Inactivation of feline calicivirus, a norovirus surrogate, by chlorine dioxide gas

The efficacy of gaseous chlorine dioxide (ClO2) against feline calicivirus (FCV), a norovirus surrogate, in the dry and the wet states on a hard surface was evaluated. We demonstrated that low-concentration ClO2 gas (mean 0.08 ppm, 0.22 microg/I) could inactivate FCV in the wet state with 0.5% fetal bovine serum (FBS) within 6 h in 45 to 55% relative humidity (RH) (> 3 log10 reductions) and FCV in the dry state with 2% FBS (percentage of FBS in the viral suspension) within 10 h in 75 to 85% RH (> 3 log10 reductions) at 20 degrees C, respectively. Furthermore, a < 0.3 ppm concentration of ClO2 gas (mean 0.26 ppm, 0.73 microg/l) could inactivate (below the detection limit) FCV in the dry state with 5% FBS within 24 h in 75 to 85% RH at 20 degrees C. In contrast, in 45 to 55% RH at 20 degrees C, ClO2 gas had little effect even when the FCV in the dry state was exposed to high-concentration ClO2 (mean 8 ppm, 22.4 microg/l) for 24 h. These results suggest that humidity plays an important role in the inactivation by ClO2 gas of FCV in the dry state. According to the International Chemical Safety Card, threshold limit values for ClO2 gas are 0.1 ppm as an 8-h time-weighted average and 0.3 ppm as a 15 min short-term exposure limit. From these data, we propose that the treatment of wet areas of human activity such as kitchens, toilets, etc., with low-concentration ClO2 gas would be useful for reducing the risk of infection by noroviruses (NV) without adverse effects. In addition, we believe that the application of a combination of a < 0.3 ppm concentration of ClO2 gas and a humidifier in places without human activity may make it possible to inactivate NV in the dry state on any surface within a contaminated room without serious adverse effects.