Many items can be considered contaminants. Contamination happens when non-recyclable items are mixed in with recyclable items; the wrong materials are put into the system (plastic grocery bags, hoses, propane tanks, etc.) or the right materials are prepared the wrong way (recyclables in plastic bags, food left in containers, etc.). Placing regular household waste in the recycling bin is a major cause of contamination.
Innocent looking paper smeared with food or grease cannot be processed with clean paper. So used napkins, paper plates, pizza boxes are NOT recyclable. Plastic grocery bags get tangled in recycling machinery and cause equipment damage. Plastic six-pack rings get tangled and also cause equipment damage.
Stick to the list of acceptable recycling materials and don't put anything else into your recycling bin. Too much contamination is the reason manufacturers reject tons of recyclables each year.
When in doubt, leave it out - and put in the trash