The primary chemical mechanism of the beneficial medical/biological action of negative air ions necessary for life was studied. Air ion deficiency is the cause of many illnesses and treatment with air ion inhalation is effective in many cases. However, its application is limited by the absence of knowledge of the primary mechanism of its action. The superoxide anion O 2 was detected in the flow of negative air ions generated by an electroeffluvial air ionizer. Earlier, the appearance of hydrogen peroxide in solutions treated with air ions was shown. The presence of these reactive oxygen species in ultralow and low concentrations (10 12-10 6 M) suggested that the primary mechanism for the beneficial medical/biological action of negative air ions is moderate activation of free radical peroxidative oxidation within a physiological range that is lower than in tissues under pathology. It was shown in patients that treatment with inhalation of negative air ions did not induce pathological changes in superoxide dismutase activity and, under simultaneous administration of a food antioxidant, led to its mild increase. The latter, along with some previous results, supports the proposed mechanism. In addition, taking the proposed mechanism into consideration, air ion doses for treatment can be selected on an individual basis and should depend on the redox state of the patient. This should achieve better results for medical treatment with ionized air.