Nano vs non-nano zinc oxide: what the safety evidence says

Walk down any Australian pharmacy aisle and you will find mineral sunscreens advertising "non-nano zinc oxide" in large type. The implication is clear enough: nano is the thing you are being protected from. It is one of the most successful pieces of ingredient marketing in skincare, and it rests on a question that health regulators in Australia and Europe have spent close to two decades working through. Their answers are more settled than the packaging suggests, and rather more interesting.

What "nano" actually means, and why it is in sunscreen at all

Nano is a measurement, not an ingredient. Zinc oxide is zinc oxide whichever size it comes in; what changes is how finely the powder has been milled and how it behaves in a formula.

The most-used definition comes from EU cosmetics law, which treats a material as a nanomaterial when it is insoluble, stays intact in the product, and more than half the particles by number have at least one dimension between one and 100 nanometres. Below that threshold of nano-sized particles, a supplier can describe the same raw material as non-nano. Australia has no equivalent labelling rule: the Therapeutic Goods Administration regulates sunscreens as therapeutic goods and sets what must appear on the label, but particle size is not something a sunscreen has to declare here. The United States has no formal nano-versus-non-nano definition for this purpose either.

The reason formulators reach for smaller particles is optical. Zinc oxide has a much higher refractive index than the oils and water it sits in, so it scatters light strongly. Coarse, micron-sized zinc leaves an opaque white film — the classic cricketer's nose. Grind the particles down and they still absorb and scatter ultraviolet, but they become far less efficient at scattering visible light, so the film turns close to transparent while its UV performance holds up. That trade — invisibility without losing protection — is the entire commercial case for nano.

The penetration question, and what the studies actually found

The worry underneath "non-nano" is intuitive: if a particle is small enough, surely it slips through the skin and gets somewhere it should not be.

The TGA has reviewed the published literature on this repeatedly, first in 2006, then in 2013, then again in 2016 covering everything to that point. Its finding across in-vitro work on human and animal skin and in-vivo studies was consistent: zinc oxide and titanium dioxide nanoparticles either do not penetrate the stratum corneum — the outermost layer of dead cells — or penetrate it only minimally. Meaningful systemic absorption, and therefore systemic toxicity, was judged highly unlikely. Cancer Council Australia takes the same line, describing the weight of evidence as showing these particles staying on the surface and in the non-viable outer layer rather than reaching living skin cells.

The reviews are not uncritical. Under UV light, both zinc oxide and titanium dioxide can generate reactive oxygen species, which is why manufacturers coat the particles with materials such as silica, aluminium hydroxide or polymers. The TGA review noted that those coatings are not indestructible — degradation has been reported in pool water containing calcium and hypochlorite ions, for instance. Its conclusion was nonetheless that neither material, used as coated ingredients in sunscreens applied as directed, is likely to cause harm, and that whatever residual risk exists is vastly outweighed by the protection against skin damage and skin cancer.

The restriction regulators do impose

Here is the part that rarely makes it into the marketing on either side, and it deserves to.

The EU's Scientific Committee on Consumer Safety assessed nano zinc oxide and concluded it can be used as a UV filter in sunscreens at up to 25 per cent without posing a risk of adverse effects after application to skin. That approval carries an explicit carve-out: it does not extend to uses that could lead to the lungs being exposed by inhalation. The SCCS position is that nano zinc oxide should not be used in sprayable products, because inhaled zinc oxide nanoparticles have been shown to cause inflammation in the lungs. EU law reflects this — zinc oxide is permitted at up to 25 per cent except in applications that may result in inhalation exposure.

So the credible concern about nano zinc is real, but it is a lung question, not a skin question, and it applies to aerosols and loose powders rather than creams and lotions.

Why "non-nano" sells anyway

Several things keep the claim thriving. Smaller-equals-more-penetrating is a persuasive mental model even where the evidence does not support it. The EU's requirement to print "(nano)" in ingredient lists gave shoppers a visible marker to react to, and marketing filled the vacuum on the other side of the world where no such marker exists. And "non-nano" pairs neatly with reef-safe and clean-beauty positioning, none of which is regulated terminology in Australia.

The claim is also slipperier than it looks. Zinc oxide in a finished emulsion does not exist as tidy individual spheres; particles clump into aggregates, and what a laboratory measures depends heavily on how the sample was dispersed before testing. The same raw material can be measured either side of the 50-per-cent line depending on method, batch and formula. A brand can hold a certificate saying non-nano in complete good faith while the particle population in the tube tells a more complicated story.

The honest bottom line, as the regulators state it: on current evidence both nano and non-nano zinc oxide are considered safe to put on skin in sunscreens used as directed, and no Australian or European authority advises avoiding one in favour of the other. Non-nano is a legitimate formulation choice with a real cosmetic cost — it is whiter on skin. Nano is a legitimate formulation choice with one real restriction — keep it out of your lungs, which in practice means treating sprays and loose powders differently from creams. What none of the evidence supports is skipping sunscreen while you decide.