They’ve become part of our daily conversation, but what are essential oils?
Essential oils are highly concentrated aromatic compounds derived from plants (usually by steam distillation or, for citrus, by cold-pressing the rind). They’re not oils in any normal sense (they’re not like olive oil), and ‘essential’ doesn’t mean that your body needs them. ‘Essential oils’ is from a much older concept of what a plant is composed of. Here’s where the name comes from, how they are made, and what modern science does and doesn’t support.
What Is An Essential Oil?
An essential oil is a plant’s easily evaporated, fragrant part, separated by distillation or pressing. For example, it’s the reason lavender smells like lavender. Essential oils are highly concentrated, so it takes a large amount of plant material to fill a small bottle. This is why they’re almost always diluted before use.
There’s a formal definition as well. The international industry standard (ISO 9235) defines a substance as an essential oil only if it was made in one of three ways: steam or water distillation, mechanical pressing of citrus peel, or dry distillation. That rules out some fragrant plant extracts you’ll often see sold alongside them, which we’ll come to below.
Are Essential Oils Actually Oils?
Not chemically. The oils you cook with, and the essential oil carrier oils used for massage, are fats. Their molecules are large and heavy, and they don’t evaporate. Essential oils are mostly made of much smaller molecules such as terpenes, and related compounds, light enough to drift into the air at room temperature. That’s the whole point – you can only smell something once it reaches your nose.
They earned the name “oil” because of their behavior in a single important way. They don’t mix with water. After distillation, they separate out and sit on the surface of the water, and they dissolve readily in fats and alcohol.
You can see the difference at home. Put a drop of an essential oil on a piece of plain paper and a drop of an oil, such as olive oil, beside it. Most essential oils evaporate and leave little or no mark. The true oil leaves a greasy ring that doesn’t completely dissipate. It’s a rough check though, not a purity test. Thick, dark essential oils such as patchouli or vetiver can leave a mark, but not a greasy one. Hard to tell just by looking.
Why Are They Called “Essential” Oils?
Medieval and Renaissance alchemists believed every substance held a pure, refined spirit that could be separated from its coarse physical matter. They called it the quinta essentia, the “fifth essence”, which is where our word “quintessence” comes from. The idea goes back to Aristotle’s fifth element, and by the 1300s alchemists were already trying to distill a quinta essentia from wine.
Its best-known champion was Paracelsus (1493–1541), the Swiss physician and alchemist, who made the quinta essentia idea central to his medicine. When distillers boiled plants and collected the fragrant liquid floating on the water, they saw it as that essence, captured. To them, it was what made a rose a rose.
The name stuck, but the meaning faded. “Essential” here means “of the essence”. It has nothing to do with essential vitamins or essential fatty acids, which really are substances your body needs.
How Are Essential Oils Made?
Steam Distillation
Most essential oils, including lavender, peppermint, eucalyptus and tea tree oils, are made this way. Plant material is packed into a still and steam is passed through it. The steam carries the plant’s volatile compounds with it into a condenser, where it cools back into liquid. The oil and water separate. Most oils float, though a few heavy ones, such as clove, sink.
The fragrant water left behind is called a hydrosol. Rose water is the best-known example.
Cold Pressing
Citrus oils such as lemon, sweet orange, grapefruit and bergamot sit in tiny glands in the peel, so they’re pressed out mechanically rather than distilled. One thing worth knowing: some pressed citrus oils, bergamot in particular, can make skin more sensitive to sunlight. An essential oil’s labeling should alert you to such sensitivities.
What Isn’t Technically An Essential Oil
Some flowers are too delicate for steam. Jasmine and tuberose, for example, are usually extracted with a solvent, which produces an absolute. Other plants are extracted with pressurized carbon dioxide, which produces a CO₂ extract. Both are legitimate and often beautiful materials, but by the standard definition they aren’t essential oils, and a good label will tell you which one you’re buying.
A Short History of Distillation
Nobody knows exactly when people first distilled plants, and some of the dates and stories you’ll see are shakier than they look.
