Chapter 64 : Routine Sampling and Temporal Variation of Legionella Concentrations in Cooling Tower Water Systems

Abstract:

This chapter presents biweekly sample data from 28 small cooling towers colonized by Legionella over a 4-month summer period. These data were used to appraise the validity of risk assessments on the basis of routine Legionella sampling and to investigate the temporal variation in concentrations of Legionella bacteria in cooling towers. All the cooling towers were maintained to comply with the Australian Standard AS 3666:1989, including weekly water treatment chemical dosage. Median Legionella concentrations and ranges were calculated for each cooling tower. Assessment of the risk of Legionnaires' disease outbreaks based solely on counts of Legionella may be misleading. The factors contributing to successful control of the bacteria are well established, and to some extent generic, although individual systems are almost always unique in some aspect. Risk assessment of cooling towers for Legionella dissemination should be focused upon the established and quantifiable criteria. The role of Legionella culture in determining risk should be subordinate, due to the absence of a quantifiable relationship between culture results, system status, and dose response.