It would depend on which plastic the slab is made of.
Polycarbonate- Lexan blocks most UV.
Acrylic- I think does not really block much unless it's specifically manufactured to do that.
https://www.palram.com/blog/construc...ted%20through.
https://www.acplasticsinc.com/inform...v-transmitting
I'd think that if the slabs were made from a UV blocking plastic the grading companies would advertise that.
But then, I think they ought to do lots of things they don't do.