Absorption is a route by which substances can enter the body through the skin. Along with inhalation, ingestion and injection, dermal absorption is a route of exposure for toxic substances and route of administration for medication. Absorption of substances through the skin depends on a number of factors, the most important of which are concentration, duration of contact, solubility, physical condition of the skin and part of the body exposed. Certain substances called carriers can be used to greatly increase the amount of other substances that is able to penetrate the skin. Dimethyl sulfoxide (DMSO) is a carrier that is frequently used to transport medication through the skin. This allows treatment to be localized, unlike with ingestion.