Essential oils consist of various chemical constituents that contribute to their aroma and therapeutic properties. They are made by plants in order to protect them from predators, pests, bacteria and fungi, and to attract pollinators. Generally, the chemical constituents contained in essential oils are made up of blocks of hydrogen, carbon and oxygen molecules. Several different classes of molecules can be found in essential oils; each varying in structure, solubility, evaporation rate and therapeutic properties.
Essential oils that are extracted from the same species of plant but vary in chemical composition are known as chemotypes. It is important to know the chemotypes of certain essential oils used in aromatherapy. For example, there are two chemotypes of basil essential oil. The main constituent of the first – known as exotic basil – is methyl chavicol, a compound that is regarded as toxic and possibly carcinogenic, and should not be used in aromatherapy. Instead, it is preferred to use the safer linalol chemotype, known as French or true sweet basil. Essential oil suppliers should clearly state which chemotype they are selling, but always check with them if in any doubt.
Terpenes are the largest class of molecules found in essential oils. They can be identified by names ending -ene. Terpene-rich essential oils are extremely volatile and are especially effective when used in inhalation. Their rapid evaporation rate means they are usually top notes – the first aromas detected in an oil blend or perfume. Several terpene constituents produce a warming and stimulating effect on the skin, which may be helpful for muscle and joint pain. Terpenes may sometimes cause skin irritation and sensitivity in some individuals.
Common terpenes include:
Alcohols all have the ending -ol. They are often less volatile than terpenes and may be middle or base notes. Alcohols possess antibacterial and antifungal properties and are generally non-toxic. They also have uplifting and sedative properties and are considered to be one of the most useful groups of chemical constituents in aromatherapy. Some alcohols may also have analgesic and antispasmodic effects.
Common alcohols include:
Phenols also have names ending in -ol. The four most common phenols are listed below; nearly all other constituents ending in -ol are alcohols. Phenols do not evaporate very quickly and usually have a very strong aroma. They generally have bactericidal properties and produce a warming, stimulating effect. Phenols are also the most irritating group of constituents, therefore essential oils containing phenols should only be used in moderation on the skin.
Common phenols include:
Aldehydes will either end in -al, or feature the word aldehyde in their name. They are derived from primary alcohols but often smell stronger than their equivalent alcohol. Aldehydes generally have calming and sedative effects, as well as antibacterial properties. They can cause irritation to the skin and mucous membranes, therefore essential oils containing high percentages of aldehydes should only be used in moderation.
Common aldehydes include:
Ketones are derived from alcohols and normally end in -one, with the exception of camphor. They have fairly high boiling points and often share a potent, minty-camphoraceous odour. Ketones typically ease nasal congestion and benefit the respiratory system. They may also possess wound-healing properties. Some of the most toxic essential oils contain ketones, such as mugwort, tansy and wormwood, which all contain thujone. However, not all ketones are toxic.
Other ketones include:
Esters are a widespread group of constituents that often have a fruity aroma. They are made from a combination of alcohols and acids and are subsequently named after both groups of molecules. Esters typically have antispasmodic, anti-inflammatory, analgesic and sedative properties and are generally non-toxic.
Common esters include:
OxidesWith a few exceptions, oxides usually keep the name of the molecule they were derived from, followed by the word oxide. One such exception is cineol, also known as eucalyptol, the most prevalent oxide found in essential oils. Oxides probably have the strongest odours of all the classes of molecules, and they give essential oils their characteristic aromas even at low percentages. With the exception of cineol, little is known about the therapeutic effects of oxides. Cineol is beneficial for the respiratory system due to its expectorant and anti-inflammatory properties. However, due to its strong odour, cineol may cause irritation of the mucous membranes in some individuals.
Common oxides include: