Assertion: sunscreen's zinc | /r/CapableOf | serve as literal physical barrier
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Source sentences:
text: In the past, mineral sunscreen formulas had one major drawback: because zinc and titanium serve as a literal physical barrier to protect your skin (opposed to chemical SPFs, which create a reaction where they absorb UV rays, transform them into heat, then release them), they were usually thick, goopy, and left skin with an icky white residue.