The principles and embodiments of the present invention relate to methods and systems for safely providing NO to a recipient for inhalation therapy. There are many potential safety issues that may arise from using a reactor cartridge that converts NO2 to NO, including exhaustion of consumable reactants of the cartridge reactor. Accordingly, various embodiments of the present invention provide systems and methods of determining the remaining useful life of a NO2-to-NO reactor cartridge and/or a break-through of NO2, and providing an indication of the remaining useful life and/or break-through.