Farnesol

Farnesol is a sesquiterpene alcohol occurring naturally in many essential oils, including neroli, rose, citronella and lemongrass. It sits in an unusual position in cosmetics: it has genuine functional properties, and it is also a regulated fragrance allergen.

The functional side

Farnesol has documented antimicrobial activity, particularly against Cutibacterium acnes, and against certain fungi. It is also a quorum-sensing molecule in fungal biology, meaning organisms use it to communicate population density, which is a genuinely interesting piece of biology.

It has reported anti-inflammatory activity, and is used as a deodorant active for the same antimicrobial reasons.

Farnesyl acetate, which appears alongside it in some formulations, is the ester form, converted to farnesol in the skin.

The allergen side

Farnesol is one of the 26 fragrance substances the EU requires to be declared on ingredient lists when present above 0.001 percent in leave-on products, because they are recognised contact allergens.

Sensitisation rates are low relative to some other fragrance allergens, but it is a documented cause of allergic contact dermatitis, and it appears on dermatological patch test series.

The tension worth naming

Farnesol appears frequently in formulations for sensitive and reactive skin, on the basis of its antimicrobial and anti-inflammatory properties. It is also an ingredient a subset of those same people may react to.

That is not a contradiction in the formulation so much as a genuine trade-off. For most people the functional benefit is real and the allergy risk is small. For someone who has previously reacted to fragrance, it is worth knowing the ingredient is there, whatever role it is playing.

Reading a label

Farnesol may appear because it was added deliberately for its antimicrobial properties, or because it is a constituent of an essential oil elsewhere in the formula, or as part of a fragrance blend. The ingredient list does not distinguish between these.

Its position is a rough guide. High in the list suggests a deliberate functional inclusion. Grouped with other declared allergens at the end suggests it arrived with the fragrance.