Use a cleaner suitable for glass and plastics.  You can find this product on Amazon.

Product maintenance 

The sealing surface is PMMA(acrylic glass, Plexiglas) and as such may attract dust over time. The surfaces should be cleaned regularly with a soft, lint free cloth. It is recommended to use an anti-static plastic and glass cleaner suited for PMMA/acrylic glass/Plexiglas, or water with a small amount of washing-up liquid. Ethanol should not be used for cleaning the product. 

The product performance may suffer from dust collection on the Sealing surface. In any such case, cleaning according to this recommendation will restore performance.