In accordance with the American Society of Heating, Refrigerating and Air-Conditioning Engineers’ (ASHRAE) sole objective to advance the arts and sciences of heating, refrigeration, air conditioning and ventilation, and their allied arts and sciences and related human factors, for the benefit of the public, the Society provides and updates a series of Position Statements and Position Papers on topics that are of concern to ASHRAE members and the general public.

Based on the scientific, medical, and engineering literature available to date, ASHRAE concludes the following:

1. Legionellae occur naturally in lakes and streams. They may be transported through potable water distribution lines and then may be aerosolized into the atmosphere. Outbreaks of individual cases of legionellosis have been traced to cooling towers and evaporative condensers, potable water services, and components, such as water heaters, showers, faucets, whirlpool baths respiratory therapy equipment, and decorative fountains.

2. The mere presence of Legionella in water or on a fixture or device is not sufficient to cause disease. For disease to develop, individuals must inhale sufficient numbers of virulent organisms to overwhelm their natural resistance. Variations in virulence of organisms and susceptibility of people exist.

3. Design and good operations, maintenance, and housekeeping procedures that prevent amplification and dissemination of Legionella should be formulated and implemented before systems are operated. These procedures should be continued rigidly thereafter. Although using these practices will not guarantee that a system or individual component will be free of legionellae, they should reduce the chance of heavy colonization with these bacteria.

4. Currently, the most reliable method of testing for the presence of viable legionellae in a system is by culturing for these organisms. No "non-legionellae" surrogate tests are available, and there is currently no correlation between total bacterial counts and legionellae concentrations. The results of single tests must be interpreted cautiously as the concentration of Legionella in a water system can increase substantially over a few days. Attention should be paid to sample handling and laboratory quality control so that the number reported reflects the number present at the time of sampling.

5. The efficacy of a specific biocide treatment in controlling legionellae can only be determined by testing specifically for the presence of legionellae in the field under actual working conditions. Laboratory trials must not be relied on exclusively as the sole proof of the efficacy of a biocide. Biocides can be health and safety hazards and must be handled in accordance with specifications on labels and material safety data sheets.

ASHRAE will continue to collect and analyze new information on the control of Legionella and disseminate this information to its members and to the general public. The Society also will maintain liaison between other professional organizations and government agencies such as the Centers for Disease Control and Prevention and the U.S. Environmental Protection Agency. ASHRAE will continue to encourage and support specific research in the control of Legionella.

Revision Date: August 3, 1998

©1998 ASHRAE. All Rights reserved.