridge, bag or sand filters. Centrifugal separators do not remove very small or low-density particles.

The particle counts for a cooling tower before and after sand filtration with five-micron media are shown in Table 1 (end of article). Notice the reduction in the number of particles after filtration, especially in the finer size range where most of the surface area and nutrient value exist.

Once a proper biocide and filtration program has been instituted, no more can be done to eliminate Legionella. Even then the possibility exists that a viable Legionella population may survive in the tower. The last step is to minimize the chance of transmitting the disease by containing the aerosol drift plume. This is accomplished through the installation of drift preventers and positioning the cooling tower for minimum drift potential. Prevailing winds and other factors important to the drift trajectory should be included as primary criteria in the determination of an appropriate location.

Due Diligence

Since you can never be certain that Legionella are not present in your cooling tower, it always is possible that someone may become infected. Therefore, owners and members of the building construction/maintenance team are constantly open to legal problems should someone become ill with Legionellosis. Concerned about this potential exposure, ASHRAE has studied the problem for several years and in 1998 published a position paper on the subject.10 In this paper they emphasize the need to maintain a clean system as the best protection against an outbreak of Legionellosis. Further, they suggest some technologies useful in maintaining a clean system and present a review of the legal benefits in implementing these suggestions. In essence, their argument, and those of others who have published similar reviews, is based on the idea of due diligence.

In order to perform due diligence, one must do everything within reason to prevent a problem. If no regulations were violated and no more can be done than was done, then no negligence has occurred. If one has performed due diligence, a tort will have little standing in court. If owners or members of the building team have not refused to use reasonably priced effective technologies and practices, a court cannot find there was a failure to consider the health and well being of the building’s users. In other words, though an outbreak might still occur, the chance is much less likely, and such an outbreak could not be attributed to any callous and negligent behavior on the part of the defendants. In all probability, attorneys will decline to pursue a case where due diligence is demonstrable. For cooling towers, due diligence is demonstrated by maintaining the cleanest possible tower conditions with effective methods of drift prevention.


The following suggestions will help establish due diligence and represent the best accepted technology to prevent infection and spread of Legionella.

Design Requirements

• When designing a cooling system, care should be taken to avoid dead legs or other traps where water may avoid exposure to disinfection procedures.

• The cooling tower location should include consideration of any aerosol plume that may be produced and should ensure that drift trajectory is away from populated areas.

• Appropriate disinfection and cleaning protocols, including chemical delivery systems and side stream filtration, should be designed into the tower system from the beginning. Retrofitting a poorly designed cooling system will significantly reduce the potential for infection and will contribute to demonstrating due diligence in the protection of the public.

General Maintenance

• General maintenance should include regular and thorough cleaning of the tower sump and fill. Any sediment accumulations should be removed and films scrubbed off of fill and basin surfaces. If a tower is to be idle for more than a few days, it should be drained. Upon refilling the tower, a shocking dose of an effective biocide should be administered followed by regular applicants as appropriate.

• A side stream filtration system should be installed and operated continuously. Filter systems that remove low-density organic particles must be used since these particles provide habitat and nutrients that stimulate bacterial growth including Legionella.

• A continued course of biocide treatment must be maintained in a fashion consistent with effective Legionella control.

Drift eliminators should be installed to discourage formation of any aerosol plume. Inhalation of aerosols containing Legionella cells is the only demonstrated pathway of transmission of this disease.13

Detailed records should be kept of all maintenance procedures. This is extremely important. A detailed logbook documenting chemical treatment and all other steps and data collection necessary to demonstrate due diligence will be critical evidence should any legal proceedings be initiated.

Editor’s Note: In view of the increasing incidence of Legionnaire’s disease, strict rules governing the use of water at RAI trade shows have been adopted. The use of water in ponds, fountains, baths, swimming pools and air conditioning systems have been banned at the upcoming Aquatech Amsterdam 2000.

About the Author:

W. Craig Meyer is a professor of environmental sciences at Pierce College, Woodland Hills, California.

Table 1: Particle Counts Before and After Sand Filtration

Particle Size

Before Filtration*

After Filtration with 5 micron media*

1 to 5 microns



5 to 10 microns



10 to 25 microns



Over 25 microns



*Data supported by Process Efficiency Products, Mooresville, NC

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