For a toxic chemical to be harmful, it must first travel from the environment to a specific target site within the body. Much of this journey happens through the deceptively simple process of diffusion: molecules migrating away from their source. Examples of diffusion are all around us, from salts dissolving into water to fumes dispersing from a smokestack at an industrial plant into the atmosphere. But diffusion becomes more complicated when the molecule must cross from one environmental compartment into another. For example, when a molecule diffuses from water into the air, properties of the molecule, air, and water all come into play to determine the rate at which diffusion occurs. Furthermore, the distance traveled by the molecule from its source and the dimension of the surface from which the molecule is diffusing also make a difference in the overall rate of diffusion.