Hydroxyethyl methacrylate (HEMA) – this is a commonly used monomer in many nail products (and many others). It is a known allergen but relatively safe at or below the recommended percentage (35%). It is mostly concerned with adhesion but products with known allergen ingredients must be used correctly and safely i.e no contact with the skin and properly cured. Developing an allergy to this, as well as to several other monomers, has other implications as they are used in dental and orthopaedic procedures. Anyone who develops an allergy is strongly recommended to have a dermatological test to discover which ingredients they are allergic to. Then, this ingredient/s must be avoided for life, and their dentist and orthopaedic consultant should be notified.