The disinfecting effect of ozonized water has been investigated. Under the precondition that the dental chair had been thoroughly sanitized, the system showed a good disinfecting effect. Finally, for reasons of practical medical treatment, the ozone concentration in air and, for reasons of hygiene in drinking water, the ozone concentrations in water were determined under various conditions. In addition, the influence of continuous-flow water heaters is discussed. The problem of continuous-flow water heaters regarding their effects on the colonisation of water by microbes proves not to be significant. The exposure of patients to disinfectants is discussed.