In schools, sick leave might to be a significant index of productivity. Elementary schools are thought to be amplifiers for i nfections in influenza in communities. In School Health and Safety Act, children are told to stay home in case of an epidemic of influenza. The humidity ratio had been reported to be a critical factor for infection of influenza. However the relationship between the really measured indoor thermal factors and the incidence of influenza is rarely investigated. Then the room

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... temperature a nd relative humidity were continuously measured in twelve classrooms in a public elementary school in Tokyo. And the period of analysis was set as 3 winter seasons from 2005/2006 to 2 007/2008. The average values of the room temperature per every winter season in all classrooms were met the School Environmental Sanitary Standard. On the other hand, the average values of indoor relative humidity in some classrooms were lower than the standard value. The high correlation coefficients were not obtained in the relationship between the average values of indoor thermal factors, i.e., air temperatur e, relative humidity, and humidity ratio per every winter season in each classroom, and the number of absence on account of flu. Focusing on the period when absence on flue had occurred, the room temperature of two days before the absence had a high correlation with the number of absence on flu. . Keywords: Influenza, Elementary schools, Humidity ratio, Relative humidity , , , Fisk 1 , 2 2 3 5 2 3 3 50.0 66.7 33.4 50.0 3 50.0 66.7% 3)