What Is in an Essential Oil: Terpenes, Linalool, Limonene and Other Compounds
Essential oils are mixtures of many compounds, mostly terpenes. Plain-language chemistry on linalool, limonene, menthol and eucalyptol, and why air exposure creates allergens.
Content last updated . This page has not been medically reviewed.
Safety first. Natural products can cause side effects and interact with medicines. Read essential oil safety before you use anything on this page.
Safety first. Knowing a compound's name does not tell you whether an oil is safe or useful. Oils can irritate or cause allergy, and they should not be swallowed. See Essential oil safety.
Plain-language chemistry
- Terpenes and terpenoids are a large family of plant chemicals. Subgroups named in the review include monoterpenes and sesquiterpenes. [1]
- Phenylpropanoids are a second family, made by a different route inside the plant. [1]
- Volatile means the compounds evaporate easily. That is why you can smell them.
Compounds people ask about
| Compound | What the sources we read say |
|---|---|
| Limonene (also written dipentene/limonene) | Found in 90 of 93 essential oils analyzed in an analysis cited by NIEHS [2]. A common fragrance terpene in everyday products [3] |
| Linalool | Found in 82 of those 93 oils [2]. Forms hydroperoxides on oxidation, and these often trigger positive patch tests [3] |
| Linalyl acetate | Found in 62 of the 93 oils; NIEHS called it specific to lavender among the compounds it tested [2] |
| Eucalyptol | Found in 79 of the 93 oils [2] |
| Menthol | Present in peppermint oil. NCCIH says it should not be inhaled by, or applied to the face of, an infant or small child [4] |
A compound listed in many oils does not make those oils the same. Amounts and combinations differ, and the result for one oil cannot be assumed for another.
Why two bottles of the same oil can differ
The chemical composition of an oil may vary within the same plant species or from plant to plant. [2] A 2026 review adds that composition depends on species, chemotype, plant part, growing conditions, harvest time and post-harvest handling. [1] A chemotype is a chemical variant within one species. This is why botanical names and plant parts matter on a label; see Essential Oil Quality and Labels.Oxidation: how a good smell becomes an allergen
Linalool and limonene are "prehaptens," meaning they are not allergens until air oxidizes them. [5] A review of the oxidized forms, the hydroperoxides, notes that they cause frequent positive patch-test reactions in people with dermatitis. [3]What the numbers show, and what they do not:
- In a US clinic, 19 of 96 patients (20%) suspected of fragrance allergy reacted to linalool hydroperoxides, and 7 of 90 (8%) to limonene hydroperoxides. [6]
- In a Swedish study of 5,773 patients with dermatitis, 7.0% reacted to oxidized linalool and 5.1% to oxidized limonene. [7]
These are people already seen for skin problems, so the figures do not describe the general public. The Swedish authors said more work on clinical relevance is needed before routine screening is recommended. [7] For oils and skin, see Essential Oil Skin Reactions.
The hormone question is a lab finding about compounds
NIEHS tested lavender and tea tree oils and eight of their components on human cells in test tubes. It reported a range of hormonal activities, and activity fell as dilution rose. [2] The clinical cases it describes involved skin use, not aromatherapy, and NIEHS says further studies are needed for aromatherapy. [2] Details are on Lavender and Tea Tree Oils: Skin Reactions and the Hormone Question.What chemistry cannot tell you
A lab report listing compounds shows composition. It does not show safety or benefit, and a compound's effect in cells is preclinical evidence only. Testing is explained in GC-MS Testing and Batch Reports.Frequently asked questions
What is the main ingredient in essential oils?
There is no single one. Reviews describe mixtures dominated by terpenes, terpenoids and phenylpropanoid derivatives. [1]
Are linalool and limonene dangerous?
They are common fragrance compounds, and they become skin allergens after air oxidation. [5] That does not mean everyone reacts. Ask a pharmacist or clinician about a known fragrance allergy.
Does a high menthol or linalool number mean a better oil?
Proper evidence not found. We read no source linking a percentage of one compound to benefit or safety.
Why do oils change over time?
Linalool and limonene need air to become allergens. [5] Storage is covered in Storing Essential Oils.
Are the compounds in oils the same as drugs?
Proper evidence not found in the sources we read. They are chemicals with their own risks, so check with a pharmacist if you take medicines.
References
- Essential Oil Quality in Medicinal and Aromatic Plants: Linking Plant Biosynthesis, Matrix Selection and Extraction Strategy. 2026 (review). Read 2026-10-22.
- National Institute of Environmental Health Sciences (NIEHS). Essential Oils. Last reviewed March 19, 2026. Read 2026-10-22.
- Ogueta IA, Brared Christensson J, Giménez-Arnau E, et al. Limonene and linalool hydroperoxides review: Pros and cons for routine patch testing. Contact Dermatitis. 2022 (review). Read 2026-10-22.
- National Center for Complementary and Integrative Health (NCCIH). Peppermint Oil. Read 2026-10-22.
- Sukakul T, et al. Fragrance Contact Allergy - A Review Focusing on Patch Testing. Acta Derm Venereol. 2024 (review). Read 2026-10-22.
- Nath NS, Liu B, Green C, Atwater AR. Contact Allergy to Hydroperoxides of Linalool and D-Limonene in a US Population. Dermatitis. 2017 (retrospective patch-test study, 103 patients). Read 2026-10-22.
- Sukakul T, Bruze M, Mowitz M, et al. Contact allergy to oxidized linalool and oxidized limonene: Patch testing in consecutive patients with dermatitis. Contact Dermatitis. 2022 (retrospective, 5,773 patients, Sweden). Read 2026-10-22.