How to identify food grade plastics. HPDE (#2 recycling symbol) is considered safest, but others are 'acceptable'. PETE (1), LDPE (4), PP (5).
http://www.wikihow.com/Identify-Food-Grade-Buckets
I just checked the bottom of an IO salt mix container & it is PP #5. No idea what my rubbermaid is 'cause it's half full of DI water, so unable to check the bottom.