INTRODUCTION

Volatile oils are complex chemical substances which give scent to the various parts of a plant. On exposure to air or steam they evaporate and are therefore called ‘volatile’ oils. In times gone by  they were considered to embody the essence or etheric properties of a plant (as opposed to the more mundane properties of  air, fire, water and earth also inherent in all living things). They are therefore also known as ‘essential’ or ‘ethereal’ oils. On extraction they are usually colourless and lighter than water. In the plant they may occur in glands and glandular hairs (eg. the Lamiacaeae - Thyme, Peppermint, Sage); oil cells (eg. the Lauraceae - Cinnamon, Cassia); oil and resin ducts (eg. Apiaceae - Cumin, Fennel, Celery and Gymnospermae - Pine, Fir); oil reservoirs (eg. citrus - orange, lemon, lime). In living plants  volatile oils may act as hormones, insect repellents, growth inhibitors for neighbouring plants, protection against disease or as pheromones to attract insects to the flower for fertilization. 

Volatile oils are extremely complex compounds, frequently having two or three hundred constituents. There is usually only a small amount of the volatile oil in a plant, for example, it takes 500 pounds of Sage, Rosemary or Thyme to produce 1 pound of oil, and up to 1 1/2 tons of rose petals to produce 1 pound of oil! (Schnaubelt).

Methods of extraction for volatile oils varies. All medicinal volatile oils are extracted by distillation with the exception of oils of lemon (expressed) and oil of cade (separation) (Trease and Evans, 1989). Water distillation is used for the more robust oils, particularly those high in terpenes. Water and steam distillation is the method most commonly used, being powerful yet gentle for more sensitive constituents. The volatile oils thus distilled float on the surface of the distillate and can be separated easily. Direct steam distillation may be used where the plant is prepared very soon after picking. Volatile oils which are readily soluble in fixed oils so may be extracted by digestion into fats. Enfleurage uses cold fats while maceration uses hot oils. Both methods require the additional use of alcohol or other solvents to dissolve the volatile oils out of the fixed oils, the alcohol then being evaporated away. Expression of oils, as with lemon, involve the use of pressure, rather like cold-pressing of olive oil. Solvent extraction involves the use of chemical solvents such as petroleum, acetone or ether. The volatile oils dissolve into the solvent which is then mixed with alcohol to extract the volatile compounds. When the alcohol is evaporated away there may remain in the volatile oil some trace of the solvent so this method is used only for commercial and food grade oils. For clinical aromatherapy the volatile oils produced in this way are not sufficiently pure (Willard 1992).

Because of their scarcity, volatile oils may be extremely expensive and adulteration is common. A good quality essential oil (except the resinous ones) will evaporate completely over time. A simple test for purity is to place a drop onto a sheet of white paper. If a yellowish discolouration remains after some hours then the oil has probably been adulterated with almond or some other fixed oil. This can be confirmed by the feel of a pure versus an adulterated oil. A pure volatile oil is light and non-greasy to the fingers, but a volatile oil adulterated with almond or other vegetable oil will feel greasy.  

The very best oils are labelled genuine and authentic meaning that they are pure, natural and complete, containing absolutely no fixed vegetable oils or synthetic substances. They have also been distilled at a reduced pressure to assure the highest quality extraction and have not been redistilled (Schnaubelt).

BIOCHEMISTRY OF VOLATILE OILS

Monoterpenes are thus hydrocarbons consisting of two isoprene units, Sesquiterpenes are hydrocarbons consisting of three isoprene units and so on.  Molecules larger than diterpenes do not occur in volatile oils because their molecular weight would be too great tp permit distillation to occur.  Monoterpenes are the most common  type of terpene in volatile oils and may be either cyclic (closed chain) or acyclic (open ended chain).  Cyclic Monoterpenes can be mono-cyclic or bi-cyclic and various side chains and may be added to form the alcohols, ketones, aldehydes, esters and so on  (Price and Price 1995). Some terpenes also occur in the glycosidal (sugar-linked) form. Thus there are glycosides (geraniol, nerol & citronellol) in the petals of some species of Rosa, glucosides and galactosides (thymol & carvacrol) in the leaves of Thyme and glucosides (eugenol, nerol & geraniol) in the leaves of Melissa (Trease and Evans 1989). 

