Norio Ogata, Tomoko Koizumi and Fumihiro Ozawa
Chlorine dioxide (ClO2) gas can protect against influenza virus infection at a concentration of 0.03 parts par million (volume/volume), which is believed to be nontoxic in humans. This suggests an opportunity for the development of a novel protective method against airborne infectious diseases. This method is especially applicable to the highly pathogenic H5N1 influenza virus, as there is currently no safe and effective protection against H5N1. However, concerns remain regarding the safety of ClO2 gas for protection against virus infection, especially whether a concentration of 0.03 parts per million is really non-toxic when it is used for humans in closed or semiclosed spaces. In view of the importance of the usefulness and necessity of low concentration ClO2 gas treatment, it is critical to determine the ?no observed adverse effect level? (NOAEL) of ClO2 gas. In 1972 Paulet and Desbrousses reported that the ?lowest observed adverse effect level? (LOAEL) of ClO2 gas was 1 part per million. We attempted to confirm their data under carefully designed experimental conditions. Here, we performed a rat hole-body inhalation toxicity study, where rats were exposed to 1 part per million ClO2 gas for 5 hours per day and 5 days per week over a period of 10 weeks. The rats were exposed to a meticulously controlled low-concentration ClO2 gas. No adverse effect was observed under these experimental conditions in contrast to the study by Paulet and Desbrousses. We conclude that the no observed adverse effect level of ClO2 gas at these experimental conditions is 1 ppm. We hope this result will help develop a preventive method against airborne microbial infectious diseases of humans.