Pandemic flu is a big concern in the world. Recently, some types of air cleaning systems with chlorine dioxide (ClO_2) gas which is effective against influenza virus A are proposed. In this study, experiments were conducted to evaluate the effects of environmental factors on ClO_2 gas concentration indoors. ClO_2 gas was generated in a room for 120 minutes constantly. ClO_2 gas concentration was measured in the center of the room. Ten types of environmental conditions were examined. Air temperature, humidity, illuminance, amount of organic matter and sound pressure level were set as experimental variables. As a result, it was showed that high-humidity environment had a great impacts on ClO_2 gas reduction. Good correlations were found between relative humidity and ClO_2 gas concentration, and between absolute humidity and ClO_2 gas concentration. Lighting also affected ClO_2 gas reduction. The other factors did not influence on ClO_2 gas concentration in this experiment.
