TPO vs TMO: The Photoinitiator Difference That Decides EU Compliance

TPO and TMO are different substances with different legal status. TPO (CAS 75980-60-8) has been banned in EU cosmetics since 1 September 2025, after being classified as a reproductive toxicant of category 1B. TMO (CAS 270586-78-2) carries no such classification, is not listed in Annex II or Annex III of the Cosmetics Regulation, and remains permitted. The chemical difference is small — TMO is TPO with one extra methyl group on each of its two phenyl rings — but the regulatory difference is the whole game.

What the EU did on 1 September 2025

Two regulations did it. Delegated Regulation (EU) 2024/197 classified TPO as a CMR 1B substance — presumed toxic to reproduction — with the classification applying from 1 September 2025. Regulation (EU) 2025/877 then moved TPO out of Annex III, where it had been allowed for professional use at up to 5%, and into Annex II, the list of substances prohibited outright. Entry 311 out, entry 1731 in.

The detail that caught people out is what the regulation does not contain: a transition period. There is no sell-through clause, no use-up allowance, no grace period for stock already purchased. From 1 September 2025, a salon applying a TPO gel to a paying client is making a product available on the market in the course of commercial activity, which is prohibited — regardless of when the bottle was bought. The Commission's own Q&A states this explicitly, because member states were interpreting it differently and enforcement was drifting.

The same package banned DMTA — N,N-dimethyl-p-toluidine, CAS 99-97-8, also sold as DMPT — as Annex II entry 1745, classified carcinogenic 1B. That is the amine accelerator in two-component acrylic liquid-and-powder systems. A salon that pulled its TPO gels but kept a DMPT monomer has not finished the job. When you audit a supplier, ask about both.

2012 — TPO classified CMR category 2 (fertility) under Regulation (EU) 618/2012, applicable from 1 December 2013
March 2014 — SCCS opinion SCCS/1528/14: safe at up to 5% in nail modelling products, but a moderate skin sensitizer
2019 — Regulation (EU) 2019/831 adds TPO to Annex III, professional use only, 5% maximum
2023 — ECHA's Risk Assessment Committee re-reviews the evidence and confirms CMR 1B
January 2024 — Delegated Regulation (EU) 2024/197 adopts the 1B classification
1 September 2025 — the classification applies, and the Regulation (EU) 2025/877 prohibition applies on the same date
1 May 2026 — Omnibus VIII (Regulation (EU) 2026/78) adds another batch of substances; none of them is a nail photoinitiator

Why 'safe at 5%' became 'banned'

This is the part suppliers usually skip, and it is the part that tells you whether they understand the system at all. TPO was not banned because new evidence appeared. It was banned because the classification changed.

Article 15(2) of the Cosmetics Regulation says a substance classified CMR 1A or 1B is prohibited in cosmetics unless a derogation is requested and granted, on the basis of a complete safety dossier and a favourable SCCS opinion. The prohibition is automatic: it follows the CLP classification on the same date, without any separate decision by the Commission. The only way out is for someone to apply. Nobody applied for TPO. Member states voted 25 in favour, 1 against, 1 abstention. The Commission had no discretion left to exercise.

The 2014 SCCS opinion did not survive the reclassification. Once a substance is classified 1A or 1B, earlier opinions are no longer valid unless they are updated through a new Article 15(2) request. That is why the industry argument — 'but SCCS said it was safe' — failed completely. It was answering a question the regulation no longer asks.

One forward-looking note. A political agreement reached in June 2026 would give future CMR bans a phase-out: six months to stop placing on the market, twelve months before products can no longer be made available. That text is still moving through the legislative process and does not apply retroactively, so it will not help anyone holding TPO stock today.

What TMO is

TMO is di-p-tolyl(2,4,6-trimethylbenzoyl)phosphine oxide. INCI name: Trimethylbenzoyl Ditolylphosphine Oxide. It also trades as TPO-R. CAS 270586-78-2, EC 884-585-5, formula C24H25O2P, molecular weight 376.4.

Structurally it is TPO plus two methyl groups — one at the para position of each phenyl ring. That is the entire difference. It is still a Norrish Type I photoinitiator: it cleaves at the alpha carbon on absorption and produces two radicals that initiate polymerisation. It absorbs in the long-wave UV and violet, which is why it works with the 365, 395 and 405 nm LEDs that nail lamps actually use.

Regulatory status: not in Annex II, not in Annex III, no harmonised CLP classification as a CMR substance. It is REACH-registered, which is why it has an EC number at all.

The honest part: what TMO's compliance actually rests on

Read this paragraph twice, because most suppliers will not write it. TMO is permitted because it has not been classified as a CMR substance — not because it has been proven safe in the way TPO was assessed. Those are different statements, and the difference matters when your safety assessor signs the CPSR.

There is no SCCS opinion on TMO. TPO had one, SCCS/1528/14. TMO does not. What exists is a REACH registration dossier, supplier toxicology data, and an unremarkable acute-toxicity picture: oral LD50 above 2,000 mg/kg in rats, dermal LD50 above 2,000 mg/kg, not classified for acute toxicity. That is a normal profile — and it is also a thinner one than a substance that has been through a full SCCS review.

