Micronized zinc oxide and titanium dioxide are widely used in sunscreens because of their invisibility advantage. The public is hence worried about the safety of micronized zinc oxide and titanium dioxide. The concerns are twofold: (1) skin penetration; (2) sufficiency for UV protection.
Skin penetration
Intuitively, when the particle sizes of zinc oxide and titanium dioxide are smaller, it is more likely for the particles to penetrate into the skin. The current scientific research, however, has shown the safety of micronized zinc oxide and titanium dioxide in this regard. TGA (Therapeutic Goods Administration of Australian Government Department of Health and Ageing) conducted a review of the scientific literature and concluded that zinc oxide and titanium dioxide nanoparticles remain on the surface of the skin and in the outer layer of the skin, and thus do not cause adverse effects.
UV protection
Zinc oxide and titanium dioxide attenuate UV and visible light by scattering and absorption. Particle sizes do affect this attenuation. At regular particle size, titanium dioxide protects skin as well as or even better than zinc oxide but titanium dioxide is more opaque and noticeable. In the micronized form, zinc oxide performs better than titanium dioxide in the UVA range. This is why some people believe that zinc oxide is a better sunscreen ingredient than titanium dioxide. To make titanium dioxide reasonably invisible for everyday use, you have to make its particle size so small that you almost lose all its protection in the UVA range.
There are some commercialized brands of micronized zinc oxide, with Z-Cote being the best known. According to some researchers in Duke University Medical Center, Z-Cote is photostable, attenuates both UVA and UVB, and works well with octinoxate (octyl methoxycinnamate), a “chemical” sunscreen that attenuates mainly UVB and is generally well tolerated by skin. These researchers are consultants for the company that makes Z-Cote. But their research is rigorous and published on Journal of the American Academy of Dermatology.
Based on current research, micronized zinc oxide is still an effective UVA and UVB filter while micronized titanium dioxide is almost a mere UVB filter especially when its particle size goes very small.
I looked at “COOLA SPF 45 UNSCENTED MINERAL SUNBLOCK perfect for babies and sensitive skin”. It is a good choice for babies. I did not include it in the post 10 Best Sunscreen Products for Sensitive Skin because those recommendations are for women to use every day. COOLA mineral sunblock for babies can be a problem for acne-prone skin. Anyway, for babies, it is good.
I have been researching for months about the best choice for sunscreens and sunblocks and have discovered Coola Suncare products. You should check out their website, a lot of updated info on sunscreens vs. sunblocks and the ingredients used in their products. The one and only cosmetics database has their Coola “BABY” ranked #1, also their lotion bars are amazing.