Essential oils are concentrated liquids that are derived from plants. They are a type of hydrophobic liquid, which, as you may have guessed, means that they repel water. They contain volatile aromatic compounds. For that reason, essential oils are also called volatile oils or ethereal oils. Essential oils shouldn't be confused with fatty acid oils such as soya, rape seed or evening primrose; their chemical properties and biochemical origins are quite different. Sometimes essential oils are called "oil of", as in "oil of clove." The word "essential" in this case means that the oil is the fragrant essence of the plant.

Here are some sample plant parts and what essential oils are derived from them:

Flowers - Chamomile. Clary sage, Clove, Geranium, Hyssop, Jasmine, Lavender, Manuka, Marjoram, Orange , Rose , Ylang-ylang

Leaves - Basil, Bay leaf, Cinnamon, Common sage, Eucalyptus, Lemongrass, Oregano, Patchouli, Peppermint, Pine, Rosemary, Spearmint, Tea tree, Thyme,

Berries - Allspice, Juniper

Seeds - Almond, Anise, Celery, Cumin, Nutmeg oil

Peel - Bergamot, Grapefruit, Lemon, Lime, Orange, Tangerine

Resin - Frankincense, Myrrh

Bark - Cassia, Cinnamon, Sassafras

Wood - Camphor, Cedar, Rosewood, Sandalwood

Root - Valerian

Rhizome - Ginger

As you may have noticed, some essential oils, such as cinnamon, may come from a combination of parts, such as leaves and bark. The oils sources in plants are extracted from cavities or ducts plus glands or hairs. Clusters of cells just below the plant's epidermis create cavities and ducts. Glands can be found on lavender florets; and geranium and mint feature leaf hairs.

Essential oils have been used since antiquity, for everything from perfume to flavoring agents to disease remedies--and even for embalming mummies! Medical claims are now regulated by most countries. Essential oils are used by many industries in today's world, including paint, manufacturing, food and beverage production, pharmaceuticals, toiletries and hygiene products. The popularity of essential oils has surged amongst consumers in the past few decades with the practice of aromatherapy.

Essential oils are produced by distillation, expression or solvent extraction. The most common essential oils, such as lavender, peppermint or eucalyptus, are produced using distillation. Distillation involves placing raw plant material into an alembic, which is placed over water. An alembic is a bulbous container with a tube descending from its top that's attached to a smaller bulbous container; see photo to the right. The raw plant material can consist of flowers, leaves, roots, seeds, wood or bark. The alembic is placed over water, and the water is heated. As the water heats, steam passes through the raw plant material; this vaporizes the volatile plant compounds. The vapors then flow through a coil where they condense into liquid form; the liquid is collected in the smaller receiving vessel of the alembic. Of course, producers of therapeutic-grade essential oils utilize distillation chambers that are much larger than alembics! For example, Young Living essential oils, one of the largest essential oil producers in the world, uses numerous distillation vats with an overall boiler capacity of more than 115,000 liters.

Most essential oils are distilled in one single process, although there are some exceptions. For example, ylang-ylang is produced through a special 22-hour distillation process called fractional distillation.

The water that is recondensed during the distillation process is called a hydrosol, hydrolat, herbal distillate or plant water essence. These are often sold as different fragrant products; you've probably seen rose water, lavender water and orange blossom water. Rose water is used extensively in India in cooling yogurt drinks called Lassi, plus in religious rituals. But believe it or not, some hydrosols possess unpleasant odors! Of course, these are not sold.

Some essential oils, namely those derived from citrus peels, are expressed mechanically. This is called cold-pressed--just like olive oil! Prior to the invention of the alembic and distillation in 800 AD by the Arab alchemist Jabir ibn Hayyan, all essential oils were produced via pressing.

