In the past five decades, it has not been publicly reported that there may be a certain relationship between fluctuation of air ions concentration (AIC) and intensity of ultraviolet (UV) irradiations in small chambers. However, our experiments with UV irradiations in small chambers indicate that 254 nm UV-C irradiation conduces to high negative AIC (NAIC) and low positive AIC (PAIC) in small chambers, and that NAIC is approximately proportional to average UV-C light intensity in small chambers. These phenomena cannot be covered by the traditional model. To address these, we developed a new model to simulate and analyze these phenomena. With this model, not only do the simulation results well agree with the experimental results, but also the linear relationship between the NAIC and the average UV-C light intensity is well interpreted. (c) 2006 Elsevier Ltd. All rights reserved.