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Sassafras root bark gives an oil that is 80 to 90 percent safrole, a substance IFRA has prohibited since 1987. Tarragon leaf gives an oil that is more than 70 percent estragole. Both are entirely natural. The question natural or synthetic therefore does not lead to an answer about safety.

A molecule does not know where it came from

Linalool distilled from coriander seed and linalool made in a factory share the same formula, the same carbon skeleton, the same alcohol group. Differences may lie in the ratio of the two enantiomers and in accompanying impurities, but the main molecule carries no label of origin. To know whether a sample of linalool came from a plant or from petroleum, a laboratory has to measure carbon isotopes: petrochemical synthetics contain essentially no carbon 14.

Body and skin respond to the molecule, not to the story about it. That is why safety systems work molecule by molecule. EU allergen labelling law counts the actual content of a substance in a product, whether it came from an essential oil or from a bottle of chemical. The 2023 regulation even names whole essential oils such as lemon, lavender, peppermint and ylang ylang.

The word natural on cosmetics has no dedicated legal definition in the EU either. Such a claim must be truthful and supported under Regulation (EU) 655/2013; standards such as ISO 16128 or COSMOS offer voluntary definitions. The library's glossary adds a short sentence at the end of this entry: natural does not mean safe.

Essential oils keep a place of their own that single substances cannot fill: the complexity of dozens of constituents, the difference between harvests, the trace of a piece of land. This essay does not rank essential oils lower. It only puts the question of safety where it belongs: the molecule, the dose, and the use.

Methyl eugenol in essential oils

Methyl eugenol smells of clove, warm wood, a little vanilla sweetness. It occurs naturally in small amounts in many essential oils: rose, nutmeg, basil, tarragon. In estragole chemotype basil its content can reach 4.2 percent; in tarragon 0.1 to 1.5 percent; in sassafras bark 0.5 to 3 percent.

The US National Toxicology Program (NTP) lists methyl eugenol as reasonably anticipated to be a human carcinogen, on sufficient evidence in experimental animals. In a two year study, rats and mice were given 37, 75 or 150 mg/kg per day by gavage; all three dose levels gave clear evidence of cancer, with liver and glandular stomach tumours increased in both sexes.

IFRA restricts methyl eugenol in every product category. Under the 51st Amendment, the maximum in fine fragrance is 0.011 percent and in body lotion 0.0015 percent. The limit applies to the total in the product, including the part that comes from essential oils. Some suppliers now offer rose and a few other oils selected for reduced methyl eugenol content.

Rosemary, a familiar bottle, gives an example of dose. The camphor chemotype of rosemary contains 0 to 0.02 percent methyl eugenol. A body lotion category product with 5 percent of that oil contains at most about 0.001 percent methyl eugenol, under the 0.0015 percent limit. At 10 percent, the figure rises to 0.002 percent and passes the limit. Same bottle; the answer changes with the dilution.

Safrole and sassafras

Sassafras is the root bark and wood of Sassafras albidum in North America and Ocotea cymbarum in Brazil, family Lauraceae. The oil distilled from it holds about 80 to 90 percent safrole. Its smell is warm, sweet, lightly woody, the smell once used in sassafras tea and root beer in America.

Safrole causes liver cancer in rats and mice. Rats fed 5,000 and 10,000 ppm in a two year study developed benign and malignant tumours; mice fed 4,000 ppm developed liver tumours. Its metabolite 1'-hydroxysafrole is a more potent liver carcinogen than safrole itself. So IFRA prohibits safrole, isosafrole and dihydrosafrole as fragrance ingredients, under a standard published in 1987, and prohibits the use of natural extracts containing them as substitutes. EU cosmetics law also prohibits safrole.

The IFRA Standard still leaves room for essential oils. Safrole arriving naturally from essential oils is accepted as long as the total of the three substances in the finished product does not exceed 0.01 percent. Safrole occurs at trace levels in some other oils; linalool chemotype ho leaf, for example, holds 0.01 to 0.9 percent.

