I got a great deal on a shark vision -r helmet a few months back, 3rd time wearing it I had an off and did this to it.
Still wear it mind, scratches are starting to annoy me so will be replacing the visor but it still gives better vision in that condition than the Duchini lid I had before.
Many years ago you could get small marks out with toothpaste but I think they were made out of perspex then and now I think it's polycarbonate, you could try but I think it may leave a matt finish, then you would get an idea of what riding in smog was like?