These are six different products, not stronger and weaker versions of one thing — and there is no legal grading system for any of them, which is the fact most of the marketing in this field depends on.
Aromatherapy involves at least six different kinds of product, which are routinely described as though they were stronger and weaker versions of one thing. They are not. They are made differently, they contain different material, and some of them carry risks the others do not.
Worth clearing up one piece of vocabulary first. "Essential" here has nothing to do with being necessary. An essential oil is the oil of a plant's essence — the volatile, aromatic fraction that evaporates and carries the smell.
Essential oils
An essential oil is the concentrated volatile fraction extracted from a plant — from flowers, leaves, bark, wood, root, seed, resin or fruit peel. It is not a fatty oil at all, despite the name, which is why it evaporates rather than leaving a greasy mark, and why it will not dissolve in water.
How it is extracted matters more than the marketing around it, because the method determines what ends up in the bottle.
Steam distillation accounts for most of what is sold. Plant material is loaded into a still, steam is passed through it, the volatile compounds leave with the vapour, and the vapour is condensed. What collects separates into two layers: a thin layer of oil, and the water beneath it. Temperature control is the skill, since too much heat alters the constituents.
Water distillation submerges the plant material in water and boils it. The water buffers the material from direct heat, which suits delicate flowers that steam would damage.
Expression, or cold pressing, is used for citrus peel and involves no heat at all — the oil sacs in the rind are simply ruptured mechanically and the oil collected. Historically this was done by hand with sponges; now it is done by machine. One consequence matters: expressed citrus oils carry non-volatile compounds called furocoumarins that distillation leaves behind, and those are what make bergamot, lemon and lime phototoxic on skin. A steam-distilled lime oil behaves quite differently from an expressed one.
Absolutes
Some flowers will not give up their scent to heat. Jasmine and tuberose are the standard examples; distilling them destroys what makes them worth having.
For these, the plant material is washed with a hydrocarbon solvent, which dissolves the aromatic compounds along with waxes and pigments. Evaporating the solvent leaves a waxy solid called a concrete, which is then washed with alcohol to separate out the aromatic fraction. What remains after the alcohol is removed is the absolute.
An absolute is therefore not a more concentrated essential oil, which is how it is often described. It is a different product: it contains non-volatile material that a distilled oil does not, it smells closer to the living flower, and it may retain traces of the solvent used. That last point is why absolutes are more at home in perfumery, and why anyone prone to skin reactions should treat them with more rather than less caution.
CO2 extracts
Carbon dioxide under pressure behaves as a solvent, and at the right temperature and pressure it will extract aromatic material from a plant. Release the pressure and the carbon dioxide returns to being a gas and leaves, taking no residue with it.
That makes CO2 extracts clean, and it makes them good at capturing compounds that heat would destroy. It does not make them essential oils: the constituent profile is broader, often including heavier molecules that distillation leaves behind, so a CO2 extract of a plant and its essential oil are different materials with different smells.
They come in two broad types. Selects are run at lower pressure and yield something closer to an essential oil. Totals extract more of the plant, including waxes, and are thick, sometimes solid, and need warming or diluting before use. The equipment is expensive, which is the whole reason these remain a premium product.
Hydrosols
Hydrosols — also sold as floral waters, hydrolats or distillates — are the water that comes off a distillation alongside the oil. They carry the water-soluble aromatic compounds and a trace of dissolved oil, which gives them a faint version of the plant's smell.
They are frequently described as mild essential oils, and that is misleading in a way that matters. A hydrosol is not a dilution of an essential oil; it contains a different set of compounds, because the ones that dissolve in water are not the ones that float on top of it.
Three practical points. They contain water and no preservative, so they spoil — they belong in the fridge, they have a short life, and a cloudy or sour-smelling bottle should be thrown away. Much of what is sold as "floral water" is water with a little fragrance added rather than a true distillate, so the label is worth reading. And they are not for internal use of any kind, a point on which some older aromatherapy writing is actively wrong.
Infused oils
An infused oil, or macerate, is made by steeping plant material in a fixed oil until the oil takes up the fat-soluble constituents. This is a different thing again from an essential oil: much weaker, non-volatile, and carrying compounds that distillation would never capture.
They are often presented as a harmless kitchen project, and there are two reasons to be more careful than that. Plant material in oil spoils, and warm oil with damp plant matter in it is a growth medium — contamination rather than simple rancidity is the risk, which is why commercially made ones exist. And some of the common macerates have specific cautions: St John's wort oil is photosensitising, so it has no place on skin that will see sunlight, and arnica is for unbroken skin only and is toxic if swallowed.
Resins and resinoids
Resin is what a tree exudes when its bark is damaged, hardening on contact with air. Frankincense and myrrh are the familiar ones, both from Boswellia and Commiphora species respectively, and both traded for millennia.
Chemically they are complex mixtures of terpenoids and resin acids rather than the simple hydrocarbons they are sometimes described as. Commercially they are harvested by controlled tapping by people who know the trees; cutting into a tree to obtain some is not a home activity and damages the tree.
Resins are too thick to dilute into a carrier oil in the ordinary way. They are either processed with solvents into resinoids, or steam distilled into an essential oil, or burned as incense. On that last one: burning resin produces smoke with fine particulates in it, so it wants ventilation and does not suit anyone with asthma.
Carrier oils
Carrier oils are fatty, non-volatile vegetable and nut oils — sweet almond, grapeseed, apricot kernel, sunflower, olive, coconut, macadamia, jojoba. They have little smell of their own and their job is to dilute an essential oil to a concentration that can safely touch skin.
One widely repeated claim about them is backwards. Carrier oils are often said to help an essential oil penetrate deeper and absorb faster. The point of a carrier is the opposite: it spreads a small amount of a potent material over a large area, reducing how much reaches any one piece of skin. That reduction is the safety mechanism.
Two things the older accounts leave out:
- Nut allergy. Sweet almond, apricot kernel, macadamia, walnut and peanut-derived oils are all in use, and none of them is suitable for someone with a nut allergy. Sunflower, grapeseed and olive are the straightforward alternatives. Any advice naming walnut oil as a carrier without mentioning this is advice to ignore.
- They go rancid. Carrier oils have a shorter life than essential oils, and a rancid one smells of old crayons and is no good on skin. Cold-pressed oils keep less well than refined ones — which is the trade-off behind the usual advice to buy them cold-pressed and in small quantities.
Quality, grades and what the label can tell you
There is no legal grading system for essential oils, which is the fact that most of the marketing in this field depends on. "Therapeutic grade", "clinical grade" and similar phrases are coined by the companies using them and certify nothing.
What does carry information:
- The Latin name. Lavender could be Lavandula angustifolia or Lavandula x intermedia, which are different oils with different constituents. A bottle labelled only "lavender" is telling you less than it could.
- The extraction method, which as above changes what is in it.
- Country of origin and batch number, because the same species grown in different places and seasons yields measurably different oil.
- A GC/MS report — gas chromatography and mass spectrometry, which lists what the batch actually contains. Suppliers who publish these per batch are telling you something; those who publish a grade name are not.
Adulteration is common and not always obvious, since a cheaper oil or a synthetic constituent can be added without changing the smell much. Price is a rough guide: rose and jasmine genuinely cost what they cost because of how much flower goes into a bottle, and a cheap one is not a bargain.
See also choosing and storing essential oils, using essential oils safely, and aromatherapy.
Changed