ABSTRACT
INTRODUCTION Essential oils are produced by blossoms, leaves, and fruits of different plants and stored in special tissues such as glandular hairs, oil cells, oil receptacles, and oil ducts. For commercial use, essential oils are derived from plant material by extraction or steam distillation. Each essential oil is a complex aromatic-smelling volatile mixture of many different compounds having low molecular weights and diverse chemical structures (see Dweck, this volume, for chemical details). Predominant compounds are monoterpene hydrocarbons, sequiterpene hydrocarbons, their corresponding oxidized products (e.g., alcohols, aldehydes, ethers, ketones, and phenols), homologues of phenylpropanoids, as well as minor amounts of diterpenoids and miscellaneous volatile organic compounds (1). Normally, essential oils are characterized by their plant origin and in some cases by predominating chemical components. For essential oils, antibacterial, antifungal, anti-inflammatory, antirheumatic, antitussive, antiviral, expectorant, immunomodulatory, sedative, and blood-circulation-enhancing effects have been described. They are able to improve the odor of cosmetic preparations, and they may act on cognition, memory, and mood (2). It is commonly accepted that the biological activity of an essential oil is the result of both its active and inactive substances. Inactive substances may influence resorption, skin penetration, rate of reaction, or bioavailability of the active compounds. In addition, several active compounds may have a synergistic effect. Furthermore, the biological activity of a given essential oil may also be influenced by factors related to the medicinal plant (e.g., genotype, chemotype, and geographical origin) as well as to environmental and agronomic conditions. The means of application depends on the pathophysiology, the desired outcome, safety, and toxicity data, as well as cultural preferences. For treating respiratory symptoms and nervous disorders, inhalation may be the best means of application, whereas topical application is the best way for treating skin diseases. Oral administration is not common, except in the case of inflammation of the oral cavity and the pharynx. For dermal application, which together with inhalation is the most common application of essential oils, percutaneous absorption is of great interest. Among the local effects, the amount of a substance remaining on the surface of the body is very decisive. Many substances are able to penetrate into the different layers of the skin and even into the bloodstream. The extent of skin permeation may be the reason why essential oils often have systemic side effects or systemic bioavailability following dermal application. Another problem concerning the cutaneous use of essential oils is the
degree of skin and mucous membrane irritation. Essential oils applied undiluted may cause irritation and toxic erythema. Therefore, it is recommended that essential oils are used only in diluted forms for external application.
