Just an addition to this would be that negatives should never be store in
the same place as your photos, albums etc. In the event of a catastrophic
experience, by having the negatives in a nother location, you do no lose
the ability to replace.
Polypropelene, melenex and mylar are all photo-safe plastics, and
documentation should accompany the pictures, pref. in an album for best
sharing and viewing.