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The history of using essential oils is extremely long but the scientific studies at the chemical compound level are still in need. Essential oils have been used in three different ways: using diffusers, which will use the olfactory system primarily; drinking, which will use primarily the digestive system; and topical application, which will enter primarily through the skin. Various effects of essential oils have been reported so far. These diversity in the influences of essential oils could be due to 1) different chemical compounds or combination of different chemical compounds used, or 2) the differences in the routes (olfactory, skin, gut-intestine) the same essential oil activated, which suggest that different routes activate different receptors and thus different chemical signaling pathways. We need to consider these different routes as well as the differences in the chemical compounds in studying the influences of essential oils. It is also very important to conduct studies at the chemical compound level rather than using the whole essential oil in order to know which chemical compound affects. Using chemical compounds will enable precise control of the concentration in studying the effects as well.
Koyama et al. (Wed,) studied this question.