Essential oil

Essential oils are concentrated, hydrophobic liquids containing volatile organic compounds extracted from plant material. 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 the oil contains the fragrance or essence of the plant. The term "essential" used here does not indicate that it is nutritionally required in the diet, as with essential amino acid or essential fatty acid.
Essential oils are most commonly extracted by steam distillation. Other processes include expression, solvent extraction, sfumatura, 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 household cleaning products.
Essential oils are often used for aromatherapy, a form of alternative medicine in which healing effects are ascribed to aromatic compounds. There is not sufficient evidence that it 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.

Production

Distillation

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 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 which is purified 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. Hydrosols include rose water, lavender water, lemon balm, clary sage, and orange blossom water.

Expression

Most citrus peel oils are expressed mechanically or cold-pressed. 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.

Solvent extraction

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 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 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.

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.

Uses and cautions

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 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 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.

Use in aromatherapy

An overview of systematic reviews found that while aromatherapy has been studied for conditions like anxiety, depression, and pain, the evidence is generally of poor quality and not convincing enough to support its effectiveness. Use of essential oils may cause harm including allergic reactions and skin irritation. There has been at least one case of death.

Use as pesticide

Research has shown that some essential oils have potential as a natural pesticide. In case studies, certain oils have been shown to have a variety of deterring effects on pests, specifically insects and select arthropods. These effects may include repelling, inhibiting digestion, stunting growth, decreasing rate of reproduction, or death of pests that consume the oil. However, the molecules within the oils that cause these effects are normally non-toxic for mammals. These specific actions of the molecules allow for widespread use of these "green" pesticides without harmful effects to anything else other than pests. Essential oils that have been investigated include rose, lemon grass, lavender, thyme, peppermint, basil, cedarwood, and eucalyptus.
Although they may not be the perfect replacement for all synthetic pesticides, essential oils have prospects for crop or indoor plant protection, urban pest control, and marketed insect repellents, such as bug spray. Certain essential oils have been shown in studies to be comparable, if not exceeding, in effectiveness to DEET, which is currently marketed as the most effective mosquito repellent. Although essential oils are effective as pesticides when first applied in uses such as mosquito repellent applied to the skin, it is only effective in the vapor stage. Since this stage is relatively short-lived, creams and polymer mixtures are used in order to elongate the vapor period of effective repellency.
In any form, using essential oils as green pesticides rather than synthetic pesticides has ecological benefits such as decreased residual actions. In addition, increased use of essential oils as pest control could have not only ecological, but economical benefits as the essential oil market diversifies and popularity increases among organic farmers and environmentally conscious consumers. some EOs are authorized, and in use, in the European Union: Melaleuca oil as a fungicide, citronella oil as a herbicide, Syzygium aromaticum oil as a fungicide and bactericide, Mentha spicata oil as a plant growth regulator; Citrus sinensis oil for Bemisia tabaci on Cucurbita pepo and Trialeurodes vaporariorum on Solanum lycopersicum; and approvals for oils of Thymus, C. sinensis, and Tagetes as insecticides are pending.

Use in food

In relation with their food applications, although these oils have been used throughout history as food preservatives, it was in the 20th century when essential oils were considered as Generally Recognized as Safe by the United States’ Food and Drug Administration.
GRAS substances according to the FDA

