Like Dollypop, I have thousands of playbills and the ones from the 1940s, especially the war years are also endangered. The ones from the 30s, despite the depression, are usually as bright and clean as new. That's simply a testament to quality. They used better materials back then. Glossy paper seems to live a long life.
I have each playbill in an acidfree sleeve as if it were a copy of Superman #1. It's the only way to guarantee survival.
BTW, my playbill from Fords Theater production of "Our American Cousin" and my pill of "Pygmalian" from His Majesty's Theater in London both have a strange problem. One side of the brochure (the underside) is clean and clear and the top side (which I guess was exposed to the air) is slowly turning to dust.
http://www.broadwayworld.com/board/readmessage.cfm?thread=972787#3631451
http://www.broadwayworld.com/board/readmessage.cfm?thread=963561#3533883
http://www.broadwayworld.com/board/readmessage.cfm?thread=955158#3440952
http://www.broadwayworld.com/board/readmessage.cfm?thread=954269#3427915
http://www.broadwayworld.com/board/readmessage.cfm?thread=955012#3441622
http://www.broadwayworld.com/board/readmessage.cfm?thread=954344#3428699