What this means in practice: TPO compliance was a hard legal boundary, because the substance was simply forbidden. TMO compliance is a judgement — the substance is not forbidden, the data supports use at nail-gel concentrations, and someone qualified has to read the file and sign. If your responsible person asks for the toxicological package before signing, that is a normal request and a supplier should be able to meet it. A supplier who tells you TMO carries no risk at all is telling you they have not read the file.

How to tell them apart on an INCI list

Check the CAS number, not the name. Both INCI names begin with 'Trimethylbenzoyl', and on a printed label sorted alphabetically they sit within a few lines of each other. Anyone scanning quickly will misread them. The CAS number cannot be made to look similar: 75980-60-8 is TPO, 270586-78-2 is TMO.

TPO — Trimethylbenzoyl Diphenylphosphine Oxide — CAS 75980-60-8 — banned, Annex II entry 1731
TMO — Trimethylbenzoyl Ditolylphosphine Oxide — CAS 270586-78-2 — permitted, no CMR classification
TPO-L — Ethyl Trimethylbenzoyl Phenylphosphinate — CAS 84434-11-7 — permitted; liquid, lower initiation efficiency than TPO
BAPO — Bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide — CAS 162881-26-7 — permitted; stronger long-wave absorption, visibly more yellowing

'TPO FREE' printed on the box is not evidence. In October 2026 the EU Safety Gate published an alert on a UK-made gel polish marketed as 'HEMA & TPO FREE' whose ingredient list contained TPO's chemical name; it was withdrawn from the Cypriot market. Regulators read the ingredient list and the CAS number. They do not read the front of the bottle.

The trap in the names, and how our documents handle it

There is a real risk here that has nothing to do with chemistry. TMO's INCI name shares its first eighteen letters with TPO's. A customs officer, a market-surveillance inspector or a buyer's compliance clerk who reads only the first half of the string can flag a compliant product as non-compliant, and the shipment stops while somebody works it out. The way to close that gap is to state all three identifiers together, every time.

Section 3 of our safety data sheet, issued by JST, reads: di-p-tolyl(2,4,6-trimethylbenzoyl)phosphine oxide, CAS 270586-78-2, EC 884-585-5, 3-8%, photoinitiator. Full chemical name, CAS, EC number, concentration and function. There is nothing left to misread and nothing for an inspector to resolve by inference. The same page is published on our About page, alongside the GMP certificate, the ISO 22716 certificate, the stability report and the heavy-metal test.

What we changed, and what stayed the same

We replaced the photoinitiator rather than reducing it. TPO is out of the formula entirely — not 'low TPO', not 'TPO only in the base coat'. Every UV-cured product in the range that used to carry TPO now carries TMO.

Cure speed unchanged: 30 seconds at 48 W LED for a standard colour layer
Gloss and clarity unchanged: TMO photobleaches the same way, so no yellow shift in the top coat
Adhesion and wear unchanged: same resin system, same balance at the nail interface
Soak-off behaviour unchanged: same removal time in acetone
Also HEMA-free and TMPTA-free across the reformulated range
Documentation that ships with it: MSDS, stability report, heavy-metal test, ISO 22716 for the site, CPNP notification number for the EU

If a buyer specifies TPO-L or BAPO instead, we can formulate to it, and we will say what it costs. TPO-L is a liquid with lower initiation efficiency, so dark shades and thick layers need a longer cure. BAPO absorbs further into the violet and cures thick white systems well, but its yellowing is several times higher, which shows on sheer and pastel shades.

What to ask your supplier

Section 3 of the SDS, not a graphic that says 'TPO free'
The CAS number of every photoinitiator in the formula, not only the one they advertise
The EC number: an EC number means the substance is REACH-registered, which is a floor, not a guarantee
A stability report and a heavy-metal test from a named laboratory, with report numbers you can verify
Confirmation of DMTA/DMPT status if they also supply acrylic systems
A written statement of who holds the EU responsible-person role for your label

One sentence version: ask for page three of the safety data sheet. A supplier who sends it within the hour is a supplier who knows their formula. A supplier who sends a graphic with a green tick is not answering the question.

None of this is unique to the photoinitiator. The same discipline — check the file, not the label — applies to every claim in a nail gel specification, from HEMA-free to the shade card itself. Is Cat Eye Gel Polish Safe? What the Risks Actually Are works through the rest of the risk list, and EU Gel Polish Compliance in 2026: TPO, HEMA and What Brands Must Check covers the wider rule set your importer will ask about.

Bottom line

TPO is gone and it is not coming back. TMO is the replacement most of the European nail industry has settled on, because it is one methyl group away from the molecule everyone already knew how to formulate with. The distinction that matters is not the marketing line — it is the CAS number on page three of the safety data sheet. Your compliance obligation sits with you, as the brand or the importer, not with your supplier's label. Get the file, check the CAS, and have your safety assessor sign against the actual data.