Common name	Botanical name of plant source
Alfalfa	Medicago sativa L.
Allspice	Pimenta officinalis Lindl.
Bitter almond, free from cyanide	Prunus amygdalus Batsch, Prunus armeniaca L., or Prunus persica Batsch.
Ambrette	Hibiscus moschatus Moench.
Angelica root	Angelica archangelica L.
Angelica seed	Angelica archangelica L.
Angelica stem	Angelica archangelica L.
Angostura	Galipea officinalis Hancock, Angostura trifoliata
Anise	Pimpinella anisum L.
Asafetida	Ferula assa-foetida L. and related spp. of Ferula
Balm	Melissa officinalis L.
Balsam of Peru	Myroxylon pereirae Klotzsch.
Basil	Ocimum basilicum L.
Bay leaves	Laurus nobilis L.
Bay	Pimenta racemosa J. W. Moore.
Bergamot	Citrus aurantium L. subsp. bergamia Wright et Arn.
Bitter almond	Prunus amygdalus Batsch, Prunus armeniaca L., or Prunus persica Batsch.
Bois de rose	Aniba rosaeodora Ducke.
Cacao	Theobroma cacao L.
Camomile flowers, German or Hungarian	Matricaria chamomilla L.
Camomile flowers, Roman or English	Anthemis nobilis L.
Cananga	Cananga odorata Hook. f. and Thoms.
Capsicum	Capsicum frutescens L. and Capsicum annuum L.
Caraway	Carum carvi L.
Cardamom seed	Elettaria cardamomum Maton.
Carob bean	Ceratonia siliqua L.
Carrot	Daucus carota L.
Cascarilla bark	Croton eluteria Benn.
[Cinnamomum cassia\|Cassia bark, Chinese]	Cinnamomum cassia Blume.
Cassia bark, Padang or Batavia	Cinnamomum burmanni Blume.
Cassia bark, Saigon	Cinnamomum loureirii Nees.
Celery seed	Apium graveolens L.
Cherry, wild, bark	Prunus serotina Ehrh.
Chervil	Anthriscus cerefolium Hoffm.
Chicory	Cichorium intybus L.
Cinnamon bark, Ceylon	Cinnamomum zeylanicum Nees.
Cinnamon bark, Chinese	Cinnamomum cassia Blume.
Cinnamon bark, Saigon	Cinnamomum loureirii Nees.
Cinnamon leaf, Ceylon	Cinnamomum zeylanicum Nees.
Cinnamon leaf, Chinese	Cinnamomum cassia Blume.
Cinnamon leaf, Saigon	Cinnamomum loureirii Nees.
Citronella	Cymbopogon nardus Rendle.
Citrus peels	Citrus spp.
Clary	Salvia sclarea L.
Clover	Trifolium spp.
Coca	Erythroxylum coca Lam. and other spp. of Erythroxylum
Coffee	Coffea spp.
Cola nut	Cola acuminata Schott and Endl., and other spp. of Cola
Coriander	Coriandrum sativum L.
Cumin	Cuminum cyminum L.
Curaçao orange peel	Citrus aurantium L.
Cusparia bark	Galipea officinalis Hancock
Dandelion	Taraxacum officinale Weber and Taraxacum laevigatum DC.
Dandelion root	Taraxacum officinale Weber and Taraxacum laevigatum DC.
Dog grass	Agropyron repens Beauv.
Elder flowers	Sambucus canadensis L. and Sambucus nigra I.
Estragole	Artemisia dracunculus L.
Estragon	Artemisia dracunculus L.
Fennel, sweet	Foeniculum vulgare Mill.
Fenugreek	Trigonella foenum-graecum L.
Galanga	Alpinia officinarum Hance.
Geranium	Pelargonium spp.
Geranium, East Indian	Cymbopogon martini Stapf.
Geranium, rose	Pelargonium graveolens L'Her.
Ginger	Zingiber officinale Rosc.
Grapefruit	Citrus paradisi Macf.
Guava	Psidium spp.
Hickory bark	Carya spp.
Horehound	Marrubium vulgare L.
Hops	Humulus lupulus L.
Horsemint	Monarda punctata L.
Hyssop	Hyssopus officinalis L.
Immortelle	Helichrysum augustifolium DC.
Jasmine	Jasminum officinale L. and other spp. of Jasminum
Juniper	Juniperus communis L.
Kola nut	Cola acuminata Schott and Endl., and other spp. of Cola
Laurel berries	Laurus nobilis L.
Laurel leaves	Laurus spp.
Lavender	Lavandula officinalis Chaix
Lavender, spike	Lavandula latifolia Vill.
