The two most commonly used iodine preparations are 10% PVP-I and Lugol’s Solution. When applied, they dry and form a thin layer on the skin surface. Both preparations contain multiple iodine species including negligible amounts of molecular iodine (I2); the species responsible for anti-microbial activity. The make up of these preparations keeps almost all of the I2 in them on the skin surface where it evaporates.
Molecular iodine does not need to be restricted to the surface of the skin. Efficacy does not benefit from any other iodine species in a formulation. On its own, the I2 species can penetrate skin layers and enters into (partitions into) the fatty tissue of the dermis/hypodermis. From within this fatty tissue, I2 is released over time as a gas.