Two approaches have proven themselves in the field of sun protection: the physical and the chemical sun filter. Both have advantages and are suitable in different situations or applications. Therefore, there is no sensible general "either-or" answer, but an optimal solution for the specific skin type and task. That is why it is worthwhile to know the properties in detail.
The physical filter effect is because fine zinc or titanium particles act like microscopic mirrors and reflect the UV radiation, so no potentially harmful effect can hit the sensitive skin cells. These mirrors shield the skin. The chemical filter (there are about 12 different substances available) is absorbed into the upper layer of the skin and converts the energy in the UV radiation into heat, which can then be released by the skin.
The physical filters are not absorbed into the skin. They are usually rinsed off with the water or rubbed into the bath towel as you dry off. Nanoparticles, as the smallest particle sizes, have been suspected of being absorbed into the user's organism anyway and are increasingly no longer used for this reason. This is usually noted on the package in order to inform consumers as best as possible ("free of nanoparticles"). In the case of nanoparticles, there is also the question of uptake in other organisms, since they are not degradable, for example plants and fish in water or subsequently in organisms in the food chain. For human users, they initially have the advantage of being harmless to health.
At the same time, zinc and titanium oxides are not easy to process. Often, the sun creams were not so comfortable to apply, but were somewhat pasty or left a whitish film on the skin. In the meantime, however, there are manufacturers who avoid this disadvantage through clever processing and selection of particle sizes. Valuable textures are pleasantly light and spread well without the undesirable "whitening effect". The ease of use is no longer a matter of principle compared to other products. It depends very much on the specific product.
Chemical filters, as mentioned, are absorbed into the upper layer of the skin. As a rule, they are not degraded by the cell metabolism, they remain in the cell until they are replaced by freshly regenerated cells during healthy skin regeneration within 4 weeks at the latest, when they dry out and continue to form the uppermost skin layer as horny cells for a few more days. The chemical filters are then not deposited in the body. Medical research and government oversight of health product safety regularly check whether limits are exceeded in terms of how much of a substance is absorbed into the bloodstream. In addition, some of the chemical filter substances have been banned regionally because their harmful effects have been observed, for example, on corals or other aquatic organisms.
Chemical filters can be processed into different textures (for example gels, oils, or light creams) that are more pleasant for certain applications or skin types. However, users must bear in mind that the chemical filters can only unfold their effect after a certain absorption time. As a rule, this is 15-30 minutes until the protective effect indicated on the package has fully developed.
The question is therefore always the desired effect, the application situation, or your own skin type. It is important to know your own sensitivity to the sun. The product should be so comfortable and pleasant on the skin that you really use it. No matter which product is used, UV protection is only achieved if it is applied correctly.
Urs Pohlman, MD PhD