Essential Oils
What are they and how are they made?
I love using essential oils in my humidifying diffuser at home and at work. I also add them to baths to help soak away aches and pains. Whether it's citrus-scented cleaning products or lavender sprays to aid with sleep, essential oils appear in a surprising number of household items. But what are they, exactly, and how are they made? In today’s post, I’ll answer these questions and more. But first, a quick poll:
What is an essential oil?
Essential oils are concentrated extracts from aromatic plants, made up of a complex blend of volatile, low molecular weight plant compounds. These include hundreds of unique substances, primarily terpenes and phenylpropenes, which are types of plant secondary metabolites. Want to learn more about these types of compounds? Check out my (admittedly nerdy) short video lecture on the topic here:
How are essential oils made?
Essential oils are extracted from plant materials using several methods, including steam distillation (the most common), hydrodistillation, cold pressing, solvent extraction, supercritical fluid extraction, and enfleurage. Let’s break these methods down.
Steam Distillation
By far the most common method, steam distillation uses an closed system apparatus that holds the plant material above a vessel of boiling water. As the water heats, steam rises and passes through the plant material, carrying its aromatic compounds. The steam then travels through a glass tube that bends downward and is cooled by surrounding coils of water, causing it to condense. This produces two layers: essential oil floating on top (most common as essential oils are typically less dense than water) and hydrosol—a fragrant water containing some of the most polar volatile compounds—beneath. To collect the essential oil, the hydrosol is drained off, leaving the oil behind.
Hydrodistillation works similarly, but instead of passing steam through the plant material, the plant material is added directly to the boiling water. The resulting steam carries both oil and water-soluble compounds, which condense into essential oil and hydrosol once cooled. These methods remain the most widely used today and were also popular in ancient times.
I made this short [40 second] TikTok video a few years ago (shared on YouTube) to showcase the hydrodistillation process for making essential oils:
Cold Pressing
This is a technique most commonly used for citrus peels (lemon, orange, grapefruit, etc.). The peel is simply mechanically pressed in cold conditions to yield high grade essential oil.
Solvent Extraction
This is a method I try to avoid whenever possible due to the potential presence of residual solvents like hexane, which we don’t want to introduce into our bodies. However, one advantage of solvent extraction is that it’s gentler on delicate plant compounds compared to the heat used in steam or hydrodistillation. For this reason, it’s commonly used in perfume making to preserve fragile aromatic notes. The process involves using chemical solvents, such as hexane or ethanol, to dissolve aromatic compounds from the plant material. After the solvent is evaporated, a concentrated, waxy substance called a "concrete" remains, which can then be further processed with alcohol to produce the final essential oil, known as an "absolute."
Supercritical Fluid Extraction
Supercritical fluid extraction (SFE) is an advanced method for obtaining high-quality essential oils from aromatic plants. Using carbon dioxide (CO₂) as a solvent, SFE operates under moderate temperatures and high pressure, preserving delicate compounds that can be damaged by heat-based methods like steam or hydro- distillation. This technique is particularly effective for extracting the complex mixture of hydrocarbons and oxygenated compounds—such as terpenes, phenols, and esters—that give essential oils their distinctive aromas, flavors, and therapeutic properties. Because CO₂ is non-toxic and leaves no harmful residues, SFE is well suited for applications in the food, pharmaceutical, cosmetic, and fragrance industries.
Enfleurage
Enfleurage is one of the oldest methods of extracting essential oils, traditionally used for delicate flowers like jasmine that can’t withstand heat. In this process, fresh petals are placed on glass plates coated with odorless animal or plant fat, which gradually absorbs their aromatic compounds. The saturated fat, or pomade, is then washed with alcohol to separate the essential oil. Though largely replaced today by more efficient methods like steam distillation and supercritical fluid extraction, enfleurage still holds artisanal value in niche perfumery for preserving the pure, subtle scent of certain flowers. Here’s a short video (by the Robertet Group, in French with English subtitles) that showcases how some modern perfumes are made in France today:
A fascinating living museum in Cairo highlights how ancient Egyptians used a related technique—steeping rose petals and other fragrant botanicals in warm animal fats—for cosmetics, perfumery, and medicine. This early form of maceration differs from the cold enfleurage later developed in 18th- and 19th-century France but shares the same goal of capturing nature’s scent.
Are there any risks associated with the use of essential oils?
Essential oils are highly concentrated plant extracts and should never be consumed directly or applied to the skin without proper dilution in a carrier oil or base substance. In fact, some essential oils should never be ingested at all. For example, consuming eucalyptus, tea tree, or wintergreen oil can lead to nausea, vomiting, gastrointestinal distress, allergic reactions, or even organ damage, depending on the dose.
Topical application also carries risks. Undiluted essential oils can trigger allergic contact dermatitis, hives, rashes, or other forms of skin irritation. Additionally, certain oils—especially citrus-based ones—can increase the skin's sensitivity to sunlight, potentially leading to burns or discoloration.
Here are some simple safety tips to keep in mind:
Always dilute essential oils in a carrier oil (like coconut, jojoba, or almond oil) before applying to skin.
Never ingest essential oils unless under the guidance of a trained healthcare professional.
Conduct a patch test on the skin before using a new oil topically.
Keep essential oils out of reach of children and pets.
The Takeaway
Essential oils are concentrated forms of plant extracts with both the potential to heal or harm. They have so many incredible uses, but must be used responsibly and in the right dose, just like any other medicine. Talk to your healthcare provider if you have questions about how to responsibly use essential oils in your daily health routines.
Which essential oils are your favorites? Which ones do you enjoy (or avoid due to allergies)? Please share in comments! I’d love to learn more about how this form of plant medicine is used by our community.
Yours in health, Dr. Quave
Cassandra L. Quave, Ph.D. is a Guggenheim Fellow, CNN Champion for Change, Fellow of the National Academy of Inventors, recipient of The National Academies Award for Excellence in Science Communication, and award-winning author of The Plant Hunter. Her day job is as professor and herbarium director at Emory University School of Medicine, where she leads a group of research scientists studying medicinal plants to find new life-saving drugs from nature. She hosts the Foodie Pharmacology podcast and writes the Nature’s Pharmacy newsletter to share the science behind natural medicines. To support her effort, consider a paid or founding subscription to Nature’s Pharmacy or donation to her lab research.
The Plant Hunter is available in hardcover, paperback, audio, and e-book formats!
Thank you very much, Dr. Quave. This overview of essential oils is fascinating. Essential oils is one of my favorite plant pleasures. I use them in many ways. So to learn more about them, their history, and how they are lovingly made ~ is most appreciated. L