Clearleaf Botanics chemotype and extraction glossary

A chemically distinct population within one botanical species, usually written after the binomial as CT. The same plant, grown in a different place or harvested at a different stage, can produce an oil dominated by a different molecule. Thyme is the standard example: CT thymol, CT linalool and CT thujanol are all Thymus vulgaris, and their safe dilution ranges differ.

See also: linalool-typethymol-typecamphor-type

An oil in which linalool dominates, typically 25-45% of the total. Found in lavender, clary sage, ho wood, basil and thyme. Linalool itself is one of the better-tolerated monoterpene alcohols and is the reason lavender tolerates a 3% ceiling, but it oxidises on air exposure into linalool hydroperoxides, which are considerably more sensitising than the parent molecule.

See also: chemotypeoxidation

An ester-dominant profile, characteristic of true lavender and bergamot. Esters are the least aggressive of the common constituent families on skin, which is why a linalyl-acetate-rich lavender is gentler than a camphor-rich lavandin at the same dilution. Esters hydrolyse over time, so an old bottle shifts toward free linalool and acetic acid.

See also: chemotypeoxidation

A profile dominated by camphor, common in rosemary, sage and lavandin. Camphor is a ketone: it is the reason rosemary CT camphor is treated more cautiously than rosemary CT cineole. Ketone-rich oils are traditionally avoided in epilepsy and around young children, and the caution is reflected in the contraindication rows on this site.

See also: chemotypecineole-type

A profile dominated by 1,8-cineole (eucalyptol), typically 45-65% in eucalyptus and present at 38-55% in rosemary CT cineole. Cineole is an oxide rather than a ketone, and it is the constituent most often associated with respiratory discomfort in infants — one of the reasons undiluted eucalyptus is never applied to a child's chest.

See also: chemotypecamphor-type

Phenol-dominant profiles, found in thyme, oregano and savory. Phenols are the most aggressive common family on skin: they are strongly irritating neat and are the oils most often implicated in dermal burns from undiluted application. Monographs for these oils carry a 2% adult ceiling rather than 3%, and the 3% row is not printed at all.

See also: chemotypesensitisation

Alcohol-dominant profiles typical of palmarosa, geranium and rose. Both geraniol and citronellol are recognised fragrance allergens under EU cosmetics labelling rules and must be declared above 0.001% in leave-on products, which is well below any therapeutic dilution — the declaration threshold is a labelling rule, not a safety limit.

See also: chemotypesensitisation

A profile dominated by the aldehyde pair geranial and neral, together reported as citral, characteristic of lemongrass, litsea cubeba and lemon myrtle. Aldehydes react quickly with skin protein; citral is one of the more frequently reported fragrance sensitisers, and oxidation of an old bottle increases its sensitising potential rather than reducing it.

See also: chemotypeoxidationsensitisation

A profile dominated by methyl chavicol, found in basil, tarragon and fennel. It is a phenylpropanoid rather than a terpene, and its metabolite has shown genotoxic potential in animal studies at high oral doses. The concern is oral and chronic, not topical at reference dilutions, but monographs for these oils say so explicitly rather than leaving it to inference.

See also: chemotypeoxidation

Steam is passed through plant material and the volatile fraction is condensed and separated. The overwhelming majority of essential oils are made this way, including lavender, rosemary, tea tree and the resin oils. It produces a chemically intact oil with no solvent residue, and yield is measured as a percentage of dry plant weight — often under 0.5% for flowers such as rose.

See also: cold-pressingco2-extraction

Fruit peel is mechanically pressed and the oil separated from the aqueous phase by centrifugation. Used for citrus. Cold pressing is the reason citrus peel oils contain furanocoumarins: distillation destroys them, expression carries them through. A distilled lime is phototoxic-free; an expressed lime is not.

See also: phototoxicityfuranocoumarin

Supercritical carbon dioxide is used as the solvent and then depressurised, leaving no residue. It extracts heavier, more lipophilic molecules that steam misses, so a CO2 extract of the same plant is not interchangeable with its distilled oil — the constituent profile, the viscosity and the hazard profile can all differ.

See also: steam-distillation

A non-immune skin reaction in which a compound absorbs UVA and releases energy into surrounding tissue, producing burning, blistering and long-lasting pigmentation. It is dose- and light-dependent, and unlike allergy it can happen on first exposure. The reaction window is roughly 12 to 18 hours after application.

See also: furanocoumarincold-pressing

The molecule family responsible for citrus phototoxicity, chiefly bergapten (5-MOP) in bergamot and lime, and oxypeucedanin in lemon and grapefruit. Steam distillation removes them; cold pressing concentrates them. This is why the same "lemon oil" can be safe at 3% on the arms or restricted to 0.5% on sun-exposed skin.

See also: phototoxicitycold-pressing

A delayed immune reaction that develops after repeated exposure. The first exposures produce no reaction; the immune system is being primed. Once sensitised, subsequent contact with the same molecule produces dermatitis within 24-72 hours, and the reaction is permanent. This is why the same oil can be fine for two years and then suddenly not.

See also: cross-sensitisationpatch-testoxidation

Sensitisation to one molecule producing reactions to chemically related ones. Reacting to geraniol raises the odds of reacting to citronellol and linalool; reacting to cinnamaldehyde is a common reason to avoid cinnamon bark, clove and cinnamon leaf together. It is the main argument for rotating oils rather than using one continuously.

See also: sensitisationpatch-test

A four-step, 48-hour self-check performed before first use of any new oil and repeated after any long gap. Dilute to 1% in carrier, apply 2 drops to the inner forearm, cover and leave 24 hours, then read at 24 and 48 hours. Redness, itching, swelling or blistering at any point means do not use it. A negative result lowers risk; it does not make a reaction impossible.

See also: sensitisationoxidation

Essential oils react with oxygen in the headspace of a partly used bottle. The products — hydroperoxides from linalool and limonene especially — are markedly more irritating and more sensitising than the original molecules. Oxidation is detected by a change in smell toward sharp, acidic or turpentine-like, or by increased viscosity. A 2019 study of commercial lavender and tea tree bottles found hydroperoxide levels far above the allergenic threshold in roughly one in five samples.

See also: linalool-typepatch-testshelf-life

The period during which an unopened, refrigerated-or-cool bottle stays within its constituent specification. Citrus peel oils are the short end at 12-18 months; resin oils such as frankincense and myrrh, and wood oils such as sandalwood and vetiver, improve with age over years. On this site every monograph prints a shelfLifeMonths figure, and it assumes a full, tightly capped, dark glass bottle kept below 20°C.

See also: oxidation

The highest concentration of a given oil this site will print for adult leave-on use on intact skin. It is set at 3% for well-tolerated oils and reduced to 2% or 1% for phenol-rich, aldehyde-rich or phototoxic oils. It is a ceiling for a short course on a small area, not a target, and it is not a recommendation to use that concentration.

See also: patch-testphototoxicity

The analytical method behind every constituent percentage published here. It separates the volatile components of a sample and identifies each by mass. A useful GC-MS report states the column and programme used, the batch, and the chemotype; a percentage quoted with none of those is a marketing figure rather than a measurement.

See also: chemotype