Phenylpropanes also occur in volatile oils, derivatives of phenylalanine, tyrosine and cinnamic acid.  They are less common than the monoterpenes, but no less significant.  Estragole (methyl chavicol) in Tarragon and Holy Basil oils, eugenol from cloves, vanillin from Vanilla, anethole from anise, cinnamaldehyde in Cinnamon bark, safrole from Sassafras, myristicin from Nutmeg  and apiole in Celery seed and Parsley oils are all examples of Phenylpropanes. 

The spatial arrangement of the molecules is very significant.  All naturally occurring molecules bend light rays, either to the right (dextro-rotatory) or to the left (laevorotatory), and this can have a significant effect on the resulting molecular effects.  For example carvone is present in the laevorotatory form in spearmint and in the dextrorotatary form in caraway, giving rise to two markedly different scents.

PROPERTIES EXHIBITED BY ALL THE SIMPLER VOLATILE OILS INCLUDE: (Mills 1986)

* Lipid solubility and therefore easy distribution throughout the  body, permitting therapeutic activity in all parts of the body. 

* General antiseptic effect combined with leucocytosis (increase in number if white blood cells). These actions are pronounced in the oils of Eucalyptus and Allium sativum (garlic). 

* Stimulation of tissues with which they come into contact. This acts as a counter-irritant and vasodilator, and when applied topically causes local anaesthesia and a rubefacient effect. 

* Internally, this local tissue stimulation acts on the taste buds to cause a reflex increase in salivation and gastric secretions. They also increase peristalsis and improve it's co-ordination, thus relieving gas and griping (a carminative  effect). 

* Stimulation of the gut also induces a reflex effect of cardiac stimulation, peripheral vasodilation and stimulation of respiration. 

Another feature of volatile oils currently being researched is their apparent ability to dissolve into the olfactory bulb on inhalation and thus act directly on the limbic system of the brain. This is the part of the brain that deals with the sense of smell, as well as controlling emotions and feelings. This connection between smell and emotion is part of the logic behind the practice of aromapsychotherapy which utilizes selected volatile oils to affect specific physiological or psychological conditions. 

FUNCTIONAL GROUPS FOUND IN VOLATILE OILS  AND THEIR GENERAL PROPERTIES (Schnaubelt)

Group	Effect	Property	Example
Ketones	mucolytic	neuro-toxic	Sage
Aldehydes	calming	irritant	Lemon grass
Esters	balancing	soothing	Lavender
Ethers	balancing	soothing	Tarragon
Alcohols	toning	energizing	Ravensare
Phenols	stimulant	hepato-toxic	Savoury
Terpenes	stimulant	skin irritant	Pine

HYDROCARBON VOLATILE OILS
These are unoxygenated terpenes (ie. they have no added side groups) and are found in practically all volatile oils. They may be Monoterpenes, Sesquiterpenes or diterpenes.  Their names usually end in ‘-ene’. 

Monoterpene hydrocarbons
Limonene from citrus rinds, Carum carvi (Caraway) and Melissa off. (Lemon balm) is the most common monoterpene hydrocarbon but others include pinene from Pinus spp. (Pine), Angelica archangelica (Angelica) & Myristica fragrans (Nutmeg),  zingiberene from Ginger and phellandrene from Foeniculum vulgare (Fennel) & Eucalyptus spp. and alpha terpene (from Elletaria cardamomum (Cardomom) & Oregano off. (Marjoram) (Willard 1992).  All the citrus family, with the exception of Bergamot, are extraordinarily rich in monoterpene hydrocarbons. 

Monoterpene hydrocarbons exhibit electro-positivity and are non-polar. As such they are associated with the following therapeutic properties: external antisepsis, anti-viral action, mucus membrane irritants and possibly some immuno-stimulatory actions internally (Schnaubelt). 

