Alpha-Isomethyl Ionone

What it is

Alpha-isomethyl ionone is a synthetic ketone with a violet and orris note, used for scent rather than for any effect on skin. It is one of the original 26 fragrance allergens the EU requires to be declared by name, and has carried that duty since the Cosmetics Regulation took effect. On a mask it is almost always present because a compounded fragrance contains it, not because it was added on its own. SCCS/1459/11 records that gamma-methylionone and methylionone are synonyms for the same substance, with one CAS number and one preferred chemical name, and reviews them together.

How it shows up on a label

What it does well

• Carries a violet and orris note in compounded fragrance

  It is odour-active at low levels and is a standard component of violet accords. This is a sensory function. No skin benefit is established, and its appearance on the label is a disclosure duty rather than a claim.

  Strong evidence

Worth knowing

• Classed as an established contact allergen, on evidence that is genuinely mixed

  SCCS/1459/11 places it among established contact allergens in humans, but its own summary is uneven. A RIFM review of four human sensitisation experiments using different protocols returned negative results throughout; the IVDK tested 986 patients between 2005 and 2008 with no positive reactions; and a Korean series of 422 consecutive patients found 2.1% positive.

  Matters most toPeople with a diagnosed fragrance allergy, particularly to violet or orris notes.

• A similarly named substance, pseudo-isomethyl ionone, is banned outright

  Pseudo-isomethyl ionone (3,6,10-trimethyl-3,5,9-undecatrien-2-one) is a different compound, prohibited as a fragrance ingredient by Annex II entry 449. The two names differ by a single word and are easy to conflate when scanning a label or a search result.

  Matters most toAnyone checking an ingredients list against the EU prohibited list.

Where the EU stands