Legionellosis (Legionnaires' Disease and Pontiac Fever)

Hazard Recognition

This page describes the conditions that promote Legionella growth in the manmade water systems that frequently serve as Legionella sources when there is a Legionellosis outbreak. It also provides information about how to recognize Legionnaires’ disease and identify Legionella hazards in the workplace, as these are critical elements of an effective water management program. This page also addresses how these sources contribute to worker exposures. The page includes section(s) covering:

Conditions Promoting Legionella Growth and Most Susceptible Water Systems
Photo Credit: OSHA  Mist cooling systems create water sprays that, if not properly maintained, can expose workers to Legionella bacteria.
Photo Credit: OSHA

Mist cooling systems create water sprays that, if not properly maintained, can expose workers to Legionella bacteria.

Low and even non-detectable Legionella levels can exist in a water source without resulting in Legionellosis cases. However, under specific, preventable conditions, Legionella bacteria can grow rapidly and reach high concentrations in water. Legionella bacteria are more likely to become suspended in the air (i.e., aerosolized) when the water is part of a system that includes the water's passing through components such as cooling towers, shower heads, and faucets. Once aerosolized, there is a risk for worker exposure that could result in Legionellosis

  • Biofilms: Slimy films of bacteria and other microorganisms can provide a source of nutrients for Legionella.
  • Stagnation: When water ceases to flow or move, biofilms can develop.
  • Scale: When mineral deposits buildup on surfaces inside water lines and containers, primarily from water containing calcium and magnesium carbonates or bicarbonates, biofilms can develop.
  • Temperature: In temperatures between 68° and 122°F (20°-50°C). Optimal Legionella growth occurs between 95° and 115°F (35°-46°C).
  • pH: 5.0-8.5.
  • Sediment: Organic particles suspended in water provide a source of nutrients for Legionella.
  • Presence of other microorganisms: Algae, amoebae, protozoa, and other organisms provide a source of nutrients.

Legionellosis outbreaks in workplaces are commonly attributable to Legionella growth in poorly maintained manmade water systems, such as:

  • Cooling towers, evaporative condensers, and fluid coolers using evaporation to remove heat.
  • Potable water systems and domestic hot water systems.
  • Humidifiers, misters, foggers, and decorative or display fountains creating water spray.
  • Spas, whirlpools, and hot tubs.
  • Cooling misters, produce misters1, and evaporative coolers.
  • Industrial processes creating aerosolized water (e.g., cooling sprays or tanks and water-based fluids used as coolants during cutting and fabrication processes).

A Legionellosis outbreak also can occur when aerosolized Legionella is disseminated throughout a workplace in an air handling system from an external or internal contaminated source.

Watch the video "Legionella Ecology and an Introduction to Environmental Health and Engineering" to learn more about where Legionella commonly grow.

Understanding Exposure Risk

Where Exposures Commonly Occur

Legionellosis outbreaks do not always occur among workers at a specific workplace. Legionella exposures are more likely to occur when traveling (e.g., in hotels and resorts or aboard cruise ships) or among patients in healthcare settings (e.g., hospitals, long-term care facilities). More than 20 percent of all reported Legionnaires' disease cases are among individuals who have traveled within 2-10 days of when symptoms begin and seven percent of Legionnaires' disease cases are among individuals who have stayed overnight in a healthcare facility within a similar timeframe.2 Dental water lines also are recognized as water sources commonly contaminated with high microorganism concentrations. These microorganisms vary widely, but could include Legionella. However, recent data suggest that there is no increased Legionnaires' disease risk among dental staff or patients compared to the general population.3

At-Risk Workers

Most healthy people do not get sick after exposure to Legionella. People over 50 years old, smokers, or those with underlying medical conditions, such as chronic lung disease or immunosuppression, are the most susceptible to infection.4 For this reason, workplace Legionella contamination should not be ruled out just because not all workers got sick, especially if there is a distinction in risk factors between those who got sick and those who did not.

The Centers for Disease Control and Prevention (CDC) Legionella Web page provides detailed information about risk factors for Legionellosis.

Detecting Cases

Legionnaires’ disease outbreaks are difficult to detect because symptoms take one to two weeks to develop and not everyone who is exposed develops the disease. As previously explained, healthier workers may remain healthy after exposure, while those with underlying medical conditions may become ill.

The Medical Information page provides more detailed information about symptoms, including a side-by-side comparison for Legionnaires’ disease and Pontiac Fever. Whenever one or more workers experience symptoms consistent with Legionellosis, consider the possibility of workplace exposure to Legionella.

Identifying Legionella and Legionellosis in the Workplace

Hazard Assessment

Employers and workers, especially maintenance workers, need to know about conditions that promote Legionella growth, identify in the where workplace growth is most likely to happen, and monitor these areas periodically for signs of bacteria growth. For example, if water heaters are operating at 140°F (60°C) or above, and if the water temperature at distant faucets is 122°F (50°C) or higher, then there is a low risk Legionella is present in these systems. If a water system is poorly maintained and operating temperatures are below recommended minimums, then there is a risk Legionella is present.