Sesquiterpene hydrocarbons
There are various chemical configurations within this grouping but all exhibit electronegativity and a non-polar nature and therefore are used therapeutically as anti-inflammatories, sedatives, anti-spasmodics, anti-allergenics  and decongestants (anti-phlogistics). Humulene from Humulus lupulus (Hops) and chamazulene from Chamomile are examples of sesquiterpene hydrocarbons.

Diterpenes
These are expectorant, anti-fungal and anti-viral.  Some are purgative when taken internally and some may influence the hormonal system eg.  Sclareol in Salvia sclarea and viridiflorol in Melaleuca viridiflora. 

ALCOHOL VOLATILE OILS
These are terpenes (mono-, sesqui- or di- terpenes) with an attached hydroxyl (OH) grouping. They are mildly electro-positive and thus generally antiseptic, stimulating and energizing (Schnaubelt).  They are usually non-irritating to the skin and non-toxic.  Their names usually end in ‘-ol’. 

Monoterpene alcohols
These have a strongly polar nature and are electro-positive. Thus they are used for their antiseptic, diuretic, heart toning and immune stimulating effects (Schnaubelt). They are much valued for their bactericidal effect and are well tolerated in skin and personal care items.  

Mentha piperita (Peppermint) is a plant with a singular amount of a monoterpene alcohol (menthol) in its volatile oil. It is used to cool irritated skin conditions (refrigerant effect), as a carminative, antiseptic, decongestant and stimulant. It is especially active against the influenza and herpes simplex viruses (Willard 1992). 

Linalol which is found especially in Lavandula spica (Spike Lavender), Coriandrum sativum (Coriander), Rosmarinus officinalis (Rosemary) Citrus auranthium (Petitgrain) and Aniba rosaeodora (Rosewood) is notably sedative, an effect not generally found with other terpene alcohols (Schnaubelt).  

Sesquiterpene alcohols
The therapeutic properties of this group are very varied and there is much research still to be done into this chemical group. In general the sesquiterpene alcohols are slightly less electropositive than are the monoterpene alcohols and exhibit the properties of liver and glandular stimulation, anti-inflammatory and anti-allergenic action and decongestant (Schnaubelt). 

Some interesting sesquiterpene alcohols include alpha santalol from Santalum album (Sandalwood) which finds therapeutic use as a urinary disinfectant and immune stimulant; and bisabolol from Chamomilla recutita (German chamomile). Bisabolol is one of the strongest anti-inflammatories and decongestants found in volatile oils and occurs in at least 4 different chemotypes (Schnaubelt). Other plants with a high proportion of sesquiterpene alcohols include Melaleuca quinquenervia (chemotype nerolidol) (Niaouli), Vetiver zizanoides (Vetiver) and Zingiber officinalis (Ginger). 

ALDEHYDE VOLATILE OILS
These contain a non-cyclic carbon double bonded to an oxygen molecule as well as bonded to a hydrogen molecule (Willard 1992). They are strongly electronegative and of almost neutral polarity. Thus they exhibit the qualities of being  anti-inflammatory, sedative, soothing and calming (Schnaubelt).  They are hypotensive, vasodilatory and antipyretic.  A particular property of the aldehyde volatile oils is their insect repellent activity.  They tend to have a very strong scent and be especially useful in perfumery.  They are often quite irritating to the skin. Their names usually end in ‘-al’. 

Herbs that contain terpene aldehydes include Melissa officinalis (Lemon balm), Eucalyptus citriodora (Eucalyptus), Cinnamonum spp. (Cinnamon), Citrus limon (Lemon peel), Cymbopogon citratus (Lemon grass) and Lippia citriodora (Lemon verbena).  Research has demonstrated increased efficacy of the sedative and soothing action with smaller doses of these oils.