- South Asia. A terracotta object in Pakistan is often described as a still and dated to around 3000 BC. Many aromatherapy texts cite it, but both what it was and its age are debated. Better documented are clay stills from the ancient Gandhara region of what is now northern Pakistan, roughly 2,000 years old.
- Mesopotamia. A cuneiform tablet from around 1200 BC names a perfume maker called Tapputi, and describes her steeping and filtering of aromatic plants. She’s the earliest perfumer whose name we know, though the method described isn’t clearly distillation.
- Alexandria. A text known as The Gold-Making of Cleopatra, written by an alchemist named Cleopatra (not the queen) sometime between the first and fourth centuries AD, contains one of the earliest known drawings of a still.
- The Islamic world. Between roughly the 9th and 11th centuries, scholars such as al-Kindi wrote about distilling perfumes, and rose water became a major trade good. Ibn Sina (Avicenna) is often credited with improving the cooling coil of the still. That credit is debated, but this time period and location is where distilling aromatics became an industry.
- Europe. In the 1500s, plant distillation took off. Hieronymus Brunschwig’s Book of the Art of Distillation (1500, expanded in 1512) was one of a flood of books on the subject, and apothecaries distilled hundreds of plants.
How Perfume Split From Medicine
For the vast majority of this history, fragrance and medicine were the same trade. That changed in the 19th century.
At the 1867 Paris International Exhibition, perfumes and soaps were shown separately from pharmaceuticals, a sign that cosmetics had become a business of their own. The following year, in 1868, the chemist William Perkin made coumarin, the sweet, hay-like scent found in tonka beans, in a laboratory. Synthetic vanillin followed in 1874. In 1882, Houbigant’s Fougère Royale became one of the first major perfumes built around a lab-made scent.
Over the next century, chemists created thousands of aroma molecules. They were cheaper and more consistent than plant extracts, and the perfume industry moved to them. As it did, the older medicinal tradition of plant aromatics faded from everyday view. (If you’re wondering how today’s fragrance oils compare with essential oils, that’s a question of its own, to be answered in a future post of this blog).
Where the Word “Aromatherapy” Came From
The plant tradition was revived in the early 20th century. The French chemist René-Maurice Gattefossé is usually credited with coining the word “aromatherapy” in 1928, and he published a book with that title in 1937. In the decades that followed, biochemist Marguerite Maury and French army doctor Jean Valnet, whose book Aromathérapie appeared in 1964, helped turn aromatherapy into a practice centered on essential oils.
How Do Essential Oils Work?
Through smell
When an essential oil evaporates, its molecules reach the smell receptors at the top of your nose. In 2004, Richard Axel and Linda Buck won the
Nobel Prize in Physiology or Medicine for discovering how those receptors work. In 1991, they identified a family of roughly 1,000 receptor genes in mice. Humans have around 400, more complex working receptor genes.
What they showed was more interesting than a simple “one smell, one receptor” model. Each receptor responds to several different odor molecules, and each molecule triggers several receptors. Your brain reads the overall pattern, which is how a few hundred receptors can tell apart a vast number of smells. A single essential oil contains dozens of compounds, so each one produces its own complex pattern.
Those smell signals travel from the nose to the parts of the brain that handle emotion and memory with fewer steps than the other senses. That’s well established, and it’s why a smell can bring back a memory before you’ve even named it. Whether particular oils have particular effects on health is a different, much more mixed area of research, and online claims are often far ahead of the evidence.
Through the skin
Because essential oil molecules are small and dissolve in fat, some of them can pass into the skin. How much varies a lot, and much of what’s applied evaporates first. One thing to note, however, essential oils are concentrated enough to irritate skin or cause lasting sensitivity, so always dilute them in a carrier oil and patch-test them; never use them topically alone. Don’t swallow them, and keep them out of reach of children and pets.
The Short Version
An essential oil is the fragrant, evaporating part of a plant, distilled with steam or pressed from citrus peel. It isn’t a fat, and “essential” is a leftover from the alchemists’ search for a plant’s essence. People have been chasing that essence for thousands of years. Modern science has explained a good deal about how we smell it, and is still sorting out the rest.
If you’d like to start with a single scent oil, our
organic essential oil single-oil pages list the plant, the part used, and the extraction method for each.
