Here are five things that, according to earth911.org, will contaminate or thwart paper mills’ attempts to recycle paper and cardboard:

- Oil and water don’t mix. That large pizza box with the oil imprint will become recycled paper with an oil stain.
- Did your napkin or paper towel blot up a water spill or nail polish remover? It could have been the latter, and, for this reason, napkins and paper towels are not recyclable.
- Plastic linings added to paper products; a perfect example is paper cups.
- Wet newspapers.
- Pressure-sensitive adhesives, such as sticky notes, postage stamps, and address labels.