Lavandin	Hybrids between Lavandula officinalis Chaix and Lavandula latifolin Vill.
Lemon	Citrus limon Burm. f.
Lemon balm	Melissa officinalis L.
Lemongrass	Cymbopogon citratus DC. and Cymbopogon lexuosus Stapf.
Lemon peel	Citrus limon Burm. f.
Lime	Citrus aurantifolia Swingle.
Linden flowers	Tilia spp.
Locust bean	Ceratonia siliqua L,
Lupulin	Humulus lupulus L.
Mace	Myristica fragrans Houtt.
Mandarin	Citrus reticulata Blanco.
Marjoram, sweet	Majorana hortensis Moench.
Mate, yerba	Ilex paraguariensis St. Hil.
Melissa
Menthol	Mentha spp.
Menthyl acetate	Mentha spp.
Molasses	Saccharum officinarum L.
Mustard	Brassica spp.
Naringin	Citrus paradisi Macf.
Neroli, bigarade	Citrus aurantium L.
Nutmeg	Myristica fragrans Houtt.
Onion	Allium cepa L.
Orange, bitter, flowers	Citrus aurantium L.
Orange, bitter, peel	Citrus aurantium L.
Orange leaf	Citrus sinensis Osbeck.
Orange, sweet	Citrus sinensis Osbeck.
Orange, sweet, flowers	Citrus sinensis Osbeck.
Orange, sweet, peel	Citrus sinensis Osbeck.
Origanum	Origanum spp.
Palmarosa	Cymbopogon martini Stapf.
Paprika	Capsicum annuum L.
Parsley	Petroselinum crispum Mansf.
Pepper, black	Piper nigrum L.
Pepper, white	Piper nigrum L.
Peppermint	Mentha piperita L.
Peruvian balsam	Myroxylon pereirae Klotzsch.
Petitgrain	Citrus aurantium L.
Petitgrain lemon	Citrus limon Burm. f.
Petitgrain mandarin or tangerine	Citrus reticulata Blanco.
Pimenta	Pimenta officinalis Lindl.
Pimenta leaf	Pimenta officinalis Lindl.
Pipsissewa leaves	Chimaphila umbellata Nutt.
Pomegranate	Punica granatum L.
Prickly ash bark	Xanthoxylum ''americanum Mill. or Xanthoxylum clava-herculis L.
Rose absolute	Rosa alba L., Rosa centifolia L., Rosa damascena Mill., Rosa gallica L., and vars. of these spp.
Rose	Rosa alba L., Rosa centifolia L., Rosa damascena Mill., Rosa gallica L., and vars. of these spp.
Rose buds	Rosa alba L., Rosa centifolia L., Rosa damascena Mill., Rosa gallica L., and vars. of these spp.
Rose flowers	Rosa alba L., Rosa centifolia L., Rosa damascena Mill., Rosa gallica L., and vars. of these spp.
Rose fruit	Rosa alba L., Rosa centifolia L., Rosa damascena Mill., Rosa gallica L., and vars. of these spp.
Rose geranium	Pelargonium graveolens L'Her.
Rose leaves	Rosa spp.
Rosemary	Rosmarinus officinalis L.
Saffron	Crocus sativus L.
Sage	Salvia officinalis L.
Sage, Greek	Salvia triloba L.
Sage, Spanish	Salvia officinalis subsp. lavandulifolia Gams
Carob	Ceratonia siliqua L.
Savory, summer	Satureia hortensis L.
Savory, winter	Satureia montana L.
Schinus molle	Schinus molle L.
Sloe berries	Prunus spinosa L.
Spearmint	Mentha spicata L.
Spike lavender	Lavandula latifolia Vill.
Tamarind	Tamarindus indica L.
Tangerine	Citrus reticulata Blanco.
Tarragon	Artemisia dracunculus L.
Tea	Thea sinensis L.
Thyme	Thymus vulgaris L. and Thymus zygis var. gracilis Boiss.
Thyme, white	Thymus vulgaris L. and Thymus zygis var. gracilis Boiss.
Thyme, wild or creeping	Thymus serpyllum L.
Triticum	Elymus repens
Tuberose	Polianthes tuberosa L.
Turmeric	Curcuma longa L.
Vanilla	Vanilla planifolia Andr. or Vanilla tahitensis J. W. Moore.
Violet flowers	Viola odorata L.
Violet leaves	Viola odorata L.
Violet leaves absolute	Viola odorata L.
Wild cherry bark	Prunus serotina Ehrh.
Ylang-ylang	Cananga odorata Hook. f. and Thoms.
Zedoary bark	Curcuma zedoaria'' Rosc.

