The surfactants triethanolamine and tetrasodium ethylenediaminetetraacetic acid (EDTA) are ingredients in some styling gels and have been reported as potential carcinogens.12 However, there are inadequate human or animal data to support the carcinogenicity of either ingredient at this time. Of note, tetrasodium EDTA has been reported to increase the penetration of other chemicals through the skin, which might lead to toxicity.12
Silicone
Silicone agents can be found in a variety of hair care products, including shampoos, detanglers, hair conditioners, leave-in conditioners, and moisturizers. Of the 22 products listed in Table 1, silicones are found in 14 products. Common silicones include dimethicone, amodimethicone, cyclopentasiloxane, and dimethiconol. Silicones form hydrophobic films that create smoothness and shine.6,8 Silicone-containing products help reduce frizz and provide protection against breakage and heat damage in chemically relaxed hair.6,7 For patients with natural hair, silicones aid in hair detangling.
Frequent use of silicone products can result in residue buildup due to the insolubility of silicone in water. Preventatively, some products include water-soluble silicones with the same benefits, such as silicones with the prefixes PPG- or PEG-, laurylmethicone copolyol, and dimethicone copolyol.7 Dimethicone copolyol was found in 1 of our reviewed products (OGX Nourishing Coconut Milk Shampoo); 10 products in Table 1 contain ingredients with the prefixes PPG- or PEG-. Several products in our review contain both water-soluble and water-insoluble silicones (eg, Creme of Nature Argan Oil Strength & Shine Leave-In Conditioner).
Oils
Oils in hair care products prevent hair breakage by coating the hair shaft and sealing in moisture. There are various types of oils in hair care products. Essential oils are volatile liquid-aroma substances derived most commonly from plants through dry or steam distillation or by other mechanical processes.13 Essential oils are used to seal and moisturize the hair and often are used to produce fragrance in hair products.6 Examples of essential oils that are ingredients in cosmetics include tea tree oil (TTO), peppermint oil, rosemary oil, and thyme oil. Vegetable oils can be used to dilute essential oils because essential oils can irritate skin.14
Tea tree oil is an essential oil obtained through steam distillation of the leaves of the coastal tree Melaleuca alternifolia. The molecule terpinen-4-ol is a major component of TTO thought to exhibit antiseptic and anti-inflammatory properties.15 Pazyar et al16 reviewed several studies that propose the use of TTO to treat acne vulgaris, seborrheic dermatitis, and chronic gingivitis. Although this herbal oil seemingly has many possible dermatologic applications, dermatologists should be aware that reports have linked TTO to allergic contact dermatitis due to 1,8-cineole, another constituent of TTO.17 Tea tree oil is an ingredient in several of the hair care products that we reviewed. With growing patient interest in the benefits of TTO, further research is necessary to establish guidelines on its use for seborrheic dermatitis.
Castor oil is a vegetable oil pressed from the seeds of the castor oil plant. Its primary fatty acid group—ricinoleic acid—along with certain salts and esters function primarily as skin-conditioning agents, emulsion stabilizers, and surfactants in cosmetic products.18 Jamaican black castor oil is a popular moisturizing oil in the African American natural hair community. It differs in color from standard castor oil because of the manner in which the oil is processed. Anecdotally, it is sometimes advertised as a hair growth serum; some patients admit to applying Jamaican black castor oil on the scalp as self-treatment of alopecia. The basis for such claims might stem from research showing that ricinoleic acid exhibits anti-inflammatory and analgesic properties in some mice and guinea pig models with repeated topical application.17 Scientific evidence does not, however, support claims that castor oil or Jamaican black castor oil can treat alopecia.