Disease Risk

What kind of exposure poses the greatest risk?

Believe it or not, most of us have been exposed to Legionella without incident! This is because healthy individuals are at little risk of illness even if exposed. Direct exposure to very high concentrations of Legionella pneumophila serogroup 1 represents the greatest risk for acquiring disease in an otherwise healthy individual. An example of direct and intense exposure occurred in Louisiana when shoppers were exposed to Legionella pneumophila serogroup 1 from a misting device at a grocery store.

Hospitalized individuals are at greater risk due to impaired health status and greater chance of exposure during procedures. Aspiration of contaminated water can cause Legionnaires’ disease in these patients.

What is the goal of risk assessments and when should disinfection be performed?

The goal of a risk assessment is to identify conditions that increase the probability of Legionnaires’ disease because of exposure to water systems colonized with disease-causing Legionella bacteria. Remediation is not always necessary and should be discussed with professionals knowledgeable in Legionnaires’ disease and its prevention and control.

Legionella Species and Serogroup Table
Species, serogroup

All isolates,

% (n = 2340)


infections % (n = 1259)

Hospital infections

% (n = 890)

Legionella pneumophila 91.4 90.7 93.6
Serogroup 1 50.5 49.6 52.5
Serogroup unknown 32.1 33.9 28.2
Serogroup 2 1.2 1.4 1.1
Serogroup 3 2.0 1.5 2.9
Serogroup 4 1.1 1.0 1.3
Serogroup 5 1.1 0.8 1.7
Serogroup 6 2.9 1.7 5.2
Serogroup 7–14 0.5 0.8 0.7
L. bozemanii 1.3 1.3 1.2
L. dumoffi 1.5 1.4 1.0
L. gormanii 0.2 0.2 0.2
L. micdadei 2.8 2.8 2.8
L. feeleii 0.2 0.2 0.2
L. longbeachae 2.2 3.3 0.7
L. jordanis 0.3 0.2 0.1

Note: Only isolates identified by culture are included. From Benin A.L., Benson R.F., Besser R.E. Clin Infect Dis 2002; 35:1039-46.

Is there a regulatory limit for Legionella in water systems?

Cooling Towers

Neither the Centers for Disease Control and Prevention (CDC) nor the Environmental Protection Agency (EPA) define an enforceable regulatory limit for Legionella in cooling towers. Outbreak investigations have documented both low colony forming units (<100 CFU / mL) and high (>1000 CFU/mL) levels of Legionella in water samples from cooling towers.

There are no evidence-based guidelines for establishing risk criteria for Legionella recovery from cooling towers. Although guidelines have been suggested from a few groups, the data used to establish action levels and disease risk is very limited. Therefore, these guidelines are overly restrictive (recommending remediation at lower levels) and should be interpreted with caution.

Other countries, and New York City and State, have adopted recommended or required actions based on the concentration of Legionella cultured from cooling tower water. The Australian guidelines are pragmatic in their approach and do not recommend high level (50 ppm) hyper-chlorination when low levels of Legionella are detected (See Cooling Tower Control Strategy). New York has a similar approach in their regulation.

Healthcare Drinking Water Systems

Healthcare facilities include hospitals, clinics, dental offices, outpatient surgery centers, birthing centers, and nursing homes. Legionnaires’ disease is a well-recognized public health problem in hospitals. Nursing homes are a growing area for concern based on the increasing number of reported cases from long-term care facilities.

In contrast to the situation for cooling towers, evidence-based data is 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) has not been shown to correlate with incidence of disease (Kool-1999).

On the other hand, risk for Legionella infections increases as the extent of colonization increases (i.e., a high percentage of water outlets yield Legionella). In two studies, Legionnaires’ disease did not occur unless 30% or more water outlets were positive with L. pneumophila (Kool-1999, Stout-2007). New York State regulations also uses the 30% cut point. The locations and method of sample collection is collection is critical, so consult with a microbiologist knowledgeable in Legionella monitoring before collecting samples.

The use of percent positivity as a risk threshold was first adopted in Pennsylvania by the Allegheny County Health Department in their 1993 Legionella prevention guideline. This approach was adopted by the Veterans Affairs Healthcare System in their 2008 directive. That directive presented a simple proactive approach to protecting patients and building occupants.

Note that complete elimination of Legionella from a hospital water system has not been shown to be necessary to prevent most cases of Legionnaires’ disease.

Which Legionella species and serogroups cause disease?

It isn’t unusual to find multiple Legionella species and serogroups in a water sample. The presence of one species has not been shown to correlate or predict the presence of another species. There are more than 60 species of Legionella, with approximately half implicated in human disease. The majority (>90%) of cases of Legionnaires’ disease reported in the U.S. are caused by Legionella pneumophila. There are more than 16 serogroups of Legionella pneumophila, but serogroup 1 is responsible for most cases (see table below). Other serogroups have caused disease, however this is rare by comparison to serogroup 1. The data in the following table remains consistent with more recent data from Europe and the US.

L. anisa is frequently isolated from environmental specimens but very rarely causes disease. Disease caused by other Legionella species, like L. anisa, occurs almost exclusively in immunocompromised individuals. Only a handful of cases attributed to L. anisa have been reported. We consider this species nonpathogenic and would NOT disinfect your water supply if L. anisa is present. For more information, see Blue-White Legionella Fact Sheet.

Proportion of Legionnaires’ disease caused by each serogroup and species of Legionella reported to the Centers for Disease Control and Prevention, United States, 1980-1998. Legionella pneumophila is responsible for > 90% of all reported cases.


“Air-handling and water systems of buildings-microbial control. Part 3: Performance-based maintenance of cooling water systems.” AS/NZS 3666.3:2000.  Standards Australia International Ltd. Sydney NSW.


Kool, J.1999. “Hospital characteristics associated with colonization of water systems by Legionella and risk of nosocomial Legionnaires’ disease: a cohort study of 15 hospitals.” Infect. Cont. Hosp. Epid. 20:798-805.


Pierre, D; Stout, JE; Yu, VL. Editorial commentary: Risk assessment and prediction for health care associated Legionnaires’ disease: Percent distal site positivity as a cut-point.


Letters to the Editor / American Journal of Infection Control. 42 (2014) 1247-53.


Stout JE, Muder RR, Mietzner S, Wagener MM, et al. “Role of environmental surveillance in determining risk for hospital-acquired Legionellosis: a national surveillance study with clinical correlations.” Infect. Cont. Hosp. Epid. 2007; 28:818-824.


Special Pathogens Laboratory. Guide to Cooling Tower Legionella Regulations: New York City and New York State. 


Department of Veterans Affairs, Veterans Health Administration. VHA Directive 2008-01: Prevention of Legionella Disease. Washington, DC, Feb. 11, 2008.