Plastic bags. Why do they get tacky on the outside sometimes?

[karykat]

I buy spices and some dry things in bulk at the coop (like flours, rice, things like that). I've noticed that over time, some of these bags will get kind of tacky on the outside.

What's going on? Are things migrating from the inside to the outside of the bag?

Is it particular kinds of bags that will do this?

I don't know whether this is a hygiene or safety problem at all. But it is kind of yukky and I'm just curious to know what the underlying phenomenon is and what the variables are.

[Nyleve]

It's true and I don't know why either. So I always always transfer all my bulk purchases to recycled glass jars for storage. Not only less gross, but also easier to identify the contents (if you remember to label them - heh heh).

The bag thing might be something about grease in the air being attracted to the plastic. I very much doubt that the contents of the bag are migrating through, but weirder things have been proven.

[Paulustrious]

All plastic bags are permeable to some extent. Some are deliberately so, such as those used for packs of prepared salads or those 'green' bags used to extend the life of vegetables.

Nyleve's point about the plastic attracting grease from the air is very plausible. Alternatively it may be a breakdown of the bag itself.