Phenylbenzimidazole Sulfonic Acid

Phenylbenzimidazole Sulfonic Acid (aka Ensulizole) is a chemical UV filter that's a bit of a unicorn in the sunscreen world: it's water-soluble where most organic filters are oil-based.

Being water-solubility is the headline because it feels lightweight and non-greasy in formulations. This makes it suitable for oily or acne-prone skin in gel/water-based products.

Ensulizole primarily protects against UVB radiation (~290-320 nm) but offers some minimal UVA protection. You'll see it often paired with UVA filters to ensure broad-spectrum coverage.

Interestingly, it can help boost SPF and stabilize finicky filters when combined with other UVB absorbers.

The FDA has approved this ingredient for OTC sunscreens up to 4% and the EU's Scientific Committee on Consumer Products concluded that up to 8% is allowed in cosmetics.

In practice, you'll most likely see it formulated around 1-4%.

Safety-wise, it is neither irritating nor sensitizing, shows no photoallergenic potential, and in vivo tests show no mutagenic potential.

You might see discussion about the "double-edged sword" effect of this ingredient. This is because lab tests done on cells in a dish (not on humans) showed Ensulizole can create small amounts of unstable molecules called free radicals that can damage DNA when exposed to UV light. It sounds scary but this is just test-tube research. There hasn't been proof that this can happen when used in a sunscreen on skin.