Question
Most of my cats (past and present) have liked to lick plastic trash bags and grocery bags. Assuming that they are not tearing off bits of plastic and swallowing them, but just licking, is there any harm in this? What are they getting out of this -- is it just a texture thing, or are they leeching some chemical out of the plastic, or what?
Answer
Why do cats lick plastic bags?
Extensive online research shows it is not just plastic bags which appear to be cat tongue attractants, but photographs as well. The activity of bag licking varies among cats; not all cats lick bags and not all bags get licked. There is no definitive answer, but most theories support @JohnCavan's answer of bags being rendered with animal fat or having acquired smells. Unfortunately, the majority of answers found are incestuous, referring to each other.
Here is the culmination of what I've discovered, with each reason listing the seemingly independent sites which support each conclusion:
Manufacturing
Due to the bio-degradable nature of the bag, it was rendered with something that created a lingering odor. This could be anything from rendered animal fats ("tallow"), lanolin, petrolium products, gelatin, or corn starch. As regards photographs, Manhattan Cats notes that gelatin is also used in the photo emulsion process, directly connecting photo licking to bag licking.
The Feline Express article breaks down bag materials as it relates to cats, noting the manufacturing process may have simply created an odor which we cannot detect, but cats prefer. Though Chron Blog proposes corn starch, Feline Express dismisses the ingredient, as corn starch bags were only manufactured for a short time among few vendors. Feline Express continues by dismissing tallow and even fish scales, referencing quotes from manufacturers stating they are not used.
Supporting Sites: Vetriscience, Chron Blog, Feline Express, Amazon AskVille, Manhattan Cats, HDW
Acquired Odor
Depending on what the bag has touched or held, there may be lingering odors from the prior contact, which the cat can detect and is trying to taste, or is simply curious about.
Supporting Sites: Vetriscience, Chron Blog
Texture
The coolness of the plastic, or the texture of the material, feels good to the feline tongue, and may even taste good as well.
Supporting Sites: HDW, Manhattan Cats
Sound
Your cat likes the sound of the plastic bag being licked. It may simulate the noise of a caught animal. This is dismissed by some sites as unlikely since cats tend to dislike noise.
Supporting Sites: Vetriscience, Manhattan Cats, Chron Blog
Diet / Cravings
Plastic bag licking is compensation for an unsatisfied dietary craving.
Supporting Sites: Chron Blog
Mental Health / Pica
Your cat may either have pica, the compulsion to eat things which are not food, or obsessive compulsive disorder. Either seems unlikely since the licking activity is apparently quite common among cats. HDW proposes the cat may be performing an orally compulsive act to replicate "nursing".
Supporting Sites: Vetriscience, HDW, Amazon AskVille
Is there any harm in it?
Per all the sites, you should not allow your cat to persist in licking a plastic bag, as this may tempt them to chew and swallow plastic pieces which may result from the activity. Consuming these pieces could potentially create intestinal blockage. Several of the cited sites refer to instances where plastic bags had to be removed from cats, or plastic pieces were found in excrement from the litter box. Additionally, you might not know the history of the bag; it could have contacted a substance poisonous to cats.
I recommend, when possible, that you do not allow your cat to continue licking plastic bags.
Answered By - JoshDM