Essential oil use is always a careful balance between potential risks and benefits. The principle of the therapeutic window helps in finding this balance.
The therapeutic window is the difference between the lowest dose or dilution that we know is effective, and the amount likely to cause an adverse reaction or toxicity. For some oils, such as Lavender, the therapeutic window is very wide – from 0.5% to 25% topically. For others, there is a very narrow window, for example 0.05% to 0.1% for Cinnamon bark.
(Safe limits for essential oils can be found in Essential Oil Safety, 2e.)
The principle of therapeutic window is explained in detail in my webinar series, Therapeutic Foundations of Essential Oils.