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Nano-sized zinc oxide

What is nano-sized zinc oxide?

Generally speaking, a nano-sized particle is a particle that is smaller than 100nm (i.e. 0.1µm). Depending on the definition used, often any particle with at least one dimension smaller than 100nm is considered a nano-sized particle.

Does nano-sized zinc oxide compare to regular zinc oxide?

Nano-sized zinc oxide is thus zinc oxide that consists of particles that are smaller than 100nm. Regular zinc oxide grades typically consist of particles that are (much) larger, sometimes above 1µm. The SEM images below give a comparison between nano-sized (image a) and regular zinc oxide (image b). Both images are taken at the same magnification and the difference in particle size is clearly visible.

a. Nanosized ZnO   a. Nano-sized zinc oxide

b. Regular ZnO b. Regular zinc oxide

What are the benefits of nano-sized zinc oxide?

The small particle size gives nano-sized zinc oxide some specific and unique properties that can be really different from those for regular zinc oxide.

Zinc oxide is traditionally known to be a good UV absorber and has been used  for decades in sunblock creams and coatings. These sunblocks and coatings were always white in color and one of the most obvious unique properties of nano-sized zinc oxide is that such systems can now be made transparent. This is because the nano-sized particles do not interfere with light so that it becomes invisible.

A typical diaper-rash cream that contains regular pharmaceutical grade zinc oxide will  leave a thick and white layer on the skin, whereas a cosmetic sunscreen formulation with nano-sized zinc oxide will completely be transparent on the skin.

Other properties that change due to the small particle size of nano-sized zinc oxide are more efficient catalyst in rubber vulcanization or a better anti-microbial perfomance

What are the current health concerns for nano sized zinc oxide and what is the scientific state of the art on these issues?

Nano-sized zinc oxide has been used for over twenty years in various applications, mainly as UV blocker in sunscreens. When the nanotechnology hype started to take off in the late 1990's, many concerns were raised about the safety of nano-sized materials in general as well as for zinc oxide in particular. Due to the existing application in sunscreens and cosmetics, the main concern was possible penetration of small particles in the skin.

Several years and many scientific studies later, it has been demonstrated that nano-sized zinc oxide does not pose a health risk and that it is very similar in terms of toxicological properties compared to 'regular' zinc oxide. More information on this is given below:

  • Skin penetration: the current weight of evidence is that zinc oxide nanoparticles remain on the surface of the skin and in the outer dead layer (stratum cornatum) of the skin, and does not penetrate through to living  skin tissue. This has been concluded by the Australian TGA, the German Federal Health Institute and others.
  • Skin irritation and sensitisation: nano and regular zinc oxide have been demonstrated to be a non-primary sensitiser 
  • Mutagenicity/genotoxicity: no mutagenic/genotoxic potential of nano-sized zinc oxide was noted in the recent gene mutation assay with Salmonella typhimurium in vitro and there was no indication for a clastogenic potential or any aneugenic activity in the mouse micronucleus assay in vivo, hence no nano-specific genotoxicity effects have been observed
  • Phototoxicity: ultimately, there is no evidence that zinc oxide micro- or nano-structured particles pose a photo-toxic or photo-genotoxic risk to humans. On the contrary, one has to consider that there is robust evidence that this substance protect human skin against UV-induced adverse effects, including DNA damage and skin cancer

Conclusion

The recent comparative study intended to simulate the consequences of systemic exposure to zinc oxide as coated or uncoated nanoparticles, as a pigment or as zinc sulphate have demonstrated that no adverse or persistence effects and hence no nano-specific effects were observed, even after injection into the blood stream. the toxicological profile of this material does not give rise to concern in human use, since the substance is not absorbed through the skin.