Alpha isomethyl ionone — sometimes called isomethylionone — is a fragrance sometimes used in cosmetics and household cleaning products — for instance in soaps, hair care products, perfumes, fabric conditioners and detergents. Dove, Olay and Nivea use it in a number of products, for instance, and it is in such scents as Paco Rabanne’s 1 Million for men and Lady Million Eau de Parfum for women.
Given the similarity of its name to methylisothiazolinone, it’s not surprising that many wonder whether it’s safe for people allergic to the isothiazolinones.
Alpha isomethyl ionone is a fragrance, isothiazolinones are preservatives, the two chemical families are not related, and from an MI-allergy perspective, it is definitely not an isothiazolinone preservative and accordingly should not cause so-called cross reactions.
That said, it is a potential skin sensitiser and allergen in its own right, and is one of the 26 fragrance allergens which EU cosmetics law has specified should be declared on lists of ingredients (in fact, alphabetically, it is the first). It isn’t found in either Fragrance Mix I or Fragrance Mix II in the contact dermatitis testing panels, so will be tested individually if you undergo patch testing. It is possible to be allergic to both it and to MI.
Cosmetics Info say that it is synthetic, therefore any product or brand claiming to be free from artificial fragrances should in theory be safe for you, if you do have an allergy to it.
In all probability, brands which pride themselves as natural or are certified by a natural or organic body will probably be alpha isomethyl ionone-free — as well as isothiazolinone free — but always double check..
So-called fragrance free cosmetics should also be free of alpha isomethyl ionone — though will not necessarily be free from MI / MCI, and will be free of other fragrances which you may not in fact react to.
As always in the above cases, make sure to check the label, and double check with the manufacturer if you can or if you are uncertain.
Other names for alpha isomethyl ionone, which may be seen on non-cosmetic products, include gammanolene and irisantheme. Its CAS number is 127-51-5. You may come across the name hyphenated or with alternate spacing / word separation — eg alpha-iso-methylionone.