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Although the skin acts as a protective barrier against many micro-organisms and chemicals, some chemicals can penetrate the skin and enter the blood stream. Whether or not a chemical is absorbed through the skin depends on its structure: chemicals need to be able to dissolve in both water and fat (lipids) to get through the skin. Those that are insoluble, or dissolve only in fats or water, and chemicals made up of very large molecules, tend not to penetrate the skin. Chemicals are more easily absorbed where the skin is thin, such as on the forearms, than through the thick skin covering the palms of the hands and soles of the feet. Chemicals are also more easily absorbed if skin is moist or damaged. Some chemicals, e.g. organic solvents, cause 'defatting' of the skin, making it a less effective barrier against further chemical exposure.

Methyl ethyl ketone (MEK), the organic solvent used by Tony Bradshaw (see p 19), is rapidly absorbed through the skin. In a study using human volunteers, MEK was applied to the skin of their forearms, and could be detected in the air they breathed out just three minutes later. During that time, the MEK had made its way through the skin, into the blood, and then to the lungs. Because MEK is soluble in water, it is absorbed through the skin even faster if the skin is sweaty. However, the amount of MEK in exhaled air accounted for only 10% of the amount applied to their skin. The other 90% was excreted in the urine, both as MEK and as its metabolites (Environmental health criteria 143 methyl ethyl ketone, WHO, 1993).

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