An essential oil is a concentrated hydrophobic liquid containing volatile (defined as "the tendency of a substance to vaporize") aroma compounds from plants. Essential oils are also known as volatile oils, ethereal oils, aetherolea, or simply as the oil of the plant from which they were extracted, such as oil of clove. An oil is "essential" in the sense that it contains the "essence of" the plant's fragrance—the characteristic fragrance of the plant from which it is derived. The term essential used here does not mean indispensable as with the terms essential amino acid or essential fatty acid which are so called since they are nutritionally required by a given living organism. In contrast to fatty oils, essential oils evaporate completely without leaving a stain (residue) when dabbed onto filter paper.
Essential oils are generally extracted by distillation, often by using steam. Other processes include expression, solvent extraction, absolute oil extraction, resin tapping, and cold pressing. They are used in perfumes, cosmetics, soaps and other products, for flavoring food and drink, and for adding scents to incense and household cleaning products.
Essential oils have been used in folk medicine throughout history. The earliest recorded mention of the techniques and methods used to produce essential oils is believed to be that of Ibn al-Baitar (1188–1248), an Al-Andalusian (Muslim-controlled Spain) physician, pharmacist and chemist.
Rather than refer to essential oils themselves, modern works typically discuss specific chemical compounds of which the essential oils are composed. For example: methyl salicylate rather than "oil of wintergreen".
Interest in essential oils has revived in recent decades with the popularity of aromatherapy, a branch of alternative medicine that uses essential oils and other aromatic compounds. Oils are volatilized, diluted in a carrier oil and used in massage, diffused in the air by a nebulizer, heated over a candle flame, or burned as incense.
Medical applications proposed by those who sell medicinal oils range from skin treatments to remedies for cancer and often are based solely on historical accounts of use of essential oils for these purposes. Claims for the efficacy of medical treatments, and treatment of cancers in particular, are now subject to regulation in most countries.
Most common essential oils such as lavender, peppermint, tea tree oil, patchouli, 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.
The recondensed water is referred to as a hydrosol, hydrolat, herbal distillate, or plant water essence, which may be sold as another fragrant product. Hydrosols include rose water, lavender water, lemon balm, clary sage, and orange blossom water. The use of herbal distillates in cosmetics is increasing.
Most citrus peel oils are expressed mechanically or cold-pressed (similar to olive oil extraction). Due to the relatively large quantities of oil in citrus peel and low cost to grow and harvest the raw materials, citrus-fruit oils are cheaper than most other essential oils. Lemon or sweet orange oils are obtained as byproducts of the citrus industry.
Before the discovery of distillation, all essential oils were extracted by pressing.
Most flowers contain too little volatile oil to undergo expression, but 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 solvents are called concretes, which are 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. Often, another solvent, such as ethyl alcohol, is used to extract the fragrant oil from the concrete. The alcohol solution is chilled to −18 °C (0 °F) for more than 48 hours which causes the waxes and lipids to precipitate out. The precipitates are then filtered out and the ethanol is removed from the remaining solution by evaporation, vacuum purge, or both, leaving behind the absolute.
Supercritical carbon dioxide is used as a solvent in supercritical fluid extraction. This method can avoid petrochemical residues in the product and the loss of some "top notes" when steam distillation is used. 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. When the extraction is complete, the pressure is reduced to ambient and the carbon dioxide reverts to a gas, leaving no residue.
Florasol is another solvent used to obtain essential oils. It was originally developed as a refrigerant to replace Freon. Although Florasol is an "ozone-friendly" product, it has a high global warming potential (GWP; 100-yr GWP = 1430). The European Union has banned its use, with a phase-out process that began in 2011, to be completed in 2017. One advantage of Florasol is that the extraction of essential oils occurs at or below room temperature so degradation through high temperature extremes does not occur. The essential oils are mostly pure and contain little to no foreign substances.
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.
An example of the medicinal value of essential oils is thymol, isomeric with carvacrol and found in oil of the common spice thyme. Thymol is part of a naturally occurring class of compounds known as biocides, with strong antimicrobial attributes when used alone or with other biocides such as carvacrol.
