When E171 is part of a food product, it passes through the digestive system without causing harm because E171 combines with the other ingredients. 

The rapid method of synthesis described here is readily scalable to the proportions required in cosmetics manufacture.

Traditionally, UV-filters are categorized as either chemical or physical. The big difference is supposed to be that chemical agents absorb UV-light while physical agents reflect it like a bunch of mini umbrellas on top of the skin. While this categorization is easy and logical it turns out it's not true. A recent, 2016 study shows that inorganic sunscreens work mostly by absorption, just like chemical filters, and only a little bit by reflection (they do reflect the light in the visible spectrum, but mostly absorb in the UV spectrum).

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