Essential oil
An essential oil is any concentrated, hydrophobic liquid containing volatile aroma compounds from plants, which are called  aromatic herbs or aromatic plants. They are also known as volatile or ethereal oils, or simply as the "oil of" the plant material from which they were extracted,  such as oil of clove.  The term essential indicates that the oil carries distinctive scent (essence) of the plant, not that it is an especially important or fundamental substance. Essential oils do not as a group need to have any specific chemical properties in common, beyond conveying characteristic fragrances. They are not to be confused with essential fatty acids.
Essential oils are generally extracted by distillation. Other processes include expression, or solvent extraction.  They are used in perfumes and cosmetics, for flavoring food and drink, and for scenting incense and household cleaning products.
Various essential oils have been used medicinally at different periods in history. Medical applications proposed by those who sell medicinal oils range from skin treatments to remedies for cancer, and are often based on historical use of these oils for these purposes. Such claims are now subject to regulation in most countries, and have grown correspondingly more vague, to stay within these regulations.
Interest in essential oils has revived in recent decades, with the popularity of aromatherapy, a branch of alternative medicine which claims that the specific aromas carried by essential oils have curative effects. Oils are volatilized or diluted in a carrier oil and used in massage, or burned as incense, for example.

# Production

## Distillation
Today, most common essential oils, such as lavender, peppermint, and eucalyptus, are distilled.  Raw plant material, consisting of the flowers, leaves, wood, bark, roots, seeds, or peel, is put into an alembic (distillation apparatus) over water. As the water is heated the steam passes through the plant material, vaporizing the volatile compounds. The vapors flow through a coil where they condense back to liquid, which is then collected in the receiving vessel.
Most oils are distilled in a single process. One exception is Ylang-ylang (Cananga odorata), which takes 22 hours to complete through a  fractional distillation.
The recondensed water is referred to as a hydrosol, hydrolat, herbal distillate or plant water essence, which may be sold as another fragrant product. Popular hydrosols are rose water, lavender water, lemon balm, clary sage and orange blossom water. The use of herbal distillates in cosmetics is increasing. Some plant hydrosols have unpleasant smells and are therefore not sold.

## Expression
Most citrus peel oils are expressed mechanically, or cold-pressed. Due to the large quantities of oil in citrus peel and the relatively low cost to grow and harvest the raw materials, citrus-fruit oils are cheaper than most other essential oils. Lemon or sweet orange oils that are obtained as by-products of the citrus industry are even cheaper.
Prior to the discovery of distillation, essential oils (EO) were extracted by pressing.

## Solvent extraction
Most flowers contain too little volatile oil to undergo expression and their chemical components are too delicate and easily denatured by the high heat used in steam distillation.  Instead, a solvent such as hexane or supercritical carbon dioxide is used to extract the oils.  Extracts from hexane and other hydrophobic solvent are called concretes, which is a mixture of essential oil, waxes, resins, and other lipophilic (oil soluble) plant material.
Although highly fragrant, concretes contain large quantities of non-fragrant waxes and resins. As such another solvent, often ethyl alcohol, which only dissolves the fragrant low-molecular weight compounds, is used to extract the fragrant oil from the concrete. The alcohol is removed by a second distillation, leaving behind the absolute.
Supercritical carbon dioxide is used as a solvent in supercritical fluid extraction. This method has many benefits, including avoiding petrochemical residues in the product. It does not yield an absolute directly. The supercritical carbon dioxide will extract both the waxes and the essential oils that make up the concrete.  Subsequent processing with liquid carbon dioxide, achieved in the same extractor by merely lowering the extraction temperature, will separate the waxes from the essential oils.  This lower temperature process prevents the decomposition and denaturing of compounds and provides for a superior product. When the extraction is complete, the pressure is reduced to ambient and the carbon dioxide reverts back to a gas, leaving no residue. Although supercritical carbon dioxide is also used for making decaffeinated coffee, the actual process is different.

## Production quantities
Estimates of total production of essential oils are difficult to obtain. One estimate, compiled from data in 1989, 1990 and 1994 from various sources gives the following total production, in tonnes, of essential oils for which more than 1,000 tonnes were produced.

# Essential oil use in aromatherapy
Aromatherapy is a form of alternative medicine, in which healing effects are ascribed to the aromatic compounds in essential oils and other plant extracts.  Many common essential oils have medicinal properties that have been applied in folk medicine since ancient times and are still widely used today.  For example, many essential oils have antiseptic properties.. Many are also claimed to have an uplifting effect on the mind. The claims are supported in some studies and unconfirmed in others.

# Dilution
Essential oils are usually lipophilic (literally: "oil-loving") compounds that usually are not miscible with water. Instead, they can be diluted in solvents like pure ethanol (alcohol), polyethylene glycol, or oils.

# Raw Materials
Essential oils are derived from various sections of plants. Some, like orange oil, are derived from any of several sections of the plant.

## Rose oil
The most well-known essential oil is probably rose oil, produced from the petals of  Rosa damascena and Rosa centifolia.  Steam-distilled rose oil is known as "rose otto" while the solvent extracted product is known as "rose absolute".

# Dangers
Because of their concentrated nature, EOs generally should not be applied directly to the skin in their undiluted or "neat" form. Some can cause severe irritation or provoke an allergic reaction.  Instead, essential oils should be blended with a vegetable carrier oil (also referred to as a base or "fixed" oil) before being applied. Common carrier oils include olive, almond, hazelnut and grapeseed. Common ratio of essential oil disbursed in a carrier oil is 0.53% (most less than 10%) and depends on its purpose.  Some EO's including many of the citrus peel oils, are photosensitizers, increasing the skin's reaction to sunlight and making it more likely to burn.
Industrial users of essential oils should consult the material safety data sheets (MSDS) to determine the hazards and handling requirements of particular oils.