As antimicrobials

The most commonly used essential oils with antimicrobial action are: β-caryophyllene, eugenol, eugenol acetate, carvacrol, linalool, thymol, geraniol, geranyl acetate, bicyclogermacrene, cinnamaldehyde, geranial, neral, 1,8-cineole, methyl chavicol, methyl cinnamate, methyl eugenol, camphor, α-thujone, viridiflorol, limonene, -linalool oxide, α-pinene, p-cymene, -caryophyllene, γ-terpinene.
Some essential oils are effective antimicrobials and have been evaluated for food incorporation in vitro. However, actual deployment is rare because much higher concentrations are required in real foods. Some or all of this lower effectiveness is due to large differences between culture medium and foods in chemistry, viscosity, and duration of inoculation/storage.

Dilution

Essential oils are usually lipophilic compounds that are immiscible with water. They can be diluted in solvents like pure ethanol and polyethylene glycol.

Raw materials

Essential oils are derived from sections of plants. Some plants, like the bitter orange, are sources of several types of essential oil.
;Bark

;Berries

;Flowers

Cannabis [flower essential oil|Cannabis]
Chamomile
Clary sage
Clove
Hops
Hyssop
Jasmine
Lavender
Manuka
Marjoram
Orange
Pelargonium
Plumeria
Rose
Ylang-ylang

;Leaves

;Peel

;Resin

;Rhizome

;Roots

Valerian

;Seeds

;Woods

, an essential oil derived from Myroxylon plants, is used in food and drink for flavoring, in perfumes and toiletries for fragrance, and in animal care products. However, national and international surveys identified balsam of Peru among the "top five" allergens most commonly causing patch test allergic reactions in people referred to dermatology clinics.

Garlic oil

is an essential oil derived from garlic.

Eucalyptus oil

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

Lavender oil has long been used in the production of perfume. However, studies have shown 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.

Rose oil

There are two major species of rose whose petals used for the production of Rose oil; Rosa damascena and Rosa centifolia.
Rose oils can be classified according to their botanical name, their place of origin and their extraction process.
Steam-distilled rose oil is known as "rose otto", while the solvent extracted oil is known as "rose absolute".
Rose otto is extracted through hydro-distillation, in which Rosa damascena petals are slowly heated with water. The heat causes the petals to release the essential rose oil. When cooled, the oil floats to the surface of the water. In contrast, rose absolute is solvent-extracted. Rose petals are combined with an organic solvent in a slowly rotating drum to draw out the aromatic matter from the flowers. After the solvent is evaporated, the waxy substance remaining is called rose concrete. Rose concrete is filtered with ethanol alcohol to remove waxes and non-aromatic components and leave behind the liquid known as rose absolute.

Toxicity

The potential toxicity of essential oil is related to the toxicity of specific chemical components of the oil, and to its level or grade of purity. Industrial users of essential oils should consult the safety data sheets to determine the hazards and handling requirements of particular oils.
Some essential oils, even at extremely low levels, can pose health threats for pregnant women, those who are breastfeeding, those with estrogen-dependent cancers or individuals with epilepsy.
Essential oil use in children can pose a danger when misused because of their thin skin and developing livers. This might cause them to be more susceptible to toxic effects than adults.
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 form. Some can cause severe irritation, provoke an allergic reaction and, over time, prove toxic to the liver. If ingested or rubbed into the skin, essential oils can be highly poisonous, causing confusion, choking, loss of muscle coordination, difficulty in breathing, pneumonia, seizures, and possibly severe allergic reactions or coma.
Some essential oils, including many of the citrus peel oils, are photosensitizers, increasing vulnerability of the skin to sunlight.

Flammability

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 Class 3 Flammable Liquids, as they have a flash points at or below 60 °C.

Gynecomastia

ic and antiandrogenic activity have been reported by in vitro study of tea tree oil and lavender essential oils. Two published sets of case reports suggest that lavender oil 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 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.

Handling

Exposure to essential oils may cause 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.

Ingestion

Some essential oils qualify as GRAS flavoring agents for use in foods, beverages, and confectioneries according to strict good manufacturing practice and flavorist standards. Pharmacopoeia standards for medicinal oils should be heeded.
The internal use of essential oils can pose hazards to pregnant women, as some can be abortifacients or cause fetal damage, and thus should not be used during pregnancy.
Some oils can be toxic to some domestic animals, cats in particular.

Pesticide residues

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.

Pregnancy

Some essential oils should not be used by pregnant or breastfeeding women, because they can be abortifacients, cause fetal damage, or disrupt hormones. Key components of essential oils and whole essential oils do not always have the same hormonal effects. Minor compounds in whole oils may have both individual or group effects.
Some essential oils may contain impurities or additives that may be harmful. Some uses of essential oils may be safe 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 a heightened sensitivity to smells and taste, and essential oils can cause irritation and nausea if ingested.

Toxicology

The following table lists the 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 derived products

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.

History

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.
Essential oils have been used in folk medicine over centuries. The Persian physician Ibn Sina, known as Avicenna in Europe, was first to derive the fragrance of flowers from distillation, while the earliest recorded mention of the techniques and methods used to produce essential oils may be Ibn al-Baitar, an Arab Al-Andalusian 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.