Yes, this is a consideration I hadn't thought of.
I do have one of those plastic covered buttons of Metropolitan Philaret in my car. So far it seems all right. But
having the plastic in heat right near one is never a good idea, for as we know the compounds exude some toxins probably over time and sped up by high temperatures outdoors.
The wood of the icon board if not coated with anything, as Fr Mark offers as an option, might be safer. Is there glue though on the back of the print which adheres it to the board ? That glue might disintegrate over time, causing the icon print to separate or crinkle.
Perhaps a safer, non-toxic version of wood glue could be used for those of us who are particular about health !
The metal icon does sound like the longest-wearing style of icon for auto use. Have you seen any around, Maria, which some of the readers who may be also concerned about this can shop for ?