## Gynaecomastia
Some essential oils, particularly lavender and tea tree oil, have been implicated in causing gynaecomastia, an abnormal breast tissue growth, in prepubescent boys.
A child hormone specialist at the University of Cambridge claimed "... these oils can mimic oestrogens" and "people should be a little bit careful about using these products". 

## Pesticide residues
There is some concern about pesticide residues in EOs, particularly those used therapeutically.  For this reason, many practitioners of aromatherapy choose to buy organically produced oils.

## Ingestion
While some advocate the ingestion of essential oils for therapeutic purposes, this should never be done except under the supervision of a professional who is licensed to prescribe such treatment.  Some very common EOs such as Eucalyptus are extremely toxic internally. Pharmacopoeia standards for medicinal oils should be heeded. EOs should always be kept out of the reach of children.  Some oils can be toxic to some domestic animals, cats in particular. Owners must ensure that their pets do not come into contact with potentially harmful essential oils.

## Smoke
The smoke from burning essential oils may contain potential carcinogens, such as polycyclic aromatic hydrocarbons (PAHs). Essential oils are naturally high in volatile organic compounds (VOCs). The internal use of essential oils should be fully avoided during pregnancy without consulting with a licensed professional, as some can be abortifacients in dose 0.510 ml.

## Toxicology
LD50 of most EO's or their main components are 0.5-10 g/kg (orally or skin test).

# Media
In 2006, the German movie Perfume: The Story of a Murderer was made on the subject of essential oils.  The story takes place in France in the 1700's.

# Notes and references
-  "ISO TC 54 Business Plan  Essential oils" (PDF). Retrieved 2006-09-14..mw-parser-output cite.citation{font-style:inherit}.mw-parser-output q{quotes:"\"""\"""'""'"}.mw-parser-output code.cs1-code{color:inherit;background:inherit;border:inherit;padding:inherit}.mw-parser-output .cs1-lock-free a{background:url("")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-limited a,.mw-parser-output .cs1-lock-registration a{background:url("")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-subscription a{background:url("")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration{color:#555}.mw-parser-output .cs1-subscription span,.mw-parser-output .cs1-registration span{border-bottom:1px dotted;cursor:help}.mw-parser-output .cs1-hidden-error{display:none;font-size:100%}.mw-parser-output .cs1-visible-error{display:none;font-size:100%}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration,.mw-parser-output .cs1-format{font-size:95%}.mw-parser-output .cs1-kern-left,.mw-parser-output .cs1-kern-wl-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right,.mw-parser-output .cs1-kern-wl-right{padding-right:0.2em} It is unclear from the source what period of time the quoted figures include.
-  Seenivasan Prabuseenivasan, Manickkam Jayakumar, and Savarimuthu Ignacimuthu (November 30 2006). "In vitro antibacterial activity of some plant essential oils". BMC Complement Altern Med. 6 (39). DOI 10.1186/1472-6882-6-39. Retrieved 2006-12-22. Check date values in: |date= (help)CS1 maint: Multiple names: authors list (link)
-  Komiya M, Takeuchi T, Harada E (September 25 2006). "Lemon oil vapor causes an anti-stress effect via modulating the 5-HT and DA activities in mice". Behav Brain Res. 172 (2): 2409. PMID 16780969. Retrieved 2006-12-24. Check date values in: |date= (help)CS1 maint: Multiple names: authors list (link)
-  Hiroko Kuriyama, Satoko Watanabe, Takaaki Nakaya, Ichiro Shigemori, Masakazu Kita, Noriko Yoshida, Daiki Masaki, Toshiaki Tadai, Kotaro Ozasa, Kenji Fukui, and Jiro Imanishi (September 15 2005). "Ambient odors of orange and lavender reduce anxiety and improve mood in a dental office". Physiol Behav. 86 (12): 925. PMID 16095639. Text " accessdate-2006-12-24
 " ignored (help); Check date values in: |date= (help)CS1 maint: Multiple names: authors list (link)
-  Lehrner J, Marwinski G, Lehr S, Johren P, Deecke L (June 2005). "Immunological and Psychological Benefits of Aromatherapy Massage". Evid Based Complement Alternat Med. 2 (2). doi:10.1093/ecam/neh087. Retrieved 2006-12-24. Check date values in: |date= (help)CS1 maint: Multiple names: authors list (link)
-  "Prepubertal gynecomastia linked to lavender and tea tree oils". New England Journal of Medicine. 356 (5): 47985. 2007. PMID 17267908.
-  "Oils make male breasts develop". BBC News. February 1, 2007. Retrieved 2007-09-09.
-  K. Bischoff, F. Guale (1998). "Australian tea tree (Melaleuca alternifolia) Oil Poisoning in three purebred cats". Journal of Veterinary Diagnostic Investigation. 10 (108). Retrieved 2006-10-17.

## Additional references
- Kurt Schnaubelt (1999). Advanced Aromatherapy: The Science of Essential Oil Therapy. Healing Arts Press. ISBN 0-89281-743-7.
- Wanda Sellar (2001). The Directory of Essential Oils (Reprint ed.). Essex: The C.W. Daniel Company, Ltd. ISBN 0-85207-346-1.
- Robert Tisserand (1995). Essential Oil Safety: A Guide for Health Care Professionals. Churchill Livingstone. ISBN 0-443-05260-3.