KETONE VOLATILE OILS
These are characterised by a side group formed from oxygen double bonded to carbon (a carboxyl group). Ketone volatile oils are moderately electronegative and strongly polar. Thus they exhibit the general characteristics of being cooling and drying. They also tend to promote tissue growth and  healing of injuries (Schnaubelt).  Ketones tend to be lipolytic, mucolytic, sedative, analgesic, anti-coagulant, anti-inflammatory, digestive tonic, expectorant and stimulant.  They should be avoided during pregnancy (Price and Price 1995).  

They may be divided into 2 categories:   

1) Monocyclic	menthone	from Mentha pulegium (Pennyroyal) & Mentha piperita (Peppermint)
	carvone	from Mentha spicata (Spearmint) & Carum carvi (Caraway)
	pulegone	from Mentha pulegium
	diosphenone	from Barosma betulina (Buchu)
2) Dicyclic	2 camphenone	From Cinnamonum camphora (Camphor)
	thujone	from Artemisia absinthium (Wormwood), Salvia officinalis) (Sage), Tanacetum vulgare (Tansy) & Thuja occidentalis (Arbor vitae) (Willard 1992).

Some ketone volatile oil molecules are neurotoxic, especially thujone. This is a major component of the volatile oil of Artemisia absinthium (Wormwood) which is used to make the alcoholic liqueur 'absinthe' There is substantial evidence now that regular consumption of this drink causes brain damage. Thujone also occurs in significant amounts in Thuja occidentalis (Arbor vitae) and Salvia officinalis (Sage). In Sage, thujone comprises 30% of the volatile oil and acts as a smooth muscle stimulator. This combined with it's marked estrogenic effects makes it definitely contra-indicated in pregnancy (Mills 1986)

2 Camphenone is obtained from the plant Cinnamonum camphora, or is synthesized from turpentine. It may also occur in some members of the Compositae and Lamiaceae families. It acts directly on the brain as a central nervous stimulant and can be used to counteract the respiratory depression that can accompany morphine or barbiturate use. When camphor is inhaled it stimulates mucous flow, and so is useful in the treatment of chronic catarrh and respiratory tract infections (Mills 1986)                    

PHENOL VOLATILE OILS
These are structurally similar to the alcohol volatile oils in that they have an OH side group (and their names usually end in ‘-ol’), but in the case of the phenols the basic molecule contains a benzene ring (ie. the isoprene unit acquires an extra carbon molecule and become cyclic as a benzene ring). Phenol volatile oils are strongly electro-positive and are often quite polar. As such they are found to be stimulating and warming, tonifying, strongly antibacterial and anti viral, immune stimulating and keratolytic/vesicant to mucous membranes and skin(Schnaubelt).

Some of the most important phenol volatile oils are thymol from Thymus vulgaris (Thyme), carvacrol from Pinus spp. (Pine) & Juniperus communis (Juniper) and eugenol from Eugenia caryophyllata (Cloves). There is a slight liver toxicity associated with phenol compounds so they should be avoided in long term use and in people with compromised liver function.

Phenolic ethers are more complex phenolic molecules.  They may occur in the cis or the trans form, with differing toxicity accordingly.  Overall, they tend to be more toxic than simple phenols and should generally be avoided.  They include safrole from Sassafras and asarone from calamus.

OTHER VOLATILE OIL GROUPINGS

Ethers
Here oxygen is not integrated into a ring structure but forms the link between two 2 carbon chains. They tend to exhibit mild electro-positivity and neutral polarity. They are used therapeutically for their balancing and soothing effect. Examples include anethole from Pimpinella anisum (Anise) and methylchavicol from Ocimum basillicum (Basil) and Artemisia dracunculus (Tarragon).

Esters
These are common in many volatile oils and frequently serve to enhance the power of the smell of the oil. They are formed by the reaction between an acid and an alcohol in the volatile oil. Generally esters are mildly electro-negative and have neutral polarity. They exhibit a spasmolytic and soothing effect via a CNS reaction and are anti-inflammatory and fungicidal (Schnaubelt).The volatile oil of Pelargonium odorantissimum (Geranium) is particularly anti fungal while other significant examples of ester volatile oils include lavender, Clary sage, Petitgrain, Bergamot, Juniper, Roman chamomile and Elecampane.

