Essential oils are organic compounds derived from aromatic plant sources such as roots, bark, flowers, leaves and seeds.
They are extremely complex substances, with each oil typically containing several hundred individual chemicals, with the vast majority at levels of less than 1%. These chemicals work together as a complete synergistic unit to maximize the effectiveness of the oil.
An oil’s chemical constituents dictate its fragrance, taste, and therapeutic properties, and a single oil can have many different uses, thanks to its complex chemical profile. Understanding the chemistry can therefore help you to get the most out of your essential oils and give you a better understanding of which oils to use, and any safety concerns.
I’m often asked how I select essential oils for both personal use and treatments. Firstly, I look at the chemical components that I need, then research the oils these are contained in and finally I will always consider the smell. Afterall, it doesn’t matter how beneficial the oils are, if you don’t like the smell you won’t use them!
This article will discuss the main chemical components, with basic information to help with your oil selection – but without too much of the mind-boggling science! I hope this will either be a good introduction for those new to aromatherapy or a useful recap for therapists!
Essential oil composition
A typical essential oil contains several hundred individual chemicals, with the great majority at levels of less than 1%.
The chemical compounds in an essential oil typically have hydrogen, carbon and oxygen as their building blocks and can be divided into two main categories:-
Hydrocarbons
These contain only carbon and hydrogen. This group is made up almost exclusively of terpenes – monoterpenes and sesquiterpenes.
From hydrocarbons a plant can make the second main group – oxygenated compounds.
Oxygenated compounds
A plant makes oxygenated compounds from hydrocarbons – mainly esters, aldehydes, ketones, alcohols, phenols, and oxides.
Some oils such as Pine are dominated by hydrocarbons whereas others such as Clove comprise mainly of oxygenated compounds. It is the oxygenated constituents that typically determine the oil’s aroma and taste. They also give them some solubility in water and considerable solubility in alcohol (Tisserand & Balacs 1995).
Main compounds found in essential oils
The main chemical families are discussed below. Please note that the percentages listed against the oils are taken from Essential Oils in Colour (Caddy 1997), and are averages based on extensive historical data and the results of hundreds of GC reports (Gas Liquid Chromatography) which analyse the chemical make-up of an oil.
Monoterpenes
This is the largest chemical family found in essential oils. Monoterpenes are found in almost every essential oil and include limonene (citrus oils) and pinene (Pine/Fir/Juniperberry). They have a very small, light molecular structure, are extremely volatile (evaporate quickly), are free flowing (not viscous) and have a strong aroma. Oils with high levels of monoterpenes are usually top notes which is why they dissipate quickly. They are also more susceptible to oxidation – especially those with high levels of limonene, meaning they will have a shorter shelf life.
Common monoterpenes include: camphene, carene, cymene, limonene, myrcene, phellandrene, pinene, sabinene, terpinene.
Common properties of monoterpenes: Antibacterial, antiseptic, antiviral, analgesic, anti-inflammatory, decongestant, rubefacient, stimulant.
Examples of oils rich in monoterpenes: Black Pepper (60%), Grapefruit (96%), Cypress (75%), Juniperberry (80%), Lemon (87%), Lime (72%), Mandarin (90%), Nutmeg (75%), Orange Bitter (90%), Orange Sweet (85%), Pine (70%), Silver Fir (90%).
Safety: can be irritating to the skin, especially if oxidised.
Sesquiterpenes
Sesquiterpenes are less common in essential oils. Their molecules are bigger and heavier making them slightly more stable than monoterpenes. This means they take longer to evaporate and have a longer shelf-life. Many oils with high levels of sesquiterpenes are derived from roots and woods or plants from the Asteraceae family and have grounding, balancing effects.
Common sesquiterpenes include: bisabolene, cadinene, caryophyllene, cedrene, chamazulene, copaene, elemene, farnesene, germacrene D, himachelene, zingiberene.
Common properties of sesquiterpenes: Antibacterial, antiseptic, analgesic, anti-inflammatory, balancing, calming, cicatrizant (skin/wound healing, cell regenerative).
Examples of oils rich in sesquiterpenes: Cade (60%), Cedarwood Virginian (60%), Cedarwood Atlas (50%), German Chamomile (35%), Ginger (55%), Helichrysum (40%), Myrrh (39%), Patchouli (50%), Yarrow (45%), Vetivert (65%), Ylang Ylang (40%).
Alcohols
Plant alcohols are either derived from monoterpenes (monoterpenols). Or to a lesser extent, sesquiterpenes (sesquiterpenols). Alcohols are common in most essential oils and are the most useful molecules in aromatherapy. They are powerful yet gentle, usually quite fluid, with pleasant aromas that are well tolerated. They generally used for their antiseptic and pain-relieving properties (physical and emotional).
Common monoterpenols include: borneol, citronellol, geraniol, lavandulol, linalool, terpineol, terpinen-4-ol, menthol, nerol.
Common sesquiterpenols include: bisabolol, cedrol, farnesol, nerolidol, santalol, zingiberol.
Common properties of alcohols: antiviral, antiseptic, anti-infective, stimulating.
Examples of oils rich in alcohols: Basil (50%), Catnip (62%), Cedarwood (30%), Coriander (70%), Geranium 63%), Lavandin (45%), Lavender (36%), Sweet Marjoram (50%), Cornmint (70%), Peppermint (42%), Myrrh (40%), Neroli (40%), Palmarosa (85%), Rose Otto (60%), Rosewood (90%), Sandalwood (80%), Tea Tree (45%), Vetivert (40%).
Phenols
Phenols are the strongest, most irritating compounds and are slow to evaporate (giving them more opportunity to penetrate the tissues). They are very chemically active with a distinct aroma (which reminds me a bit of petrol). Phenols tend to have powerful bactericidal properties and a strongly stimulating effect.
