The present disclosure relates to methods and devices to improve respiratory system health, increase resistance to infection, and increase the hypothiocyanate ion in mammalian lungs. The methods generally comprise: generating a Hydrogen Peroxide Gas that is non-hydrated and free of ozone, and directing the gas comprising primarily Hydrogen Peroxide Gas into an environment and exposing a subject to the environment such that the Hydrogen Peroxide Gas acts to the increase of the hypothiocyanate ion in mammalian lungs.