What is "Phenylbenzimidazole Sulfonic Acid" and what is its purpose?

"Phenylbenzimidazole Sulfonic Acid" functions by absorbing UVB rays within a range of 290 to 320 nm. To clarify, UVB rays are primarily responsible for sunburn, also known as "erythemal" UV rays, meaning they can cause erythema on the skin. Indeed, these rays are composed of 85% UVB and 15% UVA. UVA rays are generally implicated in the acceleration of skin ageing, a phenomenon also known as photoageing. While ensulizole is highly effective at absorbing UVB rays, its ability to filter UVA rays is limited, particularly within wavelengths between 340 and 400 nm. For this reason, it is typically combined with broad-spectrum sunscreens capable of protecting the skin from UVA rays, such as "Diethylamino Hydroxybenzoyl Hexyl Benzoate".

At the molecular level, ensulizole is used in the form of soluble salts because its free acid form has limited solubility in aqueous solution. Once applied to the skin, it forms a uniform film in the stratum corneum, where it absorbs incident light energy and dissipates it as heat, according to a absorption/emission mechanism common to organic UV filters. This transformation prevents photons from reaching the deeper layers of the skin, thereby protecting keratinocytes from oxidation and DNA damage.

When used within regulatory limits, ensulizole is neither irritating nor sensitising, and according to currently available data, it does not present any phototoxic or photoallergenic potential.

Acute toxicity studies conducted with "Phenylbenzimidazole Sulfonic Acid" demonstrate a relatively low systemic toxicity: the median lethal dose (LD50) via intraperitoneal route in rats is between 1,000 and 1,500 mg/kg, while the LD50 via skin contact is above 3,000 mg/kg, confirming very low skin contact toxicity. Subchronic toxicity studies over 13 weeks have also established a no observed adverse effect level (NOAEL) at 1,000 mg/kg/day orally. It's also worth noting that mutagenicity tests carried out in vitro have revealed no mutagenic or photomutagenic potential. Thus, the genotoxic risk has been ruled out, even in case of sun exposure.

Regarding reproductive toxicity, the data indicates no teratogenic effects, neither in the mother nor in the foetus, up to the same dose. Furthermore, ensulizole shows no oestrogenic activity, even at the maximum tested concentration of 200 mg/kg. Therefore, it does not disrupt the hormonal axis, an important point in the evaluation of UV filters, often suspected of interfering with the endocrine system. A study conducted in pregnant rats showed that, when administered orally, "Phenylbenzimidazole Sulfonic Acid" does not reach either the brain or the foetus, indicating that the blood-brain barrier and the placental barrier are not crossed. The elimination of the compound is almost complete in less than 48 hours.

Finally, in humans, a clinical study using a form of carbon-14 labelled ensulizole allowed the estimation of the systemically absorbed dose (SED) at only 0.416 µg/cm², which is less than 0.3% of the applied dose, confirming a extremely low skin absorption.

Is "Phenylbenzimidazole Sulfonic Acid" safe for the environment?

Ensulizole is generally considered to be of low toxicity to aquatic organisms. Indeed, some studies highlight a certain accumulation in the tissues of fish exposed for several weeks, particularly in the liver, plasma, and kidneys. This bioaccumulation, although moderate, is accompanied by enzymatic alterations, notably an induction of cytochromes P450, enzymes involved in hepatic detoxification. However, no signs of tissue damage have been observed, suggesting a moderate but non-pathological biological effect. Further research is therefore still necessary to better characterise the environmental cycle of "Phenylbenzimidazole Sulfonic Acid".