A very small number of oils have been found to cause sensitisation of the skin. This is not quite the same as irritation, since that is a simple reaction following contact with the irritant substance. In sensitisation, once the skin has been exposed it will be more sensitive to the same irritant in future, and even very low dilutions may cause a rash, blotches, itching or blistering. Occasionally, other, similar substances will produce a reaction once the skin has been sensitised.

In practice, this is not a major problem, since the oils which produce this effect most markedly are rarely used in therapy. The only oil in general use which will produce sensitisation in people with normal skin, is Cinnamon Bark. Sensitisation has been reported in a few instances with oils of Clove Bud and Ylang Ylang.

People with very sensitive skins, or who have a history of eczema or dermatitis should avoid using oils of Citronella, Geranium, Ginger, Pine (Scotch and Dwarf) and Terebinth, and indeed such people should use all essential oils with caution and in low dilutions until they have established that no reaction is caused by any particular oil.

If you are not sure if you may be sensitive to certain oils, always do a skin patch test first.