This balance of and reaction between constituents is also what makes one oil more or less toxic than another. The proportion of a toxic constituent in one oil may be balanced by other constituents which make the potential toxin less significant and allow the oil to be useful in therapy.
In this way, aromatherapy is not unlike homeopathy which also treats ailments with substances that are poisonous (belladonna and arsenic, for instance). The proportions are tiny, though, and when administered in correct proportions, even a poison can have a beneficial and dynamizing effect.
An experienced aromatherapist will understand enough about the composition of essential oils to know how to combine them so that these further admixtures balance each other, with an alkaline constituent in one, say, being balanced by an acid constituent in another. This is chemistry in practice. There is no way in which knowledge like this can be gained in a short aromatherapy course.