Chemical Compounds
Essential oils are complex and may contain many hundred components, some in minute quantities and it is thought that this synergistic blend adds to their effectiveness. We can only touch on the subject here but those wanting an in depth but readable text on some of the more commonly used oils will enjoy the book "Advanced Aromatherapy".

Terpenes
This is a group of chemical constituents with widely varying properties. Common Terpenes include Limonene, an anti-viral agent found in most of citrus oils. Pinene, which is a strong antiseptic found in Pine and Turpentine oils. Others, like Chamazulene (found in Camomile) possess noticeable anti-inflammatory and bacterial properties.

Esters
These are among the most common compounds found in essential oils. Esters have fungicidal and sedative properties.

Aldehydes
Most commonly found in lemon scented essences. Aldehydes generally have sedating but uplifting qualities.

Ketones
Certain Ketones are potentially toxic, so oils containing large quantities of this need to be treated with a degree of caution (see hazardous oils section). Not all Ketones are bad, for example Jasmine and Sweet Fennel contain relatively benign ones and they are very helpful with problems affecting the upper respiratory tract.

Alcohols
These substances, when found in essential oils, tend to have good antiseptic and anti-viral properties as well as uplifting qualities. Common examples include Linalol (a major constituent of Lavendar) and Citronellol (found in Rose, Lemon, Eucalyptus and Geranium).

Phenols
Phenols are bacterial and have strong stimulating effect in effect on the central nervous system. Any oil containing relatively large quantities of certain Phenols are potential skin irritants.

Oxides
Oxides are commonly found in oils of of the Camphoraceous nature which tend to have expectorant effects. Good examples include Rosemary, Eucalyptus and Tea Tree.