Wait, Walter, what is this "wait 20 mins before it becomes effective"? Did I miss something? Aren't the standard ingredients simply opaque to UV frequencies right out of the bottle/tube?
Directions: Apply liberally 15 minutes before sun exposure. Reapply: after 80 minutes of swimming or sweating; immediately after towel drying; at least every 2 hours
I’ve always heard this as time to absorb to become waterproof, but I’m not sure if it’s actual skin absorption or the liquid used to convey the sun blocking compounds to dry or evaporate. Either way, apply and then wait.
This is a good video to highlight this [0], the sunscreen part is in the second half. Probably many more like it.
This is a fairly succinct overview: https://blog.reneerouleau.com/chemical-vs-physical-sunscreen...
The molecules in sunscreen either block or absorb UV. Nothing gets “absorbed into” the skin. It just sits on top (the skin is an amazing barrier).
The fact the molecule can absorb UV doesn’t change if it’s sitting in a tube or on top of your skin.
Here's another excerpt from a different source:
> Chemical sunscreens use up to a dozen ingredients that, when applied, are absorbed in the top layer of skin. They react with the skin to absorb UV rays and convert them into energy before they can harm the skin.
> Chemical sunscreens aren’t as thick as physical sunscreens, so they are often used in sunscreens specifically made for the face as well as those found in spray bottles. Since chemical sunscreens need to be absorbed into the skin, they must be applied at least 20 to 30 minutes before heading outdoors.
> Physical sunscreens, sometimes called “natural” sunscreens, include two ingredients: zinc oxide and titanium dioxide. Together, these ingredients sit on top of the skin and deflect or reflect the sun’s rays. Think of physical sunscreens like a shield, while chemical sunscreens are absorbed into the skin. Both ingredients work well to protect from UVA and UVB (broad spectrum) rays. Since physical sunscreens are thicker, they can leave a white cast on the skin.
https://www.pennmedicine.org/cancer/about/focus-on-cancer/20...
These "chemical sunscreens" are also used in plastics to prevent UV damage. There is no "reaction with the skin" required - the molecule absorbs UV light all on it's own. And if the molecule "reacted" with the skin (i.e. create a chemical bond), that would likely alter it's UV absorption.
As per another reply, the only think I can think of is there is some "drying" of the sunscreen that prevents it from easily rubbing off. If you don't wait for it to dry, then the sunscreen won't perform as described.