Mineral UV filters not only offer broader protection to both UVA & UVB rays they are also faster acting, stable & safe. They remain on the surface of the skin, so begin protecting against the sun’s rays as soon as the sunscreen containing them is applied, while chemical sunscreens need about 20 minutes to be absorbed into the skin before they effectively protect against the sun.
Mineral UV filters are also much more photostable than chemical filters as they structurally deflect the UV radiation without chemically interacting with it. This higher photostability, means they are effective in protecting against the dangerous UV radiation for longer without needing reapplying, provided you have not gotten wet or sweated excessively which could cause the sunscreen to wipe off. This higher photostability also decreases the instances of irritation or allergic reactions.
Traditionally, mineral sunscreens have been associated with heavier & thicker formulations that do not spread as easily on the skin & leave a white cast, especially ZnO. So, while ZnO is considered the safest & best broad-spectrum UV filter it was often not preferred by consumers. Great progress has been made in sunscreen formulations to decrease this white cast effect, predominantly by using smaller particle sizes in the nanometre range.
Nano mineral UV filters offer better SPF & transparency, however, concerns over the health implications of nanoparticles have caused apprehension for products containing mineral UV filters in the nano range. While some studies suggest that in large doses nanoparticles can harm humans, the latest research suggests that nano ZnO in sunscreens cannot penetrate the skin in any significant amount. Another study found that when exposed to ZnO nanoparticles, human immune cells effectively absorbed the nanoparticles & broke them down. Nanoparticles can cause lung damage when inhaled though, so they are not recommended in spray or powder sunscreens.
ZnO is currently used in sunscreens in micron (2 to 10 µm) form as well as nano. Micron ZnO is associated with a better safety profile but is less transparent than nano ZnO, so nano ZnO is often favoured by sunscreen formulators.