There are many suggestions in astrophotography forums to use didymium filters ("Red Enhancers", made by Hoya or Marumi) to decrease the influence of sodium vapour lighting.
However, they are hard to find in Europe. I've read in a single forum post that the glass used in those filters is supposed to contain arsenic and could no longer be made or sold because of that. I could not find further sources for that claim.
Can anyone here confirm this, ideally with pointers to reputable sources? And even if there is arsenic in the glass, would it be dangerous in everyday use?