The data in the library also show caution in how to read. DNA adduct formation by safrole in human liver is 10 to 150 times lower than in mouse liver, and safrole is 8.6 times more potent as a carcinogen in mouse liver than in rat liver. Differences between species are the reason animal figures are carried over to humans through safety factors, not copied directly. The conclusion for cosmetics remains clear: sassafras oil is not used in cosmetics, and its page in the library exists mainly as a warning.

Estragole in basil

Estragole smells of liquorice, anise and tarragon leaf, sweet and herbaceous. It makes up 73 to 87 percent of tarragon oil, and about as much of estragole chemotype basil, a type produced largely in the Comoros, Réunion and parts of Madagascar under the trade name exotic basil.

The same species, Ocimum basilicum, in another chemotype gives an entirely different bottle. Linalool chemotype basil, the kind usually found in European kitchens as sweet basil, holds only 0.2 to 2 percent estragole. The species name is the same, the trade names are easily confused, and the estragole content differs by a factor of tens to hundreds. The library page notes that the certificate of analysis must be read before use.

Estragole causes liver cancer in mice and in male rats through its metabolite 1'-hydroxyestragole. The safety literature records an important detail about dose: the clearance route through O-demethylation works efficiently at low doses but is saturated at high doses. In other words, the body handles the same molecule differently depending on how much of it arrives.

Under the 51st Amendment, IFRA restricts estragole to 0.014 percent in fine fragrance and 0.0022 percent in body lotion. With estragole chemotype basil holding up to 87 percent estragole, that limit leaves very little room; the library gives this oil's maximum dermal level as 0.02 percent. In star anise and fennel, estragole comes alongside trans-anethole, a molecule with a quite different safety profile despite a similar smell.

Synthetics that have been assessed

The index of IFRA Standards in the 51st Amendment lists substances alphabetically, not by origin. The same index holds prohibitions for synthetic molecules such as acetyl ethyl tetramethyl tetralin, prohibited since 2006, or 3-acetyl-2,5-dimethylfuran, prohibited in 2023. The same index also holds prohibitions for alantroot oil, in 2006, and boldo oil, in 2009.

Each standard page points to RIFM's safety assessment of that exact substance. A synthetic molecule is usually a single substance with a defined structure; it can be tested, checked for impurities and limited directly. An essential oil is a mixture of many substances whose composition varies by harvest, region and distillation; the trade calls it a natural complex substance, and assesses it through its constituents as well as the whole mixture.

That does not make synthetics automatically safer. The very prohibitions on synthetic molecules in the index show that synthetic origin is no exemption from scrutiny. It only means the right question is not origin but which substance has been assessed, on what data, for what use.

So a label saying free of synthetic fragrance says less than it seems to, just like a label saying phthalate free. It does not tell which substance is present instead, or at what dose. The essay One Set of Standards for Every Bottle goes into how a limit figure is set.

Safety is dose and use

A single drop of essential oil, about 0.05 ml, can carry dozens of different substances. The library's safety page opens with that sentence and goes on: safety is not abstinence; safety is choosing the right oil, the right dilution, the right person and the right situation.

Those four variables have already appeared in the examples above. The right oil: linalool chemotype basil and estragole chemotype basil are two different bottles. The right dilution: camphor rosemary at 5 percent and at 10 percent gives two different answers. The right person: the safety page excludes phenylpropene ethers such as methyl eugenol, safrole and estragole throughout pregnancy because the safety margin is narrow. The right situation: one substance has twelve limits for twelve product categories.

None of the four variables is origin. A synthetic used at the wrong dose can still cause harm; an essential oil used at the right dose for the right person can be used for many years. The question natural or synthetic is the wrong question, not because it is meaningless, but because it answers a different question: one about provenance, about craft, about what a piece of land leaves in the bottle.

The right question about safety is more concrete and less romantic. Which substance. How much. On which area of skin, for how long, for whom. The standards and laws cited in this essay were written to answer exactly those questions. The essay The Names at the End of the Ingredient List continues with labelling.

A molecule carries no birth certificate.

It carries only a shape and a dose.

The rest is how people use it.