Ensulizole (Phenylbenzimidazole Sulfonic Acid)

What Is Ensulizole?

Ensulizole is a synthetic, water-soluble chemical sunscreen filter that primarily protects against UVB radiation (280–320 nm), with peak absorption around 306 nm. It is listed as Phenylbenzimidazole Sulfonic Acid on ingredient labels and is unique among sunscreen agents for its water solubility, making it especially suitable for lightweight, non-greasy formulations often preferred for oily or acne-prone skin.

Due to its solubility profile and compatibility with oil-free systems, ensulizole is commonly used in gel and water-based sunscreens, as well as daily wear products like moisturizers and tinted creams. It also acts as a stabilizer in formulations, helping to support less stable filters such as avobenzone.

How It Works

Ensulizole functions as a UV filter by absorbing UVB radiation and converting it into a less damaging form of energy, such as heat. This prevents the UVB rays from reaching the skin and causing immediate effects like sunburn or long-term DNA damage.

Compared to many oil-soluble filters, ensulizole is relatively photostable, meaning it maintains much of its UV-protective ability even after exposure to sunlight. This makes it a valuable supporting filter in multi-filter sunscreen systems.

Key Specifications

  • Structure: C₁₃H₁₀N₂O₃S
  • Chemical Class: Benzimidazole derivative
  • IUPAC Name: 2-Phenyl-1H-benzimidazole-5-sulfonic acid
  • Molecular Weight: 274.3 g/mol
  • Solubility: Water-soluble

Protection Spectrum

  • Mechanism of Action: Absorbs UVB radiation and converts it to heat
  • UVB Protection: High
  • UVA I Protection: None
  • UVA II Protection: Very limited
  • HEV/Blue Light Protection: Not applicable

Side Effects and Safety Profile

  • Skin Reactions: Generally low risk for irritation or allergic response
  • Cellular Effects: Some in vitro studies suggest it may generate free radicals when exposed to UV light, raising concerns about DNA damage in unprotected systems
  • Endocrine Activity: No established hormonal disruption from typical topical use
  • FDA GRASE Classification: Non-GRASE (Category III)

Regulatory Status:

  • United States: Approved up to 4%
  • European Union: Approved up to 8%

Formulation Considerations

Ensulizole is used to create lightweight, cosmetically elegant sunscreen products with a non-oily finish. It is often paired with oil-soluble UVA filters to achieve broad-spectrum coverage. Its ability to stabilize other filters makes it a key supportive ingredient in more complex systems.

Because it is water-soluble, ensulizole is typically included in the aqueous phase of emulsions, making it ideal for water-based sunscreens and products formulated for oily or acne-prone skin. Its inclusion can also improve the photostabilityof other active ingredients.