Glossary

The lowest concentration of a substance at which the human nose can tell that there is a smell — without yet knowing what it is. Measured in water or air, in parts per billion. Every substance has its own threshold, and they differ by factors of millions: geosmin 0.004 ppb, limonene 200 ppb.

The literature distinguishes the detection threshold (there is a smell) from the recognition threshold (you know what it is). This site uses the detection threshold because it is the figure more widely published.

The lowest concentration at which the nose not only notices a smell but can name it. Always higher than the detection threshold, usually two to five times higher.

The concentration of a substance divided by its detection threshold. A compound with an OAV above 1 can be smelled; the higher the OAV, the more it dominates. This is the number the formula reader uses to rank compounds — not percentage by weight.

The formula reader shows this number as “odour strength”; odour activity value is the formal name.

The formula reader’s display name for the odour activity value — “odour strength”. The same number.

A substance turning from liquid into vapour so that it can reach the nose. A compound that evaporates quickly is smelled early and gone early. Boiling point is a rough guide: low boiling point, fast evaporation.

The time it takes for half of a substance to evaporate from a smelling strip or from skin. The formula reader estimates it from the boiling point — a model, not a measurement.

The temperature at which a substance turns completely into vapour at atmospheric pressure. Compounds with a low boiling point leave the smelling strip early; those with a high boiling point stay a long time.

The fastest-evaporating compounds, smelled in the first few minutes and then gone. Usually monoterpenes and light esters.

Compounds that stay from a few minutes to a few hours — the body of a scent. Usually alcohols and heavier aldehydes.

The slowest-evaporating compounds, lasting many hours. Sesquiterpenes, resins, musks.

A grouping of smells by how they are perceived: citrus, floral, woody… The site’s scent dictionary uses 12 families.

How long a scent lasts on paper or skin. A strong smell is not necessarily a lasting one — geosmin is very strong but evaporates quickly.

A single substance within a mixture. A bottle of essential oil is not one substance but several dozen to several hundred mixed together, and each of them is a compound — on analysis reports often called a component or constituent.

An analysis that separates a mixture into individual substances (gas chromatography) and then identifies each one by its molecular mass (mass spectrometry). It is the standard way to find out what is in a bottle of essential oil. It does not tell you how anything smells, and it cannot tell a natural substance from an identical synthetic one.

The number of minutes a substance takes to pass through the chromatography column. Lighter compounds come out first. On an analysis report, retention time is the first column and is used to identify each compound.

Each substance appears on a chromatogram as a sharp peak. The area under the peak is proportional to the amount of the substance — that is the “area percent” on the report, roughly the percentage by weight.

The area of one peak divided by the total area of all peaks. It is the number on a GC/MS report. It is close to percentage by weight but not exactly the same, because each substance gives a stronger or weaker signal.

Putting a name to a peak by comparing its mass spectrum with a library. Whatever cannot be identified is listed as “unknown” — usually the last few percent of the report.

Checking a number against a second, independent source. The site’s data standard: a threshold is published only when it can be cross-checked.

The unique identifier of a chemical substance, assigned by Chemical Abstracts Service. Two different names with the same CAS number are the same substance; the same name with different CAS numbers means two different substances.

Two substances with the same molecular formula but different structures. Geranial and neral are isomers — both C₁₀H₁₆O, with one double bond turned the other way, and they smell different.

One part in a billion, equivalent to a microgram per litre of water. The unit of odour thresholds.

One part in a million, equal to a thousand parts per billion.

The largest family of compounds in essential oils, built from five-carbon isoprene units. Monoterpenes have ten carbons, sesquiterpenes fifteen.

A ten-carbon terpene. Light and quick to evaporate, usually a top note: limonene, pinene, myrcene.

A fifteen-carbon terpene. Heavy and slow to evaporate, usually a base note: caryophyllene, farnesene.

A compound with an –OH group. In essential oils usually soft, floral and moderately lasting: linalool, geraniol, menthol.

A compound with a –CHO group. Sharp, fresh, easily oxidised: citral, citronellal, vanillin.

A compound with a C=O group within the chain. Lasting, sometimes medicinal: camphor, carvone, menthone.

The product of an acid and an alcohol. Sweet, fruity, soft: linalyl acetate, benzyl acetate.

An aromatic ring carrying an –OH group. Strong, spicy, antimicrobial and prone to irritate skin: thymol, carvacrol, eugenol.

A compound with oxygen inside a ring. Fresh, cool, penetrating: 1,8-cineole, rose oxide.

A cyclic ester. Fatty, creamy, coconut, peach: γ-nonalactone, γ-decalactone.

A classification of compounds by functional group: alcohols, aldehydes, esters… It tells you how a compound behaves — stable or quick to spoil, strong or soft.

Passing steam through plant material so that it carries the essential oil with it, then condensing and separating. The most common method. The heat can alter a few compounds.

Mechanically pressing fruit peel to release the oil, without heat. Used for citrus. It keeps delicate compounds but also keeps waxes and photosensitising compounds.

Soaking plant material in a solvent (hexane) to take up the scent, then evaporating the solvent. It gives a concrete, then an absolute. Used for flowers that cannot stand heat.

The end product of solvent extraction: a concrete washed with alcohol to remove the waxes. Richer and closer to the living flower than a distilled oil, but not an essential oil under the ISO definition.

The intermediate product of solvent extraction: scent plus plant waxes, in a thick waxy form. Washed with alcohol, it becomes an absolute.

Using CO₂ above its critical pressure and temperature as a solvent. Low heat, no solvent residue, and it captures heavier compounds too. Expensive.

Laying petals on cold fat for days so the fat absorbs their scent, then washing the fat with alcohol. An old method, now hardly used commercially.

The water left over after distilling an essential oil. It is not diluted essential oil but a different fraction, containing the molecules that dissolve in water.

Two populations of the same plant species that, because of their environment, accumulate two different sets of dominant compounds. The same botanical name, two different smells.

A vegetable oil (jojoba, almond, coconut) used to dilute essential oils before they go on the skin. Odourless or very faintly scented.

An essential oil pre-diluted to a known concentration (10 %, 1 %, 0.1 %) so that very small amounts can be weighed. If you need 0.9 mg, weigh 90 mg of a 1 % solution.

Mass per volume, in grams per millilitre. Essential oils are usually 0.84–0.98; DPG 1.02; ethanol 0.81. Needed to convert millilitres into grams.

How perfumers write formulas: whole numbers that add up to a round thousand, so scaling to a large batch is immediate.

A substance on the European list that must be named individually on a cosmetic label when it exceeds 0.001 % (leave-on) or 0.01 % (rinse-off). The expanded list under Regulation (EU) 2023/1545 has more than eighty substances.

Some substances (bergapten in bergamot, cold-pressed lemon peel) make the skin react to sunlight, causing burns or dark patches. It is not an allergy — the mechanism is different.

The international fragrance industry association, which publishes standards limiting how much of each substance may be used in each type of product. Different from the declaration list: IFRA says how much may be used; the declaration list says whether it must appear on the label.

Mixing an essential oil into a carrier oil or alcohol to lower its concentration. Skin use is usually 1–3 %; the dilution calculator works in percentage by weight.

The international standard name of a cosmetic ingredient, written in capitals on labels: LIMONENE, LINALOOL. It is the name that must be used when declaring.