What is the acceptable upper limit of Legionella in cooling towers?

What is the upper limit of Legionella for water distribution systems?


In contrast to the situation for cooling towers, there is solid scientific data 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) does not correlate with incidence of disease (CDC, Infection Control and Hospital Epidemiology 1999, Kohler, Journal of Hospital Infection 1999, Pittsburgh Special Pathogens Laboratory [Best] Lancet 1983). On the other hand, risk for Legionella infections increase as the extent of colonization with L. pneumophila increases (i.e., a high percentage of water outlets yield Legionella). In two studies, Legionnaires’ disease did not occur unless the percentage of water outlets reached 30% or greater (Best Lancet 83, Stout, in press).

Complete elimination of Legionella from a hospital water system is not necessary to eliminate cases of Legionnaires’ disease in the hospital (Pittsburgh Special Pathogens Lab [Stout], Infection Control and Hospital Epidemiology, 1998).