The Patch Test
Some oils have been known to cause irritation, allergic reactions, or sensitivities on some people. However, all substances have been implicated to some degree in unpleasant reactions on someone… so it would be wise to try the patch test before using a new essential oil or any other cosmetic substance externally.
If you have sensitive skin or a history of allergies, then by all means use a patch test to gauge your relative sensitivity to essential oils.
Here’s how to do it:
Wash an area on your forearm about the size of a quarter and dry it carefully. Apply a drop of the essential oil. If you are very sensitive in general you may wish to dilute the oil equally with a bland carrier oil such as Olive oil and apply this dilution.
Then add a loose Band-Aid and wait 24 hours. If there is no reaction – terrific – go ahead and use the oil in your formulas. Do, however, keep all essential oils away from the eyes and from moist mucous areas in general.
From: The Aromatherapy Handbook