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 (<100 Colony Forming Units /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 guidelines for actions based on the concentration of Legionella cultured from cooling water.  The Australian guidelines are pragmatic in their approach and do not recommend high level (50 ppm) hyperchlorination when low levels of Legionella are detected (See Cooling Tower Control Strategy).  New York has a similar approach in their regulations. 

Healthcare Drinking Water Systems

Healthcare facilities include hospitals, clinics, dental offices, out-patient 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 assessments 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 of 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 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 been adopted by the national network of hospitals of the Veterans Affairs Healthcare System in their 2008 directive.

Which Legionella species/serogroups cause disease?

It is not 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 58 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 the overwhelming majority of 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 U. S. 

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.

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. 

Species, serogroup All isolates,
% (n = 2340)
Community-acquired 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. dumoffii 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 2.2 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.
See Legionella species table for complete list. 

What kind of exposure poses the greatest risk?

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 for 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 as a result of exposure to water systems colonized with disease-causing Legionella bacteria. Remediation is not always necessary and should be discussed with professionals knowledgeable in the area of Legionnaires’ disease and its prevention and control.


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

Disclaimer: The information is provided for informational purposes only and not for the purpose of providing legal or medical advice. You should contact your physician or attorney to obtain advice with respect to any particular issue or problem.