This study is to test whether a chlorine dioxide gas-generating device, which releases a low concentration gas of chlorine dioxide in a sustained manner, can protect against respiratory infections in elderly individuals living in nursing homes. Such a device is used as a deodorant for normal domestic purposes. The investigators reasoned that the antiviral and antibacterial properties of chlorine dioxide might lead to a lowering in the incidence of respiratory infectious diseases. The study is designed as a randomized placebo-controlled double-blind crossover multicentre trial involving approximately 1500 subjects.

Chlorine dioxide (ClO2), which is used as household deodorant, is a volatile gas that displays very strong oxidative activity. Indeed, the powerful oxidative activity of chlorine dioxide (Ogata, N., Biochemistry 46, 4898-4911, 2007) is responsible for its antimicrobial activity against bacteria (Benarde, M. A., et al. Appl. Mircrobiol. 15, 257-265, 1967), fungi (Morino, H., et al. Yakugaku Zasshi 127, 773-777, 2007) and viruses (Ogata, N. and Shibata, T. J. Gen. Virol. 89, 60-67, 2008). Recently, we found that the rate of absenteeism due to illness in a school was lower in classrooms where a chlorine dioxide gas-generating device was placed than in classrooms with no such device. Based upon this unexpected observation we hypothesize that chlorine dioxide gas, at a concentration low enough not to harm humans, may lower the incidence of respiratory infections by inactivating airborne microorganism within an enclosed space.

Inclusion Criteria:

• Those who who agrees with the aim of the study

Exclusion Criteria:

• Those who do not agree with the aim of the study