Why is TPO, which is so prevalent in semi-permanent nail polishes, now banned?

Diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO) is a chemical compound belonging to the photoinitiator family. Its main function is to trigger the polymerization of gels applied to nails when they are exposed to a UV or LED lamp. In other words, it is thanks to TPO that the semi-permanent nail polishes harden quickly and gain their much-appreciated durability. This property has greatly contributed to the success of these products in nail salons, as it allows for long-lasting wear of the polishes, often for two to three weeks without chipping.

Until recently, the use of TPO was strictly governed by cosmetic regulations. It was only allowed in professional settings, in nail preparations, and at a maximum concentration of 5%. This restriction was already intended to limit exposure of consumers and nail technicians to TPO, while maintaining the technical effectiveness of semi-permanent nail polishes. Despite these precautions, TPO remained ubiquitous in formulations, to the point of becoming one of the key ingredients in semi-permanent nail polishes. It is precisely this dependency of manufacturers on this molecule that makes its ban all the more significant for the industry.

In May 2025, the European regulation known as Omnibus VII introduced a new series of restrictions on cosmetic products. Among these is the ban on TPO, now classified as Category 1B CMR, meaning toxic for reproduction. Specifically, this classification indicates that TPO is suspected of impairing fertility or causing harmful effects on embryonic and fetal development. Since September 1, 2025, it has therefore been prohibited to place cosmetic products containing TPO on the market, as well as to distribute them or use them in a professional service context. The measure applies immediately, requiring nail technicians to swiftly renew their stocks.

What is the European regulation Omnibus ? This regulation updates annually the list of substances permitted or prohibited in cosmetics. The seventh update, published in May 2025, bans TPO and other active ingredients deemed of concern, thereby continuously adapting the regulatory framework.

This ban is part of a broader public health vigilance around manicure products. Already in 2023, the National Academy of Medicine had warned of potential risks linked to the use of UV and LED lamps, essential for the curing of semi-permanent nail polishes. The ban on TPO therefore does not resolve all the safety issues associated with semi-permanent nail polishes, but it illustrates the growing trend to regularly reevaluate cosmetic substances in light of new scientific data.

The ban on TPO is based on a body of toxicological studies that have demonstrated a risk to reproduction and embryonic development. In rats, an oral toxicity study showed that exposure to TPO at doses of 500 mg/kg/day induced a significant decrease in maternal weight gain, accompanied by fetal abnormalities—particularly bent hind limbs and incomplete ossification. In a complementary one‐generation reproductive study, males exposed from 200 mg/kg/day displayed reduced spermatogenesis and histological alterations in the testes. Exposed females showed a reduced number of viable litters. These data led to the establishment of a no observed adverse effect level (NOAEL) of 60 mg/kg/day and to the classification of TPO as a reproductive toxicant.

Although interesting, this study demonstrates oral toxicity of TPO rather than percutaneous toxicity. Furthermore, the doses administered to the rats were very high, well above the maximum concentration used in cosmetics, and thus potentially non-representative.

Regarding the genotoxicity and carcinogenic potential of diphenyl trimethylbenzoyl phosphine oxide, the results are more nuanced. In vitro test batteries have shown no direct mutagenicity of TPO. Among them, the Ames test, which evaluates a substance’s ability to induce mutations in various strains of Salmonella typhimurium ; chromosomal aberration assays in hamster cells, which detect structural alterations of chromosomes; and mutation assays in mouse fibroblasts, which examine whether a compound can cause gene mutations in mouse cells. These findings suggest that the molecule does not behave like a classical mutagen, capable of directly inducing DNA alterations.

Additional studies conducted on human cell lines have demonstrated a dose-dependent cytotoxicity. For example, in human lung fibroblast cultures, exposure to concentrations of 25 to 50 µM resulted in a marked decrease in cell viability and an increase in DNA fragmentation, indicative of apoptosis.

However, these various studies have been conducted in vitro or examine the effects of oral administration in rats. Therefore, they do not allow the evaluation of the compound’s penetration into the nail, a particularly impermeable medium. In this case, diffusion presents a dual challenge: first crossing the nail’s keratinized structure, then reaching the underlying skin. To date, it remains uncertain that TPO can effectively penetrate this barrier and subsequently enter the bloodstream.

An especially concerning aspect is TPO’s phototoxic potential. Because it is used in semi-permanent nail polishes in combination with UV or LED lamps, several researchers have studied its effects under irradiation. Experiments have shown that under UV light (405 nm, which is close to the salon wavelengths of 315–400 nm), TPO can generate free radicals, causing significant oxidative stress. This effect was observed after short exposure times (1–15 minutes), corresponding to typical salon exposures (< 10 minutes). This stress activates intracellular signaling pathways—particularly the JNK pathway and mitochondrial caspases—leading to cellular apoptosis. While some authors suggest a potential experimental application in anticancer phototherapy, these results primarily reinforce the idea that TPO can become problematic when activated by a light source, a context that corresponds precisely to its use in semi-permanent nail polishes.

However, the phototoxicity of TPO has only been demonstrated in vitro, and not in vivo, which again limits the conclusions.

With the ban on TPO, nail professionals must quickly adapt their practices. Several manufacturers anticipated this regulatory shift and now offer reformulated semi-permanent polishes using alternative photoinitiators deemed safer. Among the legal options are BAPO (bisacylphosphine oxide) and TPO-L, a modified, less toxic version of TPO, which preserve the curing function under UV or LED lamps without exposing users to TPO’s known risks. It is also recommended for consumers to verify the composition of the products used in salons and the absence of TPO on the ingredient list (INCI: Trimethylbenzoyl Diphenylphosphine Oxide).

Finally, it is important to remember that safety extends beyond the choice of photoinitiator. The use of UV/LED lamps must be properly managed, and exposure times should be limited.

• NARAYAN R. J. & al. The photoinitiator lithium phenyl (2,4,6-trimethylbenzoyl) phosphinate with exposure to 405 nm light is cytotoxic to mammalian cells but not mutagenic in bacterial reverse mutation assays. Polymers (2020).

• XIAO P. & al. Photoinduced free radical‑releasing systems and their anticancer properties. Photochemical & Photobiological Sciences (2022).

• KWON J.-S. & al. Cytotoxicity, colour stability and dimensional accuracy of 3D printing resin with three different photoinitiators. Polymers (2022).

• Scientific Committee on Consumer Safety (SCCS). Safety assessment of trimethylbenzoyl diphenylphosphine oxide as used in cosmetics. Cosmetic Ingredient Review (2025).

• Règlement UE n°2025/877 du Parlement européen et du Conseil. Journal officiel de l'Union Européenne (2025).