If maintenance issues are identified or a worker is diagnosed with Legionellosis, employers should consider shutting down potential sources, instituting corrective actions to improve water system maintenance, and communicating the hazard to potentially exposed workers. See the Control and Prevention page for OSHA's corrective action recommendations.

Following applicable maintenance plans should prevent Legionella growth, however, routinely looking for visible biofilms and sampling for Legionella, if appropriate, is a backup plan for hazard recognition and mitigation before anyone gets sick. If Legionella is suspected, consider seeking additional expertise from environmental health and safety professionals.

The CDC's Legionella Environmental Assessment Form guides employers through assessing facility water systems, determining whether Legionella environmental sampling is required, and developing a sampling plan.

When conducting a hazard assessment, pay special attention to:

  • Locations in the water systems where water may stagnate, such as storage tanks, and components that have been isolated and no longer maintain a significant flow of water. (known as "dead legs"), or infrequently used faucets.
  • Hot water recirculation systems.
  • Side-stream plumbing equipment not experiencing regular flow, such as expansion tanks, hammer arrestors, or by-pass lines.
  • Cross-connections between domestic and process water systems.
  • Backflow prevention devices.

The instructional video "Conducting and Interpreting the Environmental Assessment" describes useful steps for conducting Legionella-focused hazard assessments.

Legionellosis Cases in the Workplace

Employers may be alerted to suspected Legionellosis cases in their facility by employees, employees' health care providers, or the local or state health departments. In cases where the health department is not already involved, employers should immediately contact the local or state health department to make them aware of the suspected case(s). Legionellosis is a nationally notifiable disease and must be reported to the local or state health departments. Suspected and confirmed cases of Legionellosis are then reported by the state health department to the Centers for Disease Control and Prevention (CDC).

Employers should quickly take several actions to prevent more workers from getting sick including hazard assessment and water system sampling. Further actions are outlined in the Outbreak Response section of this webpage.

Employers should consult with industrial hygiene professionals familiar with Legionella in the workplace to provide guidance on hazard assessment, water system sampling and analysis, and control and prevention actions. The CDC provides guidance when identifying and working with Legionella consultants.

Employers should consult with physicians who specialize in occupational medicine and/or infectious disease to provide guidance and services, such as confidential medical consultation, educational information, and serve as a liaison between the employer and the health department. The Resources section of OSHA's Clinicians web page has links to several professional organizations, such as the American College of Occupational and Environmental Medicine (ACOEM), the Association of Occupational and Environmental Clinics (AOEC), and the American Association of Occupational Health Nurses (AAOHN), that can assist in identifying consultants.

Workers who believe they may have contracted Legionnaire’s disease or Pontiac Fever from their workplace should consult with a healthcare provider, such as their own physician or their employee health department, as soon as possible.

Cooperation between employers and workers, in partnership with the local and/or state health department, CDC, and OSHA are critical in quickly identifying Legionellosis cases and preventing additional cases.

Water System Sampling

When two or more laboratory-confirmed cases of Legionellosis are linked to a particular facility, consider the worksite as a potential source of a Legionellosis outbreak and initiate water system sampling. Because water system sampling requires specialized expertise, many employers, especially small businesses, may choose to seek the assistance of environmental health professionals. OSHA and other health officials may also collect water system samples to identify Legionella as part of their inspections or investigations.

Workers performing water system sampling must wear appropriate personal protective equipment (PPE), as described on the Control and Prevention page.

The CDC's Sampling Procedure and Potential Sampling Sites protocol provides information on recommended sampling locations and techniques for taking standard biofilm swabs and bulk water and filter samples.

Although geared toward state and local health department staff, the following CDC instructional videos discuss environmental aspects of conducting detailed facility hazard assessments.

The CDC also provides a sample data sheet to track environmental samples.

Submit the water system samples to a laboratory certified by the CDC in Legionella isolation from water system sample testing (i.e., Environmental Legionella Isolation Techniques Evaluation (ELITE)). Laboratories proficient in Legionella isolation are listed on the Legionella isolation are listed on the ELITE Members List.

Water System Sampling Analysis

Laboratories may use several different methods to analyze water system samples for Legionella, although culture techniques are the "gold standard," or best method. The table below presents the different analytical methods available for Legionella water system sample analysis.