Taken by mouth, many essential oils can be dangerous in high concentrations. Typical effects begin with a burning feeling, followed by salivation. In the stomach, the effect is carminative, relaxing the gastric sphincter and encouraging eructation (belching). Further down the gut, the effect typically is antispasmodic. Typical ingredients for such applications include eucalyptus oils, menthol, capsaicin, anise, and camphor.
Some essential oils, such as those of juniper and agathosma, are valued for their diuretic effects.[unreliable medical source?] With relatively recent concerns about the overuse of antibacterial agents, many essential oils have seen a resurgence in off-label use for such properties and are being examined for this use clinically.
Many essential oils affect the skin and mucous membranes in ways that are valuable or harmful. Many essential oils, particularly tea tree oil, may cause contact dermatitis. They are used in antiseptics and liniments in particular. Typically, they produce rubefacient irritation at first and then counterirritant numbness. Turpentine oil and camphor are two typical examples of oils that cause such effects. Menthol and some others produce a feeling of cold followed by a sense of burning. This is caused by its effect on heat-sensing nerve endings. Some essential oils, such as clove oil or eugenol, were popular for many hundred years in dentistry as antiseptics and local anesthetics.
Aromatherapy is a form of alternative medicine in which healing effects are ascribed to the aromatic compounds in essential oils and other plant extracts. Aromatherapy may be useful to induce relaxation but there is not sufficient evidence that essential oils can effectively treat any condition  Essential oils should not be interpreted to be cures for chronic disease, or other illnesses, as scientific research does not support this.. Much of the research on the use of essential oils for health purposes have serious methodological errors. In a systemic review, of 201 published studies on essential oils as alternative medicines, only 10 were found to be of acceptable methodological quality, and even these 10 were still weak in reference to scientific standards .Use of essential oils may cause harm including allergic reactions and skin irritation; there has been at least one case of death.. As such, the use of essential oils as an alternative medicine should be approached with caution and skepticism.
Essential oils are usually lipophilic (literally: "oil-loving") compounds that usually are not miscible with water. They can be diluted in solvents like pure ethanol and polyethylene glycol. The most common way to safely dilute essential oils for topical use is in a carrier oil. This can be any vegetable oil readily available, the most popular for skin care being jojoba, coconut, wheat germ, olive and avocado.
Essential oils are derived from sections of plants. Some plants, like the bitter orange, are sources of several types of essential oil.
Balsam of Peru, an essential oil derived from the Myroxylon, is used in food and drink for flavoring, in perfumes and toiletries for fragrance, and in medicine and pharmaceutical items for healing properties.[unreliable source?] However, a number of national and international surveys have identified Balsam of Peru as being in the "top five" allergens most commonly causing patch test allergic reactions in people referred to dermatology clinics.
Most eucalyptus oil on the market is produced from the leaves of Eucalyptus globulus. Steam-distilled eucalyptus oil is used throughout Asia, Africa, Latin America and South America as a primary cleaning/disinfecting agent added to soaped mop and countertop cleaning solutions; it also possesses insect and limited vermin control properties. Note, however, there are hundreds of species of eucalyptus, and perhaps some dozens are used to various extents as sources of essential oils. Not only do the products of different species differ greatly in characteristics and effects, but also the products of the very same tree can vary grossly.
Lavender oil has long been used in the production of perfume. However, it can be estrogenic and antiandrogenic, causing problems for prepubescent boys and pregnant women, in particular. Lavender essential oil is also used as an insect repellent.
The potential danger of an essential oil is sometimes relative to its level or grade of purity, and sometimes related to the toxicity of specific chemical components of the oil. Many essential oils are designed exclusively for their aroma-therapeutic quality; these essential oils generally should not be applied directly to the skin in their undiluted or "neat" form. Some can cause severe irritation, provoke an allergic reaction and, over time, prove hepatotoxic.
Industrial users of essential oils should consult the safety data sheets (SDS) to determine the hazards and handling requirements of particular oils. Even certain therapeutic grade oils can pose potential threats to individuals with epilepsy or pregnant women.
