There are no current federal or state regulations on the occurrence and control of Legionella in drinking water. The USEPA mandated that surface waters (lakes rivers, reservoirs, or groundwaters under the direct influence of surface water) used as drinking water supplies be properly filtered and disinfected under the 1989 Surface Water Treatment Rule. The rule sets a Maximum Contaminant Level Goal (MCLG - a non-enforceable guideline) of zero Legionella organisms for drinking water. It is generally assumed that if surface water was treated according to the techniques outlined in the rule, Legionella will be controlled.  Recent experience has pointed to the problem of Legionella growth being more prevalent in building water systems – which are outside the jurisdiction of the EPA regulations

A number of International, federal, state and country guidelines have been compiled in order to make the best recommendations for Legionella monitoring and control.  Below is a summary of these Legionella risk management, monitoring and control guidelines.
Georgia, USA
Hong Kong, China
The Netherlands
Ontario, Canada
United Kingdom
Florida, USA
New Zealand

The ideal scenario is to have no Legionella present in a water system, as there is no known absolutely safe level of Legionella. However, low concentrations and low frequency of occurrence of Legionella have been associated with low risk. Guidelines were compiled for action levels from 13 different organizations and categorized into three different levels to form a basis for responding to risks of Legionella in water - categorized as Effective Maintenance, Potentially Hazardous and Immediate Action Required. The ranges are broad, spanning three orders of magnitude or more, indicating diverse approaches for Legionella control. The Occupational Safety and Health Administration (OSHA) doesn't have specific standards for Legionnaires' disease, but has suggested the guidelines in the table below to assess the effectiveness of water systems maintenance.  These guidelines are intended to apply only to water systems being used by healthy individuals and are not necessarily right for people who have weak immune systems.  Guidelines with lower trigger values could be used when high-risk populations are present (e.g., hospitals, nursing homes, etc.).

Levels of Risk/Action

Legionella Presence (cfu/ml)

Effective Maintenance

<1 cfu/ml

Potentially Hazardous

1 cfu/ml – 100 cfu/ml

Immediate Action Required         

> 100 cfu/ml

  * cfu/mL- colony forming units per milliliter