Zinc oxide (ZnO) and titanium dioxide (TiO2) are highly effective UV filters and have been used as ingredients in sunscreens for over 30 years.
However, in their macroparticulate (bulk) form in sunscreens, they are visible on the skin as an opaque layer, which discourages consumers from their use. Sunscreen manufacturers have attempted to address this undesirable visual effect by decreasing the particle size of these metal oxides to the much smaller nanoparticle (NP) form. When used in this NP form, these oxides retain their sunscreen properties but cannot be seen on the skin.
These nanoparticles of ZnO and TiO2 are particularly useful in sunscreens because of their ability to filter both UVA and UVB, providing broader protection than any other sunscreen agent. However, concerns understandably have existed as to whether the small particle size of the NP oxides are able to penetrate the skin to reach viable cells, and if so, whether they have any potential toxicity.
The Australian Therapeutic Goods Administration (TGA) continuously monitors the emerging scientific literature in this area and works cooperatively with international regulatory agencies to ensure that appropriate regulatory action is undertaken if any unacceptable risk of harm/toxicity is identified. A review on these issues was first published by the TGA in 2006, updated in 2009, 2013 and now again in 2017.
Key points from the 2017 updated review include:
- There is conclusive evidence from in-vitro studies that in the presence of UV light, both ZnO and TiO2 NPs can induce reactive oxygen species (ROS) that have the capacity to damage cellular components of the skin.
- The current weight of evidence suggests that TiO2 and ZnO NPs do not reach viable skin cells (even in compromised skin) or the generally circulation, but remain on the skin surface and in the outer layer of the stratum corneum, a surface layer of non-viable keratinised cells. It is therefore highly unlikely that if sunscreens are used as is intended, NPs from sunscreen applied to the skin will not achieve significant concentrations in the systemic circulation.
- Since, the compounds are not absorbed through the skin, it is highly unlikely that NPs can induce the cytotoxic responses or pathological outcomes found in the in vitro or animal studies, respectively.
The data from the reviewed in vitro experiments should be interpreted with caution given that the findings from studies conducted in cell lines are of limited value in assessing the potential toxicity NPs pose to humans from topically applied sunscreens. Similarly, the limitations of the reviewed animal studies, where NPs were administered at relatively high concentrations through exposure routes that are not relevant in the context of sunscreen use and at high frequency, should also be acknowledged.
Given the majority of studies found no evidence of skin penetration of NPs when applied dermally, it is highly unlikely that the high systemic NP concentrations attained in these experimental animals would be achieved in people, even if accidental intake occurred via these non-dermal routes. Therefore, any deductions made regarding the safety of topically applied sunscreens in humans by extrapolating these findings in animals to humans, are of limited value.
On current evidence, neither TiO2 nor ZnO NPs are likely to cause harm when used as ingredients in sunscreens.
The current state of knowledge strongly indicates that the minor risks potentially associated with NPs in sunscreens are vastly outweighed by the benefits that NP-containing sunscreens afford against skin damage and, importantly, skin cancer.