The uptake and release of molecular iodine by the skin: chemical and bactericidal evidence of residual effects caused by povidone-iodine preparations.

The residual effect of iodine-based disinfectants is caused by a dynamic back-diffusion which is the reverse of the absorption occurring during application. A very sensitive photometric method was used to measure the iodine flux of the back-diffusion after treating the skin with povidone-iodine preparations and Lugol's solution. After removal of the preparation the intensity of the iodine flux decreases with time and correlates with the amount of iodine being resorbed. The latter depends on the concentration of free molecular iodine of the preparation, the contact time and the thickness of the horny layer of the treated skin. With Lugol's solution (approximately 170 ppm free molecular iodine) iodine flux could be observed 12-24 h after an application of 20-60s, while with low-level povidone-iodine preparations (0.5-20 ppm free iodine) the measurable flux lasted only 0.5-1 h after an application time of 3 min. The bactericidal activity of the back-diffusing iodine from skin previously treated with a commercial povidone-iodine preparation was assayed by using an inoculation with Micrococcus luteus. This showed a logarithmic reduction of 0.4 in bacterial concentration. Since the skin was washed with soap for 1 min after application this bactericidal action reflected the residual effect of the back-diffusing iodine.