The effects of negatively-charged air conditions were analyzed as one of the approaches to improve health and quality of life. We previously reported that the use of a charcoal coating and application of an electric voltage yielded predominantly negatively-charged particles in an experimental room, and that 2.5 hours of living in these conditions caused a slight activation of the immune system (slight elevation of serum interleukin (IL)-2), regulated blood flow, and stabilized the autonomic nervous system when compared with control conditions (no dominance of negatively-charged particles). In this study, we expanded the previous study and placed 15 subjects in negatively-charged air conditions for two weeks during the night and analyzed various biological parameters. Although individual biological reactions differed from subject to subject, natural killer (NK) cell activity increased significantly following living in negatively-charged air conditions. Taken together, the results of the previous investigation and those of this study show that repeated elevation of IL-2 (although it immediately returned to the baseline level) causes chronic and recurrent stimulation to NK cells and results in the steady activation of NK cells. Negatively-charged air particles may be a good tool to improve health and quality of life.