Zinc oxide is an inorganic sunscreen filter and one of the most common sunscreen filters that targets both UVA and UVB rays.
When it comes to inorganic filters (often called physical filters or mineral filters) we just have zinc oxide and titanium dioxide, both forms of different minerals.
This compound is insoluble in water and tends to leave a white-cast on the skin. Furthermore, contrary to the popular belief most zinc oxide is actually produced synthetically and does not reflect sun rays at all since it absorbs most UV rays before it reflects them (It just reflects a 4-5% of UV rays).1Cole C, Shyr T, Ou-Yang H. Metal oxide sunscreens protect skin by absorption, not by reflection or scattering. Photodermatol Photoimmunol Photomed. 2016;32(1):5-10. doi:10.1111/phpp.12214
Zinc oxide spectrum range and stability
This filter is broad spectrum, since it covers UVA I (320 to 400 nm), UVA II and UVB (280 to 320 nm) rays.
Zinc oxide is photostable and degrades slowly compared to other sunscreen filters, although its nano form is more affected and could degrade faster by UV than its large form.
When it comes to its SPF and PPD protection, zinc oxide is often combined with titanium dioxide and although they can achieve a good SPF value to protect against UVB rays, their UVA protection can’t compete with modern chemical sunscreen filters or with avobenzone. So, if you’re planning to use a sunscreen to prevent photoaging it would be better to choose a chemical one.
Although zinc oxide is broad-spectrum it needs titanium dioxide or other chemical filters in the same sunscreen formula in order to reach to a decent SPF protection.
Zinc oxide free radical damage
When it comes to free radical damage, both zinc oxide and titanium dioxide (the only two available physical filters) have shown to produce free radical damage when they are exposed to the light.
This phenomenon happens due that both zinc oxide and titanium dioxide are photocatalysts substances, that means, that when they are exposed to the light they absorb it and produce a chemical reaction.
This chemical reaction produces DNA and free radical damage on the skin and induce cell dying. Although part of this damage could be prevented by using coated zinc oxide (the most effective are silica-based coatings) and antioxidants in the sunscreen formula, not all manufacturers use them to prevent it.2Smijs TG, Pavel S. Titanium dioxide and zinc oxide nanoparticles in sunscreens: focus on their safety and effectiveness. Nanotechnol Sci Appl. 2011;4:95-112. Published 2011 Oct 13. doi:10.2147/NSA.S19419
Zinc oxide allergies and safety
Zinc oxide does not produce any type of allergy and its absorption is relatively low compared to all sunscreens filters (It just reaches the stratum corneum in its nano form).3Newman MD, Stotland M, Ellis JI. The safety of nanosized particles in titanium dioxide- and zinc oxide-based sunscreens. J Am Acad Dermatol. 2009;61(4):685-692. doi:10.1016/j.jaad.2009.02.051 Furthermore zinc oxide does not have any effect on the endocrine system.
When it comes to skin irritation, zinc oxide comes as a great alternative to chemical sunscreens for those who get skin irritation or are trying to repair their skin barrier since it is an anti-irritant.
Interestingly, zinc oxide has antibacterial activity and it is currently being researched due to its future potential as a treatment for wounds, ulcers and microbial infections by inducing free radical damage to the bacteria when UV exposure occurs.4Siddiqi KS, Ur Rahman A, Tajuddin, Husen A. Properties of Zinc Oxide Nanoparticles and Their Activity Against Microbes. Nanoscale Res Lett. 2018;13(1):141. Published 2018 May 8. doi:10.1186/s11671-018-2532-3
The safety concerns come with the production of free radical damage on the skin and when it comes in [nano] form.
Nowadays most zinc oxide sunscreens come in nanoparticles form, a smaller form of zinc oxide compared to the original form. This happens since zinc oxide feels heavy and produced a big white-cast on the skin, so manufactures convert it to nano form in order to achieve a cosmetically elegant formula with less white-cast and less heavy.
Those concerns come since it is supposed that nano forms can be inhaled and damage lungs or that they can penetrate the skin in deeper levels, but as it has been mentioned before there’s a lack of research right now and there are no concerns except for a higher production of free radicals.
Zinc oxide environmental effects
Zinc oxide does not posse any risk for environment, except its nano form which can be ingested together with food by certain aquatic species damaging their digestion.5Croteau MN, Dybowska AD, Luoma SN, Valsami-Jones E. A novel approach reveals that zinc oxide nanoparticles are bioavailable and toxic after dietary exposures. Nanotoxicology. 2011;5(1):79-90. doi:10.3109/17435390.2010.501914