[Current status and issues of hygiene management in bathtubs of public bathhouses and accommodations: Considerations based on the detection of Legionella in bath water].

Objective　Most legionellosis outbreaks in Japan have been attributed to bath water. This study aimed to examine the relationship between hygiene control and the detection of Legionella, as well as the problems associated with the use of adenosine triphosphate (ATP) as a pre-test to obtain information that will be useful for monitoring bathtubs in public bathhouses and accommodations.Methods　From fiscal years 2017 to 2024, monitoring was conducted in bathtubs in public bathhouses and accommodations in Takaoka City, Toyama Prefecture. Interviews and simple water quality tests were conducted. Legionella testing was conducted when ATP exceeded 80 relative light Unit (RLU), Legionella testing was conducted. Tested bathtubs were analyzed to assess the relationship between Legionella detection and six hygiene control indicators as well as ATP levels.Results　Legionella was detected in 29 of the 94 bathtubs analyzed. Regarding the concentration of free residual chlorine at the time of water sampling and the cleaning and disinfection of the hair catcher, there was a 20.7%-23.2% difference in compliance between Legionella-detected and non-detected bathtubs. When the concentration of free residual chlorine at the time of water sampling and cleaning, and disinfection of the hair catcher did not meet the standards, the detection rate was the highest at 85.7% (6/7). The detection rate in circulating bathtubs that used tap water was 85.7% (6/7). However, in circulating bathtubs that used groundwater, the detection rate was only 25.0% (19/76). In contrast, in the non-circulating bathtubs that used groundwater, the detection rate was 36.4% (4/11).Conclusion　It is difficult to prevent Legionella multiplication only by controlling the concentration of free residual chlorine. If the hair catcher is cleaned and disinfected less than once a week, the risk of Legionella growth may increase. Immediate improvement is required if the concentration of free residual chlorine and cleaning and disinfection of the hair catcher do not meet these standards. To use ATP as a pre-test, it is necessary to investigate ways to improve the detection rate in bathtubs that use groundwater.