Organic versus inorganic sunscreens

Organic UV filters are a group of carbon-containing compounds designed to absorb UV radiation. To maintain effectiveness over time, they should not alter chemically after repeated UV irradiation. Yet when many common organic UV filters absorb UV radiation and enter an excited state, they are often unable to release energy quickly to return to their ground state. This is when they are stable and ready to absorb additional UV energy.

Unstable UV filters photodegrade over time in the sun, losing their absorption capability and generating photoproducts, which haven’t been investigated for their safety on human skin. One example is Avobenzone (INCI: Butyl Methoxydibenzoylmethane). Although it’s common and globally approved UVA filter, Avobenzone is notoriously unstable under UV radiation. Hallstar, represented locally by Millchem, was one of the first companies to discover its photodegradation and dedicated many years of research to developing a range of solutions to stabilise the filter. The result was cutting-edge photostabilisation products SolaStay S1 and Polycrylene, as well as the new China compliant photostabiliser, AvoBrite.

Inorganic UV filters

These are a group of mineral oxides such as titanium dioxide (TiO2) and zinc oxide (ZnO). They are often referred to as physical filters, making them inorganic. There is a common misconception that inorganic filters are safer than organic ones because they only reflect, scatter and refract sunlight. Yet TiO2 and ZnO are both semiconducting materials capable of absorbing UV energy, which contributes most of their UV shielding ability. After absorbing UV energy, these mineral particles become photoactive. TiO2 and ZnO are well-known active photocatalysts used extensively in heterogeneous photocatalysis to destroy environmental pollutants that are organic in nature.1 When photoactivated by UV light, they are known to generate highly oxidising radicals such as hydroxyl (OH) and other reactive oxygen species like H2O2 and singlet oxygen, 1O2. H2O2 and 1O2 are known to be cytotoxic and/or genotoxic. OH radicals photogenerated from photoactive TiO2 specimens extracted from commercial sunscreen lotions induce damage to DNA plasmids in vitro and to whole human skin cells in cultures.2 To restrict the photoactivity of inorganic UV filters, all TiO2 and ZnO used in sunscreens needs to be surface treated. Unfortunately, the quality of such treatment is not well controlled or monitored and is often insufficient to inhibit all their photoactivity. As a result, common inorganic sunscreens used today convey a false sense of security to the consumer.