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Hydrophobic liquid containing volatile aroma compounds from plants

An essential oil is a concentrated hydrophobic liquid containing volatile (easily evaporated at normal temperatures) chemical compounds from plants. Essential oils are also known as volatile oils, ethereal oils, aetheroleum, or simply as the oil of the plant from which they were extracted, such as oil of clove. An essential 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 or usable by the human body, as with the terms essential amino acid or essential fatty acid, which are so called because they are nutritionally required by a living organism. Essential oils are generally extracted by distillation, often by using steam. Other processes include expression, solvent extraction, sfumatura, absolute oil extraction, resin tapping, wax embedding, and cold pressing. They are used in perfumes, cosmetics, soaps, air fresheners and other products, for flavoring food and drink, and for adding scents to incense and household cleaning products. Essential oils are often used for aromatherapy, a form of alternative medicine in which healing effects are ascribed to aromatic compounds. Aromatherapy may be useful to induce relaxation, but there is not sufficient evidence that essential oils can effectively treat any condition. Improper use of essential oils may cause harm including allergic reactions, inflammation and skin irritation. Children may be particularly susceptible to the toxic effects of improper use. Essential oils can be poisonous if ingested or absorbed through the skin. 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.

Taken by mouth, many essential oils can be dangerous in high concentrations. Typical effects begin with a burning feeling, followed by salivation. Different essential oils may have drastically different pharmacology. Some act as local anesthetic counterirritants and, thereby, exert an antitussive (cough suppressing) effect. Many essential oils, particularly tea tree oil, may cause contact dermatitis. Menthol and some others produce a feeling of cold followed by a sense of burning. In Australia essential oils (mainly eucalyptus) have been increasingly causing cases of poisoning, mostly of children. In the period 2014–2018 there were 4,412 poisoning incidents reported in New South Wales.

The potential toxicity of essential oil is related to its level or grade of purity, and to the toxicity of specific chemical components of the oil. Industrial users of essential oils should consult the safety data sheets to determine the hazards and handling requirements of particular oils. The European Commission's Scientific Committee on Consumer Safety dismissed the claims against tea tree oil as implausible, but did not comment on lavender oil. In 2018, a BBC report on a study stated that tea tree and lavender oils contain eight substances that when tested in tissue culture experiments, increasing the level of estrogen and decreasing the level of testosterone. Some of the substances are found in "at least 65 other essential oils". The study did not include animal or human testing.

Exposure to essential oils may cause contact dermatitis. 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.

Concern about pesticide residues in essential oils, particularly those used therapeutically, means 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.

Some essential oils may contain impurities and additives that may be harmful to pregnant women. Certain essential oils are safe to use during pregnancy, but care must be taken when selecting quality and brand. Sensitivity to certain smells may cause pregnant women to have adverse side effects with essential oil use, such as headache, vertigo, and nausea. Pregnant women often report an abnormal sensitivity to smells and taste, and essential oils can cause irritation and nausea when ingested. 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

The resins of aromatics and plant extracts were retained to produce traditional medicines and scented preparations, such as perfumes and incense, including frankincense, myrrh, cedarwood, juniper berry and cinnamon in ancient Egypt may have contained essential oils. In 1923, when archaeologists opened Pharaoh Tutankhamun’s tomb, they found 50 alabaster jars of essential oils. while the earliest recorded mention of the techniques and methods used to produce essential oils may be Ibn al Baitar (1188–1248), an Arab Al Andalusian (Muslim 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, such as referring to methyl salicylate rather than "oil of wintergreen". Essential oils are used in 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 or diffuser, heated over a candle flame, or burned as incense.