Essential Oil use in children can pose a danger when misused because of their thin skin and immature livers. This might cause them to be more susceptible to toxic effects than adults. 
The flash point of each essential oil is different. Many of the common essential oils, such as tea tree, lavender, and citrus oils, are classed as a Class 3 Flammable Liquid, as they have a flash point of 50–60 °C.
Estrogenic and antiandrogenic activity have been reported by in vitro study of tea tree oil and lavender essential oils. Case reports suggest the oils may be implicated in some cases of gynecomastia, an abnormal breast tissue growth in prepubescent boys. The European Commission's Scientific Committee on Consumer Safety dismissed the claims as implausible in 2008. However in 2018, a study led by J. Tyler Ramsey of the American National Institute of Environmental Health Sciences found that these oils contain eight substances that affect human hormones, increasing the level of oestrogen and decreasing the level of testosterone. Some of the substances are found in "at least 65 other essential oils".
Exposure to essential oils may cause a contact dermatitis. Essential oils can be aggressive toward rubbers and plastics, so care must be taken in choosing the correct handling equipment. Glass syringes are often used, but have coarse volumetric graduations. Chemistry syringes are ideal, as they resist essential oils, are long enough to enter deep vessels, and have fine graduations, facilitating quality control. Unlike traditional pipettes, which have difficulty handling viscous fluids, the chemistry syringe, also known as a positive displacement pipette, has a seal and piston arrangement which slides inside the pipette, wiping the essential oil off the pipette wall.
Essential oils are used extensively as GRAS flavoring agents in foods, beverages, and confectioneries according to strict Good Manufacturing Practice (GMP) and flavorist standards. Pharmacopoeia standards for medicinal oils should be heeded. Some oils can be toxic to some domestic animals, cats in particular. The internal use of essential oils can pose hazards to pregnant women, as some can be abortifacients in dose 0.5–10 ml, and thus should not be used during pregnancy.
There is some concern about pesticide residues in essential oils, particularly those used therapeutically. For this reason, many practitioners of aromatherapy buy organically produced oils. Not only are pesticides present in trace quantities, but also the oils themselves are used in tiny quantities and usually in high dilutions. Where there is a concern about pesticide residues in food essential oils, such as mint or orange oils, the proper criterion is not solely whether the material is organically produced, but whether it meets the government standards based on actual analysis of its pesticide content.
The use of essential oils in pregnancy is not recommended due to inadequate published evidence to demonstrate evidence of safety. Pregnant women often report an abnormal sensitivity to smells and taste, and essential oils can cause irritation and nausea.
The following table lists the LD50 or median lethal dose for common oils; this is the dose required to kill half the members of a tested animal population. LD50 is intended as a guideline only, and reported values can vary widely due to differences in tested species and testing conditions.
|Common Name||Oral LD50||Dermal LD50||Notes|
|Neem||14 g/kg||>2 g/kg|
|Lemon myrtle||2.43 g/kg||2.25 g/kg|
|Frankincense||>5 g/kg||>5 g/kg||Boswellia carterii|
|Frankincense||>2 g/kg||>2 g/kg||Boswellia sacra|
|Indian frankincense||>2 g/kg||>2 g/kg||Boswellia serrata|
|Ylang-ylang||>5 g/kg||>5 g/kg|
|Cedarwood||>5 g/kg||>5 g/kg|
|Roman chamomile||>5 g/kg||>5 g/kg|
|White camphor||>5 g/kg||>5 g/kg||Cinnamomum camphora, extracted from leaves|
|Yellow camphor||3.73 g/kg||>5 g/kg||Cinnamomum camphora, extracted from bark|
|Hot oil||3.80 g/kg||>5 g/kg||Cinnamomum camphora, oil extracted from leaves|
|Cassia||2.80 g/kg||0.32 g/kg|
In 2002, ISO published ISO 4720 in which the botanical names of the relevant plants are standardized. The rest of the standards with regards to this topic can be found in the section of ICS 71.100.60 
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A suspected link between abnormal breast growth in young boys and the use of lavender and tea tree oils has been given new weight, after a study found eight chemicals contained in the oils interfere with hormones.