In our living environment, there are various microorganisms that are thought to affect human health. It is expected that excessive microbial suppression can have a negative effect on human health and that the appropriate control of the microbiome is beneficial to health. To understand how the physical environment, such as temperature and relative humidity, or housing itself affects the microbiome in a rural house, we measured temperature and humidity and collected microbial samples in a traditional Japanese house with a thatched roof. The relative humidity of outdoor air was over 60% most of the day throughout the year. Indoor and outdoor air temperature and humidity were closer to each other in summer than in winter. The DNA concentration of indoor surfaces correlated with the relative humidity, especially with the lowest annual relative humidity. In the thatched roof, outside surface relative humidity often reached 100%, and the occurrence of condensation can affect the DNA concentrations. A high percentage of archaea were detected in the house, which is not a common characteristic in houses. In addition, the microbial community was similar outdoors and indoors or in each room. These characteristics reflect the occupants’ behaviour, including opening the windows and partitions in summer. In the future, it will be necessary to conduct continuous surveys in various houses, including traditional and modern houses, in Japan.

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Microorganisms in our living environment may affect human health. Microbial suppression by air cleaners and disinfecting agents may provide protection from pathogenic materials. However, excessive microbial suppression can negatively affect human health; thus, an appropriate level of microbiome control is beneficial. It is not well understood how physical environmental conditions, such as temperature and relative humidity, and human lifestyles and behaviors affect indoor microorganisms. To understand the relationship between physical environmental conditions and microbial communities in the human living environment, we measured temperature and relative humidity and collected microbial samples in modern and traditional Japanese houses. In this study, bacteria and fungi were the target microorganisms. In both houses, the DNA concentration of microorganisms on floor surfaces was high when the average relative humidity of the room was high. The same tendency was observed for the beam and pillar surfaces in the traditional house. Although more careful consideration is needed for some indoor surfaces, such as storage ceilings and air conditioner outlets, seasonal changes in relative humidity and DNA concentrations of microorganisms on indoor surfaces exhibit some correlation.