Each successive exposure to the offending substance causes a greater reaction than the previous one did until eventually the person begins to notice the rash or other symptom. Each further exposure to the substance will create a further, more serious reaction. It is possible for some essential oil constituent chemical to fit into this latter category. Indeed, we know little about the metabolism of many of these chemicals as they move, or try to move, inward from the surface of the skin. The most common reactions which we will see, however, are one or another sort of dermatitis, a rash, eczema, etc.
Allergies are unpredictable things.
Some, especially those of childhood, may disappear in time. It will be better for us to err on the side of caution, however, and assume that once a person has an allergic reaction to a substance, the next encounter with this substance will most likely produce a worse reaction. One of the things I found from the Botanical Dermatology Database is that virtually every plant ever used to treat skin conditions has also been responsible for causing dermatitis in someone. We must always remember that there is no such thing as a perfectly safe substance. Someone somewhere will react negatively to anything.