Cosmetics are complex mixtures of various ingredients designed to cleanse, moisturise, protect, or enhance the appearance of the skin, hair, and nails. While most of these ingredients are generally well-tolerated, certain substances have a higher propensity to cause allergic reactions in susceptible individuals. Among the most frequently implicated allergens in cosmetics are fragrances and preservatives. Their widespread use and inherent chemical properties contribute to their significant role in causing allergic contact dermatitis.
Fragrances are ubiquitous in a vast array of cosmetic products, including cleansers, moisturisers, makeup, shampoos, and perfumes. They are added to impart a pleasant scent to the product, enhance the consumer experience, and sometimes to mask the odours of other ingredients. However, fragrances are not typically single chemical entities but rather complex mixtures of dozens, if not hundreds, of individual fragrance chemicals, both natural and synthetic. This complexity significantly increases the likelihood of containing one or more substances that can act as allergens. Common fragrance allergens include components like balsam of Peru, cinnamic aldehyde, eugenol, isoeugenol, hydroxycitronellal, and oak moss absolute. Individuals can become sensitised to one or more of these fragrance chemicals through repeated exposure. Once sensitisation occurs, subsequent exposure, even to small amounts, can trigger an allergic reaction characterised by itching, redness, swelling, and sometimes blistering at the site of contact. The face, neck, and hands are common areas affected due to the frequent application of fragranced products to these sites. Notably, fragrance allergies can sometimes present as a more generalised dermatitis if the allergen is widely distributed through product use or via transfer from hands to other parts of the body. Identifying the specific fragrance component(s) responsible for an allergic reaction can be challenging due to the complex nature of fragrance mixtures. While ingredient lists are mandatory in many regions, specific fragrance allergens are not always individually listed under the general term “fragrance” or “parfum,” although regulations in some areas are moving towards more detailed labelling. This lack of transparency can make avoidance difficult for fragrance-sensitive individuals. Patch testing with a standard fragrance mix and, in some cases, individual fragrance components is essential for identifying the specific allergens to which a person is sensitised.
Preservatives are another major category of allergenic ingredients commonly found in cosmetic formulations. They are essential components of many cosmetic products, particularly those containing water, as they prevent the growth of bacteria, fungi, and yeasts that can contaminate the product and potentially cause skin infections or product degradation. The cosmetic industry’s shift towards water-based formulations has further increased the necessity for effective preservatives. However, many preservatives are low molecular weight, biologically active compounds that can penetrate the skin and act as allergens in susceptible individuals. Allergic contact dermatitis due to preservatives can occur in any area of the body where the preserved product is applied, but it is frequently seen on the face, neck, hands, and axillae. Several classes of preservatives are known to be common allergens. Formaldehyde-releasing preservatives are a significant group. These substances, such as quaternium-15, imidazolidinyl urea, diazolidinyl urea, DMDM hydantoin, and methenamine, slowly release formaldehyde, which acts as the antimicrobial agent. Formaldehyde itself is a well-known allergen, and individuals can become sensitised to formaldehyde or to the formaldehyde-releasing preservatives themselves. Another important group of preservative allergens are the isothiazolinones, particularly methylchloroisothiazolinone (MCI) and methylisothiazolinone (MI), often used in combination (MCI/MI) or individually. These preservatives are effective against a broad spectrum of microorganisms and are found in a wide range of personal care products, including moist wipes, shampoos, conditioners, and liquid soaps. The frequency of allergic reactions to isothiazolinones has been a concern, leading to regulatory restrictions on their use in some regions. Parabens (e.g., methylparaben, ethylparaben, propylparaben, butylparaben) were previously very widely used preservatives but have become less prevalent due to consumer concerns, although they can still cause allergic reactions in some individuals. Other preservative allergens include iodopropynyl butylcarbamate and phenoxyethanol. Similar to fragrance allergies, identifying the specific preservative allergen requires patch testing with a series of common preservatives. Once identified, strict avoidance of products containing that specific preservative, or cross-reacting preservatives, is crucial for preventing future allergic reactions. In addition to fragrances and preservatives, other cosmetic ingredients such as certain dyes (e.g., p-phenylenediamine in hair dyes), emulsifiers (e.g., lanolin alcohols), and plant extracts can also act as allergens in some individuals. However, fragrances and preservatives remain the most frequently identified culprits in allergic contact dermatitis from cosmetics due to their widespread use and inherent allergenic potential. Enhanced labelling and greater awareness of these common allergens can help consumers make informed choices and reduce the risk of allergic reactions.