Diffusion

Essential oils are highly concentrated substances.  In some cases, thousands of pounds of plant matter are required to distill a single ounce of pure essential oil.  "Less is more" is a common reminder among aromatherapists to heed the strength of the essential oil.  

In general, inhalation is the safest way to use essential oils - and it is quite safe.  Inhalation can be as simple as a drop of EO on a tissue nearby, a drop in a bowl of water for steaming one's face, or a couple of drops in a high end diffuser.  Diffusion allows a larger space to be affected by a very low concentration of the scent.  

Is aromatherapy effective by inhalation and diffusion?  For certain uses, yes!  This method works because aromatic molecules, when inhaled through the nose, bind with olfactory receptors which lie very close to the limbic section of the brain.  The limbic area is the brain's seat of emotions and behavior. According to Jennifer Peace Rhind (2012),

Via the sense of smell and the limbic system, aromatics can act directly on the mind and emotions, and different odours will induce different states, such as sedation, stimulation, happiness, relaxation, apathy, depression, irritation, sensuality, euphoria, etc.  These effects will vary between individuals; therefore it is important to account for individuality when selecting essential oils for therapeutic use.

Studies have shown the effects of diffused rosemary and lavender essential oils on academic tasks.  These effects are not only measurable, but they are repeatable, giving scientific credence to the historic uses both rosemary and lavender.  

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Buchbauer, G., Jirovetz, L., Jáger, W., Plank, C., & Dietrich, H. (n.d.). Fragrance compounds and essential oils with sedative effects upon inhalation. Journal of Pharmaceutical Sciences J. Pharm. Sci., 660-664.

Diego, M., Jones, N., Field, T., Hernandez-Reif, M., Schanberg, S., Kuhn, C., . . . Galamaga, R. (n.d.). Aromatherapy Positively Affects Mood, Eeg Patterns of Alertness and Math Computations. Int J Neurosci International Journal of Neuroscience, 217-224.

Rhind, J. (2012). Theoretical Perspectives. In Essential oils: A handbook for aromatherapy practice (2nd ed., p. 55). London: Singing Dragon.