When used in cosmetics as a sunscreen active ingredient, ZnO is found in one of two forms, micron or nano. Micron formulations of ZnO contain larger particles in the size range of 2 to 10 µm, while the smaller particle size in the nano formulations range from 10 to 40 nm.
Comparison of the particle sizes of ZnO in the micron & nano forms
There are benefits & negatives to both forms of conventional ZnO. The large size of the particles in the micron form of ZnO make it the safer option of the two, since the particles are unable to permeate through the skin, as opposed to the smaller sized particles in nano ZnO. Nano ZnO is also more harmful to the environment as the small particles may accumulate in plant roots.
Despite micron ZnO being considered the safer option, nano ZnO is the form more commonly used by formulators in sunscreen. This is due to nano ZnO’s superiority as an effective UV filter additive. Nano ZnO provides a higher protection to UV rays per percentage mass & can cover a higher surface area per percentage mass when compared to micron ZnO. It also creates a more visually & aesthetically pleasing sunscreen since it has a high level of transparency.
The efficacy of ZnO as a UV filter & nano ZnO’s superiority in transparency, depend on the particles not agglomerating. This is a problem, as nano ZnO exhibits a very high level of uncontrolled agglomeration & micron ZnO has also been shown to exhibit uncontrolled agglomeration.