Sesquiterpene lactones
A compound is called a 'lactone' if it contains an ester group integrated into a carbon ring system. The sesquiterpene (15 carbon) lactones are found to have very strongly mucolytic effect as well as being decongestant and anti-inflammatory (Schnaubelt). In the Asteraceae (Dandelion) family the sesquiterpene lactones have been found to exhibit significant anti-tumour effects, possibly by a from of natural chemotherapy.  Inula helenium (Elecampane) is a good example of a volatile oil notably high in sesquiterpene lactones.

Oxides
The only oxide of note that has been discovered is 1-,8-cineole, otherwise known as eucalyptol.  It occurs abundantly in Eucalyptus spp. and Melaleuca spp. and is a powerful expectorant and anti-viral, and slightly irritant to the skin.

SUMMARY OF CHEMICAL CONSTITUENTS AND ACTIONS  

Aldehydes (strongly electro-negative, moderately polar)
Sedative, anti-inflammatory, anti-viral, cooling, soothing and relaxing. 
E.g. Helichrysum, Melissa, Lemon grass, Thuja, Sage, Cumin, Hyssop, Verbena, E. Citriodora 

Ketones (moderately electro-negative, strongly polar)
Cell regenerative, mucolytic, drying, tonifying, potentially neurotoxic.
E.g. Pennyroyal, Peppermint, Caraway, Wormwood, Sage, Thuja, Tansy, Camphor, Buchu 

Esters (electrically neutral and of medium polarity)
Spasmolytic, fungicidal, soothing to the skin, anti-inflammatory.
E.g. Geranium. Lavender, Clary sage, Petigrain, Bergamot, Roman chamomile, Inula 

Monoterpene hydrocarbons (strongly electro-positive and non-polar)
Anti-viral, stimulating, irritant, drying, immuno-stimulatory.
Eg. Citrus and evergreen needle oils, Angelica, Nutmeg, Fennel, Oregano 

Phenylpropanes (moderately electro-positive and moderately polar)
Stimulating, and anti-bacterial.
E.g. Basil, Tarragon, Nutmeg, Parsley, Anise, Cinnamon and Cloves 

Sesquiterpene (strongly electro-negative, of variable polarity)
Hepatic and glandular stimulants, anti-inflammatory and anti-allergic, decongestant.
Eg. Sandalwood, Chamomile

CHEMOTYPES IN VOLATILE OILS
Plants which are botanically related, either by family or even genus, frequently exhibit similar medicinal value. Thus, for example, most members of the Apiaceae are rich in volatile oils with strong carminative and warming properties. However, the converse may also be true to the extent that sometimes even plants of the same species may have different therapeutic effects according to their unique biochemical individuality. This is called the chemotype of the plant or chemical polymorphism. Two important examples of the significance of chemotypes are Rosemary and Thyme. 

Yarrow:            (Achillea millefolium) Bulgaria             R
Anti-inflammatory, soothing, healing.  Regulates menstrual cycle.

Ylang-Ylang (extra):            (Chananga odorata genuina) Madagascar 
Sedative and anti-spasmodic.   Relaxes body, mind and spirit. Alleviates anger and frustration. Anti-depressant, aphrodisiac.  This is the first distillation and very powerful.

©         Avoid in hypertension

n         May cause skin irritation

l         Avoid in pregnancy

R         May cause sun sensitivity

TOXICOLOGY OF ESSENTIAL OILS

‘Babysafe’ essential oils
These are often rich in terpene alcohols and esters and are generally safe for long term use.

 

Generally safe essential oils
Can be safely used in small amounts for specific conditions for prescribed periods of time. May occasionally be slightly irritant to the skin (eg Lemon grass) and citrus oils may cause photosensitivity and skin irritation. Any oil rich in phenols should not be used in high doses or for long periods of time.

 

Safe when used appropriately
These oils should never be used undiluted and internal use should not be attempted except by  experienced practitioners.