Some plants, particularly their flowers, contain very little volatile oil, so the expression process is ineffective. And some plants' chemical components are too delicate for the high heat required for the distillation process. That's where solvent extraction comes in. Solvents, such as supercritical carbon dioxide or hexane, are used to extract the volatile oils. Extracts from hexane are called concretes; these are a combination of essential oil, oil-soluble plant material, resins and wax. Although concretes are quite fragrant, they also contain large amounts of non-fragrant resin and wax. Ethyl alcohol dissolves the fragrant low-molecular weight compounds to extract fragrant oil from concretes. Then the ethyl alcohol is removed during a second distillation, which leaves what is called the absolute. Using supercritical carbon dioxide helps prevent petrochemical residues from appearing in the final product. Supercritical carbon dioxide doesn't directly yield an absolute; rather, it extracts the wax and essential oils that comprise the concrete. Then liquid carbon dioxide is used in conjunction with lowering the extraction temperature, which separates wax from the essential oils. The lower temperature prevents chemical compounds from decomposing to yield a superior product. Once the extraction process is complete, the pressure is reduced so that the carbon dioxide reverts to gas form, which leaves no residue.

Essential Oils Safety Precautions

Generally speaking, full-strength essential oils should not be applied directly to the skin because they're so concentrated. This is called their "neat" form. Certain oils may instigate severe irritation or an allergic reaction. Rather, essential oils should be blended with a vegetable carrier oil before application. These carrier oils are called base oils or fixed oils. Preferred carrier oils are organic and/or cold pressed. Commonly used carrier oils include almond, apricot kernel, avocado, grapeseed, jojoba, hazelnut, sesame or wheat germ. A typical ratio of essential oil to carrier oil usually ranges from 0.5 to 5 percent, depending on the use; a 3 to 5 percent dilution is 3 drops of essential oil per 1 teaspoon of carrier oil. As a matter of fact, industrial handlers of essential oils consult material safety data sheets about the potential hazards and handling requirements for certain essential oils. Also, some essential oils, such as those derived from citrus peels, are photosensitizers; in other words, they increase the likelihood of becoming sunburned.

Essential oils are remarkably potent. So here are some important safety precautions:

• Never assume that an essential oil possesses the same properties as its plant
• If you have sensitive skin, epilepsy, heart ailments, kidney problems, or any serious medical condition, do not use essential oils except under the supervision of your physician
• Read and follow all label directions and cautions
• Do not apply neat, or undiluted, essential oils directly to the skin; they must be diluted with a carrier oil
• Always conduct a test of diluted essential oil on the inner arm before using; do not use if redness or irritation occurs
• Keep essential oils out of eyes, ears, nose, mouth, wounds and any body opening
• Keep out of reach of children and pets; do not use on children or pets except under the supervision of a pediatrician or veterinarian
• Keep away from sources of flame, heat and ignition
• You should never use these essential oils, as they possess very high oral and dermal toxicity: bitter almond, buchu, camphor, sassafras, calamus, horseradish, mugwort, mustard, pennyroyal, rue, savin, savory, southernwood, tansy, wormseed and wormwood
• These essential oils are potential skin irritants: allspice,basil, bay, birch, bitter almond, black pepper, cassia, cinnamon leaf, cinnamon bark, citronella, clove bud, costus, cumin, fennel, fir needle, ginger, lemon, lemongrass, lemon verbena, melissa, myrrh, oak moss, orange peppermint, oregano, parsley seed, peppermint, pimento berry, pine, tagetes, tea tree, red and wile thyme and wintergreen
• You should never use these essential oils on the skin: ajowan, cassia, elecampane and origanum
• These essential oils will render skin more sensitive to sunlight: angelica, bergamot and all citrus oils
• You should not use these essential oils during pregnancy: aniseed, basil, bitter almond, birch, cedarwood, clary-sage, cypress, basil, marjoram, clove, fennel, hyssop, jasmine, juniper, lemon, myrrh, lemon verbena, peppermint, rose, rosemary, sage, sweet marjoram, thyme and wintergreen
• You should not use these essential oils if you have high blood pressure: hyssop, rosemary, sage or thyme
• You should not use these essential oils if you have epilepsy: Sweet fennel, hyssop, sage, rosemary and wormwood

There are a few essential oils that can be used for dietary purposes. However, you should never ingest an essential oil without the recommendation of a licensed professional, especially during pregnancy, as they can cause miscarriage. That's because some essential oils are internally toxic, such as eucalyptus. Remember, essential oils are a very highly concentrated form of the plant; their properties can change.