Essential Oils vs Synthetic Fragrances — Which One Is Safer for Your Skin?

“I choose cosmetics with essential oils because I prefer natural ingredients” — it's a sentence repeated by many conscious consumers. Intuition suggests that something natural must be gentler and safer than “chemicals.” Yet dermatology and toxicology tell a very different story: essential oils are among the most common causes of allergic contact dermatitis (ACD) triggered by cosmetics.

What essential oils actually are

Essential oils are volatile, complex mixtures of chemical compounds obtained from plants by steam distillation, cold pressing (citrus fruits) or solvent extraction. A single essential oil can contain anywhere from 100 to over 400 different chemical compounds — terpenes, terpene alcohols, aldehydes, esters, phenols and ketones.

For example, lavender oil (Lavandula angustifolia), according to the ISO 3515 standard, contains:

• Linalool — 25–38% (a terpene alcohol, an allergen listed in EU Annex III).
• Linalyl Acetate — 25–45% (an ester that can hydrolyse into linalool).
• Lavandulol — 0.3–1.5%.
• Terpinen-4-ol — 0.5–8%.
• 1,8-Cineole (eucalyptol) — up to 2.5%.
• Plus dozens of other compounds in trace amounts.

This means that more than 50% of lavender oil's composition consists of substances from the EU list of 26 allergens. And this isn't the exception — it's the rule for most essential oils.

Oxidation — when a safe ingredient becomes an allergen

One of the most important discoveries in the field of contact allergy in recent decades is the mechanism of terpene autoxidation. In their pure, fresh form, linalool and limonene have a relatively low allergenic potential. The trouble begins when they come into contact with atmospheric oxygen.

Research by Professor Ann-Therese Karlberg's group at the University of Gothenburg (published, among others, in Contact Dermatitis, 2008–2016) showed that:

• Linalool oxidises into linalool hydroperoxides — potent contact allergens. After 10 weeks of exposure to air, the concentration of hydroperoxides rises from 0% to over 20% of the sample content.
• Limonene (d-limonene) oxidises into limonene hydroperoxide and carvone — both with a far higher allergenic potential than pure limonene. Patch-test studies have shown that up to 3.5% of patients with suspected ACD react positively to oxidised limonene.

The practical takeaway: an old essential oil is more allergenic than a fresh one. A bottle of oil sitting on a shelf for a year — opened repeatedly and exposed to light and air — contains far more oxidation products than a freshly opened product.

The 26 EU allergens — natural and synthetic treated equally

The list of 26 fragrance allergens that must be declared under Annex III of EU Regulation 1223/2009 does not distinguish an ingredient by its origin. Linalool from lavender oil and linalool synthesised in a laboratory are the exact same chemical molecule — and they are subject to the same regulations.

The difference is that synthetic linalool is a single, pure substance, whereas linalool from an essential oil comes accompanied by hundreds of other compounds, many of which have not been fully studied toxicologically.

Of the 26 EU allergens, the following commonly occur in essential oils:

Synthetic fragrances — controlled purity

The advantage of synthetic fragrance substances is their consistency and purity. Synthetic linalool is linalool and nothing more — without the hundreds of “accompanying” compounds with unknown safety profiles. The manufacturer knows precisely the concentration, the degree of purity and the potential level of impurities.

This doesn't mean synthetics are free of risk. Some synthetic fragrance substances raise concerns:

• Lilial (Butylphenyl Methylpropional) — banned in the EU since March 2022 due to its reprotoxic effects (harmful to fertility).
• HICC (Hydroxyisohexyl 3-Cyclohexene Carboxaldehyde / Lyral) — banned since 2021 as a strong contact allergen.
• Polycyclic musks (HHCB, AHTN) — concerns regarding bioaccumulation and endocrine activity.

Natural ≠ safer — what does the science say?

A systematic review published in Contact Dermatitis (de Groot and Schmidt, 2016) covered more than 75 studies and clearly demonstrated that essential oils are among the most common causes of contact allergy among cosmetic ingredients. Tea tree oil, lavender oil and ylang-ylang oil ranked at the top.

In its 2019 guidelines, the European Society of Contact Dermatitis (ESCD) recommends caution when applying undiluted essential oils to the skin and emphasises that “the natural origin of an ingredient is no guarantee of dermatological safety.”

Practical tips — how to minimise the risk?

1. Store oils in dark glass, tightly sealed — this minimises oxidation.
2. Don't use oils older than 1–2 years — oxidation products accumulate over time.
3. Don't apply undiluted oils to your skin — IFRA and dermatologists recommend diluting to a maximum of 1–5% in a carrier oil.
4. Check the INCI composition — use our ingredient search to assess a product's allergen profile.
5. If you're prone to allergies — consider products with synthetic fragrances of a known safety profile instead of complex essential oils.

You'll find more on reading cosmetic labels in our guide to the INCI list.