The 33rd Epidemiological Bulletin of the Robert Koch Institute (RKI), published on August 15, 2024, presents the TESLI study (Temperature Reduction and Legionella Incidence), which investigates whether the inflation-driven lowering of operating temperatures in domestic hot water storage systems in 2022 led to an increase in the incidence of reported cases of Legionnaires’ disease. The study also illustrates the trends in average drinking water temperatures over time and the proportion of households that lowered their water temperature. These data were correlated with the incidence of Legionnaires’ disease cases at the state level. WATERcontrol was one of the six laboratories approached that provided the RKI with anonymized large-scale drinking water datasets from commercially used drinking water installations for analysis purposes.

Legionnaires’ disease is the most common form of legionellosis, which manifests as pneumonia caused by Legionella bacteria. Legionella are environmental microorganisms found in water that thrive particularly well at temperatures between 25 and 45°C and can also colonize technical drinking water installations (TWI), which are relevant to human health. Above 55°C, their growth is inhibited. During the energy crisis in 2022, there was a significant increase in energy prices, which also made the heating of drinking water more expensive. The RKI investigated in the TESLI project (Temperature Reduction and Legionellosis Incidence) whether there were changes in drinking water temperatures in 2022 and whether a rise in the incidence of Legionnaires’ disease cases could be observed in this context (ecological study design). According to the Drinking Water Ordinance (TrinkwV), large-volume central TWIs must be regularly tested for their Legionella concentration, and the water temperature is also measured during this process. Therefore, the RKI researchers contacted large, nationwide laboratories that provided the relevant data. Die durchschnittliche Wassertemperatur lag zwischen 2015 und 2023 durchgehend im empfohlenen Bereich (≥ 55°C). For TWIs that are not subject to the Drinking Water Ordinance (TrinkwV), users can adjust the hot water temperature themselves (e.g., households with instantaneous water heaters or gas boilers, as well as single- and two-family homes).

The German Environment Agency (UBA) provided a series of recommendations to reduce the costs of hot water, such as installing water-saving fixtures or using hot water according to demand. However, the UBA also pointed out that a minimum temperature of 60°C should still be maintained at the outlet of the water heater (at the so-called hot water supply) and 55°C throughout the entire piping system to prevent increased Legionella growth in the TWI. It is unclear whether, and if so, how often and for how long operators of regulated systems or households lowered the drinking water temperature to save energy. Es bestand die Befürchtung, dass eine Senkung der Trinkwassertemperatur auf ein Niveau, welches das Legionellenwachstum fördert, auch zu einem relevanten Anstieg von Fällen von Legionärskrankheit führen könnte.

Through a nationwide survey of nearly 2,000 individuals, the RKI found that 28% of the respondents had lowered their drinking water temperature for an extended period. This corresponds to approximately 11.5 million people in single- or two-family homes alone. About 15% of the respondents reported having installed a water-saving fixture. When aggregated by federal state, the research team found no correlation between the average drinking water temperature (in regulated TWIs) or the proportion of people with a prolonged reduction in drinking water temperature (in unregulated TWIs) and the incidence of Legionnaires’ disease.

Due to the different responsibilities for setting drinking water temperatures, two approaches were chosen. The first approach was based on laboratory data from regulated TWIs: Commercially used systems must be regularly tested for their Legionella concentration according to the Drinking Water Ordinance (TrinkwV), with publicly used TWIs (e.g., in swimming pools or nursing homes) being tested annually and commercially used TWIs (e.g., in residential buildings) being tested every three years as part of a routine inspection. Water temperature measurements are also required during these inspections. The operators commission accredited laboratories to sample their TWIs and test the samples for Legionella. Therefore, the RKI contacted large laboratories that, ideally, accept sampling orders on a regional or national level and could provide data on the following parameters. The researchers included 48,447 routine inspections from 41,574 distinct TWIs in their analysis, provided by six laboratories. The data spanned from 2015 to 2023, with the majority of the routine inspections taking place between 2019 and 2023 (see Fig. 1). A total of 274,171 samples were collected for the routine inspections (an average of 5.7 samples per inspection). The majority of the routine inspections came from two laboratories (A and B), with WATERcontrol being one of the two major data providers. The tested TWIs were distributed across all federal states, with the majority located in North Rhine-Westphalia (26%), Baden-Württemberg (19%), and Bavaria (12%).

The detailed report can be found at https://www.rki.de/DE/Content/Infekt/EpidBull/Archiv/2024/Ausgaben/33_24.pdf?__blob=publicationFile.