According to Austrian legal hygienic regulations, a test bacterium (Pseudomonas aeruginosa) must be reduced in pool water by the power of a 3,5 log reduction within 30 seconds. As the disinfection capability of chlorine decreases significantly with increasing pH value, the maximum pH value for pool water is set at 7,8. Given its chemical characteristics, thermal water, however, often has a higher natural pH value and also a high buffer capacity so that the pH value can only be reduced by adding inadmissibly high quantities of chemicals such as sulphuric acid. The treatment of thermal water with high mineral concentrations leads to a reduction of the naturally occurring CO2 content. This results in a shift of the carbonate hydrogencarbonate balance and thus to an increase of the pH value up to 8,7 which influences the inactivation rate of microorganisms negatively and thereby favors the survival of pathogens. Includes 2 references, figures.