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The essential oils are obtained by extraction from aromatic plant material, that is rich in essences (not to be confused with the term, therefore, aromatic, used in organic chemistry for structures highly unsaturated).
The essences are produced by plants for many reasons, and in some cases maybe even as scrap. The strongest case they want to play the essences allelopatica function, antibiotics, attraction of pollinators, and act as intermediaries of energy reactions.
The essential oils are concentrated and complex mixtures of chemicals: the use without the supervision of a doctor or a pharmacist can be dangerous: the application of pure essential oils on the skin can lead to inflammation and lesions of the skin, their ingestion (depending on the type of oil and the amount ingested) is potentially lethal.