Do PSA slabs offer UV protection?

I have some raw autographed pieces that I keep on display and I am concerned about fading. Does anyone know as if PSA slabs offer UV protection and, if so, what level (in terms of percentage)? I am thinking of getting some of the higher end items slabbed simply to better protect and preserve them. Thoughts?
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Comments
What the plastic will NOT do is prevent your items from fading - use common sense when deciding where to display and what kind of light it will be exposed to.