In general, pure essential oils can be subdivided into two distinct groups of chemical constituents; the hydrocarbons which are made up almost exclusively of terpenes (monoterpenes, sesquiterpenes, and diterpenes), and the oxygenated compounds which are mainly esters, aldehydes, ketones, alcohols, phenols, and oxides.
Terpenes - inhibit the accumulation of toxins and help discharge existing toxins from the liver and kidneys.
Esters - are the compounds resulting from the reaction of an alcohol with an acid (known as esterification). Esters are very common and are found in a large number of essential oils. They are anti-fungal, calming and relaxing.
Aldehydes - are highly reactive and characterized by the group C-H-O (Carbon, Hydrogen, Oxygen). In general, they are anti-infectious with a sedative effect on the central nervous system. They can be quite irritating when applied topically (citral being one example), but may have a profound calming effect when inhaled.
Ketones - are sometimes mucolytic and neuro-toxic when isolated from other constituents. However, all recorded toxic effects come from laboratory testing on guinea pigs and rats. No documented cases exist where oils with a high concentration of ketones (such as mugwort, tansy, sage, and wormwood) have ever caused a toxic effect on a human being. Also, large amounts of these oils would have to be consumed for them to result in a toxic neurological effect. Ketones stimulate cell regeneration, promote the formation of tissue, and liquefy mucous. They are helpful with such conditions as dry asthma, colds, flu and dry cough and are largely found in oils used for the upper respiratory system, such as hyssop, Clary sage, and sage.
Alcohols - are commonly recognized for their antiseptic and anti-viral activities. They create an uplifting quality and are regarded as non-toxic.
Terpene Alcohols stimulate the immune system, work as a diuretic and a general tonic, and are anti-bacterial as well.
Sesquiterpene Alcohols are anti-inflammatory, anti-bacterial, anti-mycotic, and ulcer-protective (preventative).
Phenols - are responsible for the fregrance of an oil. They are antiseptic, anti-bacterial, and strongly stimulating but can also be quite caustic to the skin. They contain high levels of oxygenating molecules and have anioxidant properties.
Oxides - According to The American Heritage™ Dictionary of the English Language, an oxide is "a binary compound of an element or a radical with oxygen".
All pure essential oils have some anti-bacterial properties. They increase the production of white blood cells, which help fight infectious illnesses. It is through these properties that aromatic herbs have been esteemed so highly throughout the ages and so widely used during the onsets of malaria, typhoid, and of course, the epidemic plagues during the 16th century. Research has found that people who consistently use pure essential oils have a higher level of resistance to illnesses, colds, flues, and diseases than the average person. Further indications show that such individuals, after contracting a cold, flu, or other illness, will recover 60-70 percent faster than those who do not use essential oils.
The information on this page is from the Reference Guide for Essential Oils by Connie and Alan Higley. For more information on this subject, see also the book, The Chemistry of Essential Oils Made Simple, by David Stewart.