Hi andrew,
You can read more about the pros and cons of different types of mattress protectors in post #89 here.
There are thousands of types of polyurethane products and it’s one of the most common materials in modern society (from flexible foam to rigid molded plastics to some of the membranes used in mattress protectors). There is more about the different types of membranes used in mattress protectors in post #2 here but none of the MSDS (material safety data sheets) for polyurethane I have seen have indicated any type of cancer risk and they are usually completely inert so I think you may be getting some misinformation somewhere. Some of them are even GOTS certified organic or Oeko-Tex certified (which is the same testing protocol for VC’s and harmful substances used to test most of the latex that is available in the market).
Phoenix