Air-cleaning systems using a low concentration of chlorine dioxide (ClO 2) gas for prevention against infection by the influenza virus have been developed. ClO 2 gas is easily decomposed by indoor climate factors. Three experiments were conducted to evaluate the effects of indoor environmental factors on the decrease of ClO 2 gas concentration. The reaction (decay) rate constant was also determined. High air temperatures and light irradiation accelerated the decrease of ClO 2 gas concentration; the latter had the greatest influence on the decrease of ClO 2 gas concentration among the conditions. A quicker decay of concentration and greater value of the reaction rate constant of ClO 2 was found when a UV lamp was used; this is because the gas absorbs irradiation in the UV range. ClO 2 gas concentration was significantly reduced when an evaporative humidifier was employed due to adsorption and a chemical reaction between the gas and the filter in the humidifier.
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