If you’ve sat in a nail salon recently, you may well have encountered TPO or trimethylbenzoyl diphenylphosphine oxide to give it its full chemical name. You won’t have seen the name on the bottle. But if you’ve had your gelled fingers under a blue-violet lamp, TPO could well have been part of the process.

TPO is what chemists call a photoinitiator – basically, a chemical that reacts when it’s hit with UV light. When your nails go under the lamp, TPO breaks apart and helps link tiny liquid molecules together, turning the polish into that solid, shiny, long-lasting gel layer.

It’s smart chemistry, and it’s one reason gel manicures last so much longer than normal nail polish. But recently, the EU banned TPO because research suggests it might increase the risk of cancer and could be harmful to the reproductive system.

Meanwhile, alternatives such as benzophenone and other common photoinitiators come with concerns of their own.

Benzophenone, for instance, is listed as a possible endocrine disruptor, meaning it may interfere with hormones. Another common substitute for TPO, called TPO-L is harmful to aquatic life and may cause skin allergies. None of this is hidden. The European Chemicals Agency maintains a public database where anyone can look up chemicals and find their hazard classifications and environmental data.

The point is not that nail varnish is dangerous. It is that even everyday products involve........

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