Indicators of low legionella colonization in Danish primary schools and approaches to minimise infection risks

Legionella in hot water systems poses significant health risk from infections. Standard preventive measures are high water temperatures and regular water flow, which conflict with the goals of sustainable energy and water use. In 2025, 22 primary schools in the Danish Capital Region were re-investigated for the presence of Legionella and for characteristics of hot water systems. The aim is to identify characteristics for hot water systems that are associated with low vs high Legionella colonization. The results show that the average Legionella concentrations in the A and B samples had decreased by 61% and 79% respectively since 2023. A tap water temperature of 55 °C or higher was associated with Legionella concentrations below 1,000 CFU/L. However, some schools with lower water temperatures showed low Legionella concentrations. These systems typically show smaller dimensions, i.e. small hot water tanks sized to the number of pupils, recently renovated installations, fewer students. Other results show that low Legionella concentration in A-samples correlate with low contents in B-samples, from taps and from showers. No other physiochemical indicators could explain the variations between schools. By inclusion of significant indicators, a heigh explanation gradient for a linear regression is seen between water temperature and Legionella concentrations.