I’m not sure about this, hopefully a paper conservator will chime in, but I’m thinking the mylar may serve to trap the acidity as the paper degrades. Interleaving with buffered paper and boxing are great ideas, but the interleaves may absorb acidity that much faster within a mylar enclosure. Encapsulation is a useful access measure, but perhaps some of the newspapers can be copied so that the images and info of interest can be accessible on a more stable paper.
