There are four different routes by which essential oils can access the body. These include direct absorption through the skin, inhalation by the lungs, inhalation by the olfactory system, and direct ingestion. These will each be discussed separately below.

• ABSORPTION THROUGH THE SKIN

    Many sources claim that when essential oils are applied during massage the diluted blend of essence and vegetable oil is absorbed through the pores of the skin into the bloodstream and interstitial fluid. Once the oils enter the bloodstream their effects on the body vary according to the oil being used. They can have physiologic effects such as stimulating circulation, increasing the elimination of toxic substances, increasing the secretion of endogenous chemicals such as bile and estrogen, decreasing blood pressure, and activating the immune system. It is also believed that essential oils may play a role in the treatment of pain and depression by stimulating the release of endorphins and activating the body’s natural opioid system.

    Although the theory of absorption is widely accepted among proponents of aromatherapy, there is no scientific evidence to support it. At least one web-based aromatherapy source openly admits that this theory of absorption is a myth (The Guide to Aromatherapy). Sylla Shepperd-Harger, author of the book "The World of Aromatherapy" states clearly that, from proper aromatherapy experiments where oils are applied to the skin of a person with a separate air supply, there is NO evidence that significant amounts of the oils go through the skin. Some studies do reveal that very small amounts of essential oils show up in the blood but the miniscule amount has no proven therapeutic value. Similar studies by dermatologists reveal the same conclusion. In fact, they have reported that those oils that do have penetrating effects are the same ones that are likely to cause irritation and allergic reactions.

    Where it is unlikely that essential oils are absorbed into the blood, it is likely that some oils do have analgesic properties with direct topical application. One study published in Alternative Therapies demonstrates that one of the components in essential oils that produce an analgesic effect is 1.8-cineole, also known as eucalyptol. This chemical can be found in eucalyptus, rosemary, lavender, chamomile, and cardamon.

    The beneficial effects of aromatherapy with massage most likely come from the therapeutic power of touch and direct inhalation of the oils, not from their absorption into the blood. Touch is used in the practice of medicine to convey compassion, provide comfort, and ease pain. It has been described as the "first and most fundamental means of communication". Massage has been shown to improve a patient’s ability to relax and make pain more bearable. Even without considering the potential analgesic effects of essential oils, aromatherapy with massage may play a role in altering the perception of pain by drawing attention to or away from the site of pain depending on which situation best addresses the patient’s psychological needs.

    This theory introduces another very important concept to consider in the scientific evaluation of aromatherapy, the placebo effect. A placebo is any putatively inactive substance or maneuver tested in controlled studies for comparison with presumed active drugs or proven treatment prescribed for the relief of symptoms to meet a patient’s demands. Placebos have been shown to help patients with anxiety, depression, tension, headache, cough, insomnia, the common cold, arthritis, ulcers, hypertension, nausea, and a number of other conditions. The two components of the placebo effect that are thought to contribute to the subjective and objective changes associated with them are the anticipation of results, or suggestibility, and spontaneous change, the body’s inherent ability to heal. For a more detailed explanation of the placebo response and links to other sites, see Dr. Jeffries’ Homeopathy site at the Creighton Alternative Medicine page.

• INHALATION INTO THE LUNGS

    Another route by which essential oils access the body is inhalation. During use with massage, baths, compresses, and vaporization the vapors of the essential oil are inhaled by the respiratory system and absorbed into the bloodstream through the lining of the lungs. The essence can then circulate through the body and produce it’s therapeutic effects. Although this theory is widely accepted, there is little scientific evidence to support the absorption of essential oils into the bloodstream by this route.

• INHALATION BY THE OLFACTORY SYSTEM

    The most widely accepted theory explaining how essential oils access the body to produce their beneficial effects is through olfaction. We all know about the power aromas and odors possess to generate emotion and change behavior. Scientists believe that olfaction can influence mood, thought, sexual behavior, cognition, vigilance, mental health, and possibly general overall health. Olfaction is a chemosense, meaning that smell is accomplished when aromas, or chemical molecules, floating through the air reach the olfactory system in the body. The olfactory system is a complex web of mucous membranes, cilia, nerve cells, nerve fibers, and specific parts of the central nervous system. The chemical components of essential oils first access the olfactory system via the cilia of the nasal epithelium. These cilia project into the nasal cavity from the receptor cells and are bathed in lipid-rich mucous secreted by Bowman’s glands. Olfaction is then thought to be mediated by a combination of odorant binding proteins, odorant receptors, G-protein activation, ion channel activation, cAMP, Phospholipase C, IP3/DAG systems, and calcium concentrations. The other end of the receptor cell projects to the olfactory bulb which then transmits the signal to the limbic system of the brain which is made up of the amygdyla and the hippocampus. The amygdyla mediates our emotions and the hippocampus is responsible for our memory. Together they control our emotional, hormonal, metabolic, and stress responses all of which may be influenced by smell.

    Exactly how this occurs is not completely understood, however, the effect of odors on the brain’s electrical activity have been "mapped" using computers and EEGs. A study by Tim Jacobs, professor of cell physiology at Cardiff University,  compared the effects of two essential oils on the alpha waves of the brain. One oil, Rosemary, was known to have a stimulant effect and the other, Ylang-Ylang, was known to have a soothing effect. The study showed that the stimulant depressed alpha wave activity whereas the soothing aroma increased alpha wave activity. This data is consistent with the expected results. Another study by Jahangeer, Melluier, and Caston published in Physiology and Behavior looked at the influence of olfactory stimulation on nociceptive behavior in mice. They hypothesized that because the cerebral localization of pain and the olfactory pathways are anatomically and physiologically linked, olfactory stimulation can modify the response of the mice to painful stimuli in the presence of positive (attractive), negative (aversive), or neutral odors. Their results confirmed that a relationship does exist between olfaction and the perception of pain in mice. The authors of this study suggest that this relationship is likely mediated by the endogenous opiod system as well as by neurons containing substance P, GABA, and dopamine within the olfactory tract and the limbic system. More research is needed to verify the validity of this argument but this study does provide some objective data to support the potential benefits of aromatherapy in the treatment of pain.

• INGESTION BY MOUTH

    Also called aromatic medicine, the practice of ingesting essential oils through the mouth requires the training of and the prescription by a primary care provider or certified aromatherapist. This route of administration is rarely used in the United States but is commonly used in England, France, and New Zealand. Caution should be used to avoid potential toxicity.