Common phenols include: anethole, carvacrol, chavicol, estragole (methyl chavicol), eugenol, thymol.
Common properties of phenols: antibacterial, antiseptic, antimicrobial, antiviral, immuno-stimulant, mind stimulant, rubefacient.
Examples of oils rich in phenols: Basil ct Methyl Chavicol (90%), Cinnamon Leaf (86%), Clove Bud (90%), Fennel (62%), Oregano (70%), Tarragon (70%), Thyme ct Thymol (40%).
Safety: Oils rich in phenols are irritating to the skin and mucous membranes. Only use in low dilution and for short periods of time. Always blend with alcohol-rich oils. Avoid during pregnancy.
Aldehydes
Aldehydes are the most recognizable component, as they have a potent citrus-like scent which is a key contributor to an oil’s overall aroma. They are quite unstable and are susceptible to oxidation.
Common aldehydes include: cinnamaldehyde, citral, citronellal, geranial, neral.
Common properties of aldehydes: Anti-fungal, anti-inflammatory, antiseptic, antiviral calming/sedative yet uplifting.
Examples of oils rich in aldehydes: Citronella, Lemon Eucalyptus (80%), Lemongrass (80%), May Chang, Melissa (50%).
Safety: Oils high in aldehydes should be used in low dilution (1%) as they can be irritating to skin and mucous membranes.
Ketones
Some of the most common toxic compounds are ketones (e.g. thujone, pulegone), and this is why oils such as Thuga and Pennyroyal are avoided in aromatherapy. Not all ketones are toxic however, and some have great therapeutic benefits particularly in relation to the upper respiratory system, for easing congestion and aiding the flow of mucus. Ketones are distinctive in their fragrance and often share a potent, minty-camphoraceous odour. They are highly penetrating and should be used with care.
Common ketones include: Camphor, carvone, fenchone, jasmone, menthone, pincocamphone.
Common properties of ketones: Cicatrisant, decongestant, expectorant, lipolytic (breaks down fat), mucolytic, sedative (some).
Examples of oils rich in ketones: Caraway (54%), Dill (50%), Eucalyptus dives (45%), Hyssop (46%), Spearmint (55%), Rosemary (25%), Sage (35%), Tagetes (50%),
Safety: Should be used with care as they can build up in the body. Note contraindications for each individual oil and take particular care during pregnancy.
Esters
Esters are probably the most widespread group found in essential oils – there are not many oils that don’t have an ester content. They have an intensely fruity aroma and are generally non-toxic and gentle in action, making them ideal for use with the young, old or infirm. Esters are a go-to component – they have so many therapeutic benefits but are especially good for relaxation and balancing the mood.
Common esters include: benzyl benzoate, bornyl acetate, geranyl acetate, linalyl acetate, methyl salicylate (main component in Deep Heat).
Common properties of esters: antifungal, antispasmodic, anti-inflammatory, analgesic, calming (for physical body and nervous system), cicatrisant, sedative.
Examples of oils rich in esters: Benzoin (70%), Bergamot (40%), Sweet Birch (99%), Roman Chamomile (75%), Clary Sage (70%), Helichrysum (40%), Jasmine (54%), Lavender (45%), Bergamot Mint (60%), Petitgrain (55%), Sweet Thyme (40%), Wintergreen (99%).
Safety: Sweet Birch and Wintergreen contain high levels of methyl salicylate which is thought to be more powerful than aspirin. Both oils have serious safety considerations and should be used with extreme care.
Oxides
Oxides have the strongest aromas of all the molecules and are my favourite components – being bracing and breathing! The most common oxide is 1,8-cineole, also called eucalyptol. Found in most varieties of Eucalyptus oil and other camphoraceous oils such as Rosemary, Tea Tree and Cajeput, cineole is in a class of its own, with its potent expectorant properties and distinctive scent.
Oxides are drying molecules that remove damp from an area, making them a perfect choice for respiratory issues.
Common oxides include: bisabolol oxide, caryophyllene oxide, 1,8-cineole, linalool oxide, pinene oxide, rose oxide.
Common properties of oxides: Anti-inflammatory, cooling, bronchiodilator, decongestant, expectorant, mucolytic.
Examples of oils rich in oxides: German Chamomile (35%), Eucalyptus smithii (78%), Lavender Spike (34%), Wild Marjoram (55%), Niaouli (60%), Ravensara (60%), Rosemary (30%).
Safety: Cineole can be irritating to skin and mucous membranes.
Chemotypes
Sometimes essential oils obtained from botanically identical plants can have a significantly different set of chemical components and therapeutic actions. This can be due to different growing conditions, location, climate etc. When this happens, the different oils are classed as chemotypes (ct). Thyme (Thymus vulgaris) is a great example, with Red Thyme (Thyme ct Thymol) containing more phenols (thymol) and Sweet Thyme (Thymus ct linalool) containing more alcohols (linalool). Basil, Marjoram and Rosemary are also examples of chemotypes.
Jeni Broughton
Consultant Aromatologist & Sports Massage Specialist
Disclaimer & Safety Advice
Read other articles by Jeni Broughton
References / Useful Reading
Tisserand, R., & Balacs, T. (1995). Essential Oil Safety, A Guide For Health Care Professionals. New York: Churchill Livingstone.
Caddy, R. (1997). Aromatherapy Essential Oils in Colour. Amberwood Publishing Ltd.
Lawless, J. (1995). The Illustrated Encyclopedia of Essential Oils. Element Books.
Price, S., & Price, L. (2012). Aromatherapy for Health Professionals (4th ed.). Churchill Livingstone.
Clarke, S., (2008). Essential Chemistry for Aromatherapy (2nd ed.). Churchill Livingstone.