An essential oil is a concentrated hydrophobic liquid containing volatile 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.
Essential oils have been used medicinally 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.
雖然香氣濃郁，但是凝香體含有大量不具有香味的蠟和樹脂。通常使用另一個溶劑，例如乙醇，用來從凝香體中提取芳香油。將酒精溶液冷卻至−18 °C（0 °F）超過48小時，使蜡和脂類沉澱。The precipitates are then filtered out and the ethanol is removed from the remaining solution by evaporation, vacuum purge, or both, leaving behind the 原精.
超臨界二氧化碳在超臨界流體萃取中作為溶劑。此方法可避免石化殘留於產品中，和避免使用蒸汽蒸餾時損失前調。 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. 較低溫度防止了化合物的分解和變性。當萃取完成後，將壓力降低，使二氧化碳揮發，不會留殘餘物。
Carvacrol, a terpene found in oregano oil, inhibits the growth of several bacteria strains including Escherichia coli and Bacillus cereus.[需要可靠醫學來源] In Pseudomonas aeruginosa, it causes damages to the cell membrane of these bacteria and, unlike other terpenes, inhibits their proliferation.[來源請求] The cause of the antimicrobial properties is believed to be disruption of the bacteria membrane. Carvacrol is a potent activator of the human ion channels transient receptor potential V3 (TRPV3) and A1 (TRPA1).[來源請求]
Another 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. In addition, naturally occurring biocidal agents such as thymol can reduce bacterial resistance to common drugs such as penicillin. Numerous studies have demonstrated the antimicrobial effects of thymol, ranging from inducing antibiotic susceptibility in drug-resistant pathogens to powerful antioxidant properties.[需要可靠醫學來源] Research demonstrates that thymol and carvacrol reduce bacterial resistance to antibiotics through a synergistic effect,[需要可靠醫學來源] and thymol has been shown to be an effective fungicide, particularly against fluconazole-resistant strains.[需要可靠醫學來源] Carvacrol and thymol have been demonstrated to have a strong antimutagenic effect.[需要可靠醫學來源] In addition, there is evidence that thymol has antitumor properties.[需要可靠醫學來源] Though the exact mechanism is unknown, some evidence suggests thymol effects at least some of its biocidal properties by membrane disruption. Thymol has been shown to act as a positive allosteric modulator of GABAA in vitro.
Studies have shown that certain essential oils[哪個／哪些？] may have the ability to prevent the transmission of some drug-resistant strains of pathogen, specifically Staphylococcus, Streptococcus and Candida.
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.[來源請求]
Different essential oils may have drastically different pharmacology. Those that do work well for upper respiratory tract and bronchial problems act variously as mild expectorants and decongestants.[需要可靠醫學來源] Some act as locally anesthetic counterirritants and, thereby, exert an antitussive effect.
Some essential oils, such as those of juniper and agathosma, are valued for their diuretic effects.[不可靠的醫學來源？] 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.
Use in aromatherapy编辑
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 appears to be useful to induce relaxation, especially when administered with massage. Use of essential oils may cause harm including allergic reactions and skin irritation; there has been at least one case of death.
尤加利精油，亦稱為桉樹精油，而市售精油大多數是萃取自藍桉葉。Steam-distilled eucalyptus oil is used throughout 亞洲、非洲、拉丁美洲和南美洲 as a primary cleaning/disinfecting agent added to soaped mop and countertop cleaning solutions; it also possesses insect and limited vermin control properties. 注意，桉樹有幾百種品種, 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.
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.
雌激素和抗雄激素活性已經有in vitro相關研究，並以茶樹油和薰衣草精油。 Case 研究結果建議the oils可能與 in some cases of 男性乳房发育症相關，異常的乳房組織生長於青春時期的男性。然而，這些主張已經遭受質疑[不可靠的醫學來源？] and the European Commission's Scientific Committee on Consumer Safety has dismissed the claims saying "Since the hormonal active ingredients of Tea Tree Oil were shown not to penetrate the skin, the hypothesized correlation of the finding of 3 cases of gynecomastia to the topical use of Tea Tree Oil is considered implausible." 
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 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|
Standardization of its derived products编辑
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 
|維基教科書中的相關電子教程：Complete Guide to Essential Oils|
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