1 Introduction
Per- and polyfluoroalkyl substances (PFAS) are a large group of synthetic fluorinated organic compounds that have wide industrial, professional and consumer uses. Their popularity is due to their chemical and thermal stability, a feature that is imparted by the carbon-fluorine bond, which is one of the strongest bonds in chemistry. This bond also makes PFAS highly resistant to biodegradation by most microorganisms [1]. Wastewater treatment plants (WWTPs) are therefore unable to degrade PFAS sufficiently, making them significant sources of PFAS release into the environment [2].
Aliphatic fluorocarbons are around 1.5 times more hydrophobic than aliphatic hydrocarbons [3] and, at least in the case of interaction with protein residues, the difference is attributed to the hydrophobic surface area rather than a difference in mechanism of hydrophobicity [4]. This increased hydrophobicity combined with their outstanding stability make PFAS useful ingredients in cosmetics that are marketed as waterproof and long-lasting. In terms of the lipophilicity or lipophobicity of aliphatic fluorocarbons as compared to aliphatic hydrocarbons, the situation is more complex and may vary depending on the functional groups present [5].
2 PTFE as a cosmetic ingredient
Unlike Perfluorooctanoic acid (PFOA) and Perfluorooctanesulfonic acid (PFOS), which have been well studied and largely phased out due to their persistence and toxicity [6], 31 PFAS are still added as ingredients in cosmetics based on the on a survey from the US EPA that covered use during a period of one year, through March 2023 [7]. This list includes polytetrafluoroethylene (PTFE, Fig. 1), which, according to a recent study by researchers at Nantes Université in France, was the most frequently found PFAS in 765 cosmetic products studied [8]. This paper reviews current knowledge about the safety of this PFAS substance, as used in cosmetics, and addresses both the regulatory environment and the future challenges.
PTFE is a fluoropolymer more commonly known as Teflon and is used in cosmetics as a bulking agent and for its friction-reducing properties [9]. It may be found in various cosmetic products including foundations, mascara and lipsticks, where it improves product texture and longevity. As a polymer, PTFE is not required to be registered under the European regulation on the Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH). This lack of regulatory impetus limits the amount of company-generated safety data and its public dissemination. The most comprehensive published safety assessment to date, as it relates to its use in cosmetics, is the one conducted by the US Cosmetic Ingredient Review (CIR) expert panel [9].
3 PTFE safety studies
In the CIR report, maximum use concentrations of 13% in leave-on products and 2.4% in rinse-off products were based on FDA’s Voluntary Cosmetic Registration Program (VCRP) database. In an acute oral toxicity study in rats, a low-molecular-weight PTFE resin did not result in any deaths or clinical effects up to 17 g/kg [9, 10]. In a 90-day study in rats fed 25% PTFE in their diet, there were no microscopic tissue changes. Nonetheless, rats fed unsintered PTFE resin had an increase in liver size [9, 10]. Nine-day inhalation studies using a 20% dispersion of low molecular weight PTFE resin in CCl2F–CClF2, in rats, showed no evidence of pathology after repeated exposure. Laboured breathing, incoordination and irritation of the nose were attributed to the propellants and the dispersing (not identified) [9, 10]. Mice exposed to an undisclosed concentration of PTFE for 6 months showed skin lesions, fur loss and a 50% weight reduction [11].
Regarding skin irritation and sensitization, in a 48-hour semi-occlusive patch test on 26 human subjects with 7.6% PTFE as well as a 24-hour semi-occlusive patch test on 15 subjects with 12% PTFE, skin irritation was not observed [9]. A human repeated insult patch test (HRIPT) on 206 subjects with 9% PTFE showed no evidence of skin sensitization [9]. A concentration of 20% low molecular weight PTFE resin in CCl2F–CClF2 was applied to the skin of 10 guinea pigs (duration and dose per area not stated) with no evidence of skin sensitization [10]. 20% of the same substance was also applied in the eyes of an undisclosed number of New Zealand White rabbits with mild conjunctival irritation subsiding in less than 72 hours and mild corneal injury observed at 24 hours but not 48 hours. It was concluded that the reactions observed were not greater than those observed with CCl2F–CClF2 alone [10].
In terms of genotoxicity and teratogenicity, anticohesive coating materials containing 60-73% PTFE tested negative in both the Ames test and micronucleus assay [9, 12]. A developmental toxicity study on 10 rats (5 males and 5 females) using the same materials administered once a day at doses of 1.25 g/kg and 6.25 g/kg, during gestation days 7 to 16, did not induce birth defects in rats [12].
As for carcinogenicity, fibrosarcomas, sarcomas and fibroadenomas (some malignant) were observed in multiple subcutaneous implantation studies of PTFE disks, films, and surgical mesh, in rats and mice [13–20]. These results should be taken with caution since subcutaneous implantation may not be directly relevant to the topical application of cosmetics. Moreover, it must be noted that tetrafluoroethyelene, the monomer used to make PTFE, is classified as a potential human carcinogen [21, 22], while PFOA and its substitute GenX, which may be used as processing aids in the production of PTFE [23], are on the EU Candidate List of Substances of Very High Concern for Authorisation (SHVC list) [24]. Therefore, the levels of these impurities must be kept to a minimum.
4 Prospects
Overall, more research would be needed on PTFE, following OECD test guidelines, and under Good Laboratory Practice (GLP), with well-defined substances in terms of molecular weight and impurity levels. Moreover, there is currently no data on dermal absorption, distribution, metabolism and excretion. Data on endocrine disruption and aquatic toxicity is also lacking. In terms of environmental fate, PTFE is neither readily biodegradable nor inherently biodegradable [25]. The concerns surrounding PFAS include their potential for endocrine disruption, reproductive toxicity, carcinogenicity, and their persistence in the environment. Additionally, there is a general lack of comprehensive high-quality data on these substances. PTFE, along with other PFAS, may therefore face a precautionary ban in the EU as part of a far-reaching PFAS restriction proposal [26]. There is a concern that such a ban would apply to certain substances simply due to a data gap rather than being a result of scientific evidence [27]. This was perhaps a problem in the making, as polymeric PFAS, such as PTFE, were never required to register under EU REACH, and a revision of the regulation to include an obligation to register certain polymers has been postponed [28].