Aromatherapy is a form of alternative meicine that uses volatile liquid plant materials, known as essential oils (EOs), and other aromatic compounds from plants for the purpose of affecting a person's mood or health. Essential oils differ in chemical composition from other herbal products because the distillation process only recovers the lighter phytomolecules. For this reason essential oils are rich in monoterpenes and sesquiterpenes, as well as other VOC substances (esters, aromatic compounds, non-terpene hydrocarbons, some organic sulfides etc.).