Legionella Analytical Methods

Analytical Method Description
Cultured samples
  • Uses special agar culture media, such as buffered charcoal yeast extract (BCYE), to grow Legionella.
  • With proper sampling, positive Legionella culture results reliably indicate that the bacteria were present at the sampling site (i.e., culture methods have a very low false-positive rate).
  • Can determine how many Legionella colony forming units (CFU) there are in a certain volume or area of water (usually reported per milliliter (mL) of sample).
  • Requires up to 14 days after sample submission to obtain results.
  • Quantitative results (i.e., how much Legionella is found) are highly dependent on sampling techniques, culture methods, and other factors.
Genetic typing with cultured samples
  • Identifies specific strains (i.e., serogroups) for Legionella species.
  • Good for use in an outbreak situation because it can match patient serogroup and subtype to bacteria found in a water source (strong evidence supporting an associated epidemiological link).
Direct fluorescent antibody (DFA) stain
  • Stains the organism with a fluorescent dye to determine organism quantity.
  • Results available in 1-2 days.
  • Technique is unable to distinguish between live and dead Legionella bacteria, so the test identifies bacteria killed by biocidal disinfectants in addition to bacteria that are still capable of causing infections.
  • May have some cross-reactivity with other bacteria.
  • Potential for false-positive and false-negative results.
DNA amplification assay
  • Uses a polymerase chain reaction (PCR) process to amplify and then detect DNA portions unique to Legionella.
  • Reproducible, accurate, and precise results in one day.
  • Gives results for both live and dead Legionella bacteria.
  • Prone to false negatives.
  • May not correlate with culture results.

Water System Sampling Results

Once water system sample analysis is complete, the analyzing laboratory will provide results indicating which samples identified Legionella bacteria, the concentration of bacteria in each sample, and other measures of overall water quality (e.g., total bacterial counts, total dissolved solids, and pH). Employers, inspectors, and others involved in assessing workplace Legionella hazards can use sampling results to determine what steps, if any, should come next. However, interpreting water system sampling results is complex and may require additional expertise from environmental health and safety professionals familiar with Legionella sampling and control measures.

Negative Sampling Results

A Legionella hazard is unlikely if all water system sample results from well maintained and correctly operated water systems are negative or show only acceptably low Legionella levels. What constitutes "acceptably low" may differ from system to system, worksite to worksite, or by other factors. The Control and Prevention page provides additional guidance on this topic.

Positive Sampling Results

In general, positive sampling results indicate that Legionella is growing in the sampled water system or part of the sampled water system.

Historically, control measures have often been based on the concentration, or the number of colony forming units (CFU) per mL, of Legionella in water system samples. However, water system sampling depends on the skill and experience of the person performing the sample collection and analysis, and can produce very different quantitative results from the same water system when different techniques are used. For example:

  • Biofilm swabs from water system surfaces may find significantly higher concentrations of Legionella compared to water samples collected from a faucet, water heater drain, or other outlet.
  • Some techniques, such as using an acid buffer or different types of growth media, for eliminating non-Legionella bacteria from samples may affect the amount of Legionella bacteria identified in a sample.
  • Concentrating water samples using filtration versus spinning the sample tubes (i.e., centrifugation) also may affect the amount of Legionella bacteria identified in a sample.

Samples that find Legionella growth may prompt disinfecting the water system and then re-testing to determine of the disinfection steps eliminated the Legionella hazard. In some instances, employers may need to limit workers' and others' access to the contaminated water system or areas affected by potential Legionella-containing aerosols or implement other controls. During an outbreak, further genetic analysis of water system samples alongside clinical specimens from patients with suspected or confirmed Legionellosis can match the patients' Legionella serogroup and subtype to bacteria found in a water source-an increasingly useful way of connecting cases to exposures. However, not all of these actions are appropriate in every instance and, as emphasized previously on this webpage, control measures should follow an environmental health expert's recommendations.

Sampling results may also mean different things in different types of water systems. For example, water quality measurements (e.g., total bacterial counts, total dissolved solids, and pH) are not proven indicators for Legionella levels in cooling towers.

Additional Information

For more information on Legionella hazard recognition, review the American Industrial Hygiene Association (AIHA) Recognition, Evaluation, and Control of Legionella in Building Water Systems guidelines.

1 Epidemiologic Notes and Reports Legionnaires' Disease Outbreak Associated with a Grocery Store Mist Machine -- Louisiana, 1989. Morbidity and Mortality Weekly Report, 39(7), 108-110 (1990).

2 Legionella (Legionnaires' Disease and Pontiac Fever). Centers for Disease Control and Prevention, U.S. Department of Health and Human Services.

3 Estrich, C. G., Gruninger, S. E., & Lipman, R. D. Rates and predictors of exposure to Legionella pneumophila in the United States among dental practitioners: 2002 through 2012. The Journal of the American Dental Association, 148(3), 164-171 (2017); Zemouri, C., de Soet, H., Crielaard, W., & Laheij, A. A scoping review on bio-aerosols in healthcare and the dental environment. PloS One, 12(5), e0178007 (2017).

4 Legionella (Legionnaires' Disease and Pontiac Fever). Centers for Disease Control and Prevention, U.S. Department of Health and Human Services.