Despite Australia having the highest incidence of skin cancer in the world, a significant portion of Australians are shying away from using nanosunscreens – although nano-zinc oxide (ZnO) is a >2-fold better broad-spectrum ultraviolet (UV) filter than bulk ZnO. Such metal oxide sunscreens also have many advantages over organic chemical sunscreens, which can cause irritant reactions, have greater absorption through the skin, can be photoactivated and/or broken down by UV light; and as most organics only block narrow parts of the UVB range, mixtures of organics are used.
The photobleaching of painted surfaces of coated steel by some nanosunscreens containing titanium dioxide (TiO2) or ZnO, are of limited relevance to human exposure. Several studies have shown negligible penetration of these nanoparticles through the outer dead cell layer (stratum corneum) of healthy human skin. Nanosunscreens are formulated to remain on the skin’s surface, which is constantly shedding its outer layer of dead cells, while ZnO NPs also partially dissolve in acidic sweat. Importantly, zinc is an essential metal (cofactor in >200 enzymes) and zinc trafficking in the body is carefully regulated – so absorbed zinc from sunscreen is minuscule compared to the natural optimal nutritional levels of zinc in the body. Furthermore, UV light itself causes ROS generation and DNA mutations within the crucial basal layer of the epidermis, where DNA damage to dividing skin cells are most likely to result in skin tumours. UVA also penetrates into the dermis, where it causes immunosuppression. The often-mentioned in vitro studies, that show ZnO NPs are slightly positive for causing DNA damage using two specific cell exposure systems, have been difficult to relate to the whole body situation, because of an identified experimental artefact known as “psuedo-photoclastogenicity”. Consequently, the potential for direct nanosunscreen effects on this basal layer are relatively minimal.
As part of the Australian Consortium contributing to the OECD’s recent safety testing programme for manufactured nanomaterials, we investigated the worst-case scenario of human skin and immune cells directly encountering nanosunscreen nanoparticles – we found that ZnO and TiO2 NPs are as well tolerated as zinc ions and conventional chemical sunscreens in human cell test systems. Therefore, excessive UV light is a well-known and greater risk for skin damage and cancer compared to a perceived risk from using nanosunscreen that has not been supported by the scientific literature. It is crucial that people do not stop using the most effective broad spectrum sunscreens as part of their sun protection measures.