Until comparatively recently it was believed that the skin was an efficient barrier that prevented external substances from entering the body. This theory was challenged back in the 1960's with the development of the drug DMSO which was shown to transport other substances through the skin barrier and into the bloodstream.
The ability of many chemicals to be absorbed through the skin is now widely accepted and in some cases is used as a route of medication. Drugs such as nicotine and female hormones are now routinely administered in the form of a skin patch and are readily absorbed into the body. Studies have shown that many substances can be absorbed through the skin and that up to 60% of some substances applied to the skin may enter the body through this route.