[Contamination of hot water supply in office buildings by Legionella pneumophila and some countermeasures].

An assessment of the contamination by Legionella species of hot water supplied to office buildings was made based on 80 samples of water from 3 types of systems as follows: 20 samples supplied from instantaneous heaters, 20 samples from hot water storage type systems, and 40 samples from circulation type systems. Legionella spp. were detected in two samples (10.0%) from hot water storage systems and five samples (12.5%) of the circulation type. The number of Legionella spp. was 2.0 x 10-8.4 x 10(2) CFU/500 ml. All isolated strains were identified as Legionella pneumophila. All hot water samples with Legionella spp. showed a temperature range from 41-55 degrees C. Contamination may easily occur in hot water storage and circulation type systems, due to their presence for prolonged periods. Hot water may be therefor be a source of Legionnaires' disease. It is clear from the present study that Legionella spp. survived over long periods and proliferated considerably. Elimination of this organism requires careful cleaning of hot water storage tanks but this alone may not be sufficient. Heat treatment for 20 hours at 70 degrees C was found to be adequate for complete elimination. Experimentally, complete sterilization from Legionella spp. within 5 minutes was demonstrated by heating at 60 degrees C. As a countermeasure temperature of hot water should be maintained at more than 55 degrees C, as the best means for elimination while also giving consideration to the prevention of scalding.