These studies have underscored the significance of patch testing in individuals diagnosed with FFA and have identified common allergens prevalent in this patient population. They have suggested that patients with FFA are more likely to have positive patch tests, and in some cases patients could experience improvements in scalp pruritus and erythema with allergen avoidance; however, we emphasize that a causal association between contact allergy and FFA remains unproven to date.
Most Common Allergens Pertinent to FFA
Preservatives—In some studies, patients with FFA have had positive patch tests to preservatives such as gallates and methylchloroisothiazolinone/methylisothiazolinone (MCI/MI).14 Gallates are antioxidants that are used in food preservation, pharmaceuticals, and cosmetics due to their ability to inhibit oxidation and rancidity of fats and oils.17 The most common gallates include propyl gallate, octyl gallate, and dodecyl gallate. Propyl gallate is utilized in some waxy or oily cosmetics and personal care items including sunscreens, shampoos, conditioners, bar soaps, facial cleansers, and moisturizers.18 Typically, if patients have a positive patch test to one gallate, they should be advised to avoid all gallate compounds, as they can cross-react.
Similarly, MCI/MI can prevent product degradation through their antibacterial and antifungal properties. This combination of MCI and MI is used as an effective method of prolonging the shelf life of cosmetic products and commonly is found in sunscreens, facial moisturizing creams, shampoos, and conditioners19; it is banned from use in leave-on products in the European Union and Canada due to increased rates of contact allergy.20 In patients with FFA who commonly use facial sunscreen, preservatives can be a potential allergen exposure to consider.
Iodopropynyl butylcarbamate also is a preservative used in cosmetic formulations. Similar to MCI/MI, it is a potent fungicide and bactericide. This allergen can be found in hair care products, bodywashes, and other personal products.21
UV Light–Absorbing Agents—A systematic review and meta-analysis conducted in 2022 showed a significant (P<.001) association between sunscreen use and FFA.22 A majority of allergens identified on patch testing included UVA- and UVB-absorbing agents found in sunscreens and other products including cosmetics,11,12 such as drometrizole trisiloxane, ethylhexyl salicylate, avobenzone, and benzophenone-4. Drometrizole trisiloxane is a photostabilizer and a broad-spectrum UV filter that is not approved for use in sunscreens in the United States.23 It also is effective in stabilizing and preventing the degradation of avobenzone, a commonly used UVA filter.24
Fragrances—Fragrances are present in nearly every personal and cosmetic product, sometimes even in those advertised as being “fragrance free.” Hydroperoxides of linalool, BoP, and fragrance mix are common allergens that are found in a variety of personal care products including perfumes, cosmetics, and even household cleaning supplies.25 Simultaneous positive patch tests to BoP and fragrance mix are common due to shared components. Linalool can be found in various plants such as lavender, rose, bergamot, and jasmine.26 Upon air exposure, linalool auto-oxidizes to form allergenic hydroperoxides of linalool. Among patients with FFA, positive patch test reactions to fragrance chemicals are common and could be attributed to the use of fragranced hair products and facial cosmetics.
Hair Dyes and Bleaches—Allergic reactions to hair dyes and bleaches can result in severe ACD of the head/neck and, in rare cases, scarring alopecia.27 Chemicals found in these products include paraphenylenediamine (PPD) and ammonium persulfate. The most common hair dye allergen, PPD also is used in some rubbers and plastics. Ammonium persulfate is a chemical used in hair bleaches and to deodorize oils. One case study reported a patient with FFA who developed chemically induced vitiligo immediately after the use of a hair color product that contained PPD.28 However, without patch testing to confirm the presence of contact allergy, other patient-specific and environmental risk factors could have contributed to FFA in this case.