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Background / Aims ClO2, the so-called "ideal biocide", could also be applied as an antiseptic if it was understood why the solution's rapid killing of microbes does not cause any harm to humans or to animals. Our aim was to study both theoretically and experimentally its reaction-diffusion mechanism to find the source of that selectivity. Methods ClO2 permeation measurements through protein membranes were performed and the time delay of ClO2 transport due to reaction and diffusion was determined. To calculate ClO2 penetration depths and estimate bacterial killing times, approximate solutions of the reaction-diffusion equation were derived. Additionally, as a preliminary test, three patients with infected wounds were treated with a 300 ppm high purity ClO2 solution and the healing process was documented. Results The rate law of the reaction-diffusion model predicts that the killing time is proportional to the square of the characteristic size (e.g. diameter) of a body, thus, small ones will be killed extremely fast. For example, the killing time for a bacterium is on the order of milliseconds in a 300 ppm ClO2 solution. Thus, the few minutes of contact time (owing to the volatility of ClO2) is quite enough to kill all bacteria, but short enough to keep ClO2 penetration into the living tissues safely below 0.1 mm, minimizing cytotoxic effects. Pictures of successful wound healings confirm these considerations. Various properties of ClO2, advantageous for an antiseptic, are also discussed. Most importantly, bacteria are not able to develop resistance against ClO2 as it reacts with biological thiols which play a vital role in all living organisms. Conclusion Selectivity of ClO2 between humans and bacteria is based not on their different biochemistry, but on their different size. Preliminary clinical results encourage further research with this promising local antiseptic., Comment: 32 pages incl. 12 pages of Supplementary information; 6+1 figures, 3 tables