When choosing different sunscreen products, consumers take many factors into consideration, including SPF values, broad spectrum claims, UVA-PF value or PA+ rating, and the type of filters used: organic versus inorganic.
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.
However, many common organic UV filters, when they absorb UV radiation and enter their energy-rich excited states, are often unable to quickly release the excited state energy to return to their ground state where they are stable and ready to absorb additional UV energy.
These unstable UV filters will photodegrade over time in the sun, losing their absorption capability and generating photoproducts that have not been studied for their safety on human skin.
An example is avobenzone (INCI: Butyl methoxydibenzoylmethane).
While a very common and globally-approved UVA filter, it is notoriously unstable under UV radiation.
Hallstar was one of the first companies to discover the photodegradation of avobenzone and dedicated many years of research to developing a range of solutions for its stabilisation.
Inorganic UV filters, on the other hand, are a group of mineral oxides such as titanium dioxide (TiO2) and zinc oxide (ZnO).
They are often referred to as ‘physical’ filters, as compared with ‘chemical’ filters, which are organic compounds.
There is a common misconception that inorganic filters only reflect, scatter and refract sunlight, and as such are safer to our skin than organic filters.
In fact, both titanium dioxide (TiO2) and zinc oxide (ZnO) are semiconducting materials capable of absorbing UV energy, which contributes most of their UV shielding ability.
After absorbing UV energy, these mineral particles are promoted to their conducting band and become photoactive.
In addition, TiO2 and ZnO are well-known active photocatalysts used extensively in heterogeneous photocatalysis to destroy environmental pollutants that are organic in nature.
When photoactivated by UV light, TiO2 and ZnO are known to generate highly oxidising radicals such as •OH, and other reactive oxygen species (ROS) such as H2O2 and singlet oxygen, 1O2, which are known to be cytotoxic and genotoxic.
Hydroxyl (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.
To restrict the photoactivity of inorganic UV filters, all TiO2 and ZnO used in sunscreens must be surface treated.
Unfortunately, the quality of such treatment is not well controlled, and is often insufficient to inhibit all their photoactivity.
Therefore, inorganic sunscreens commonly used in today’s market convey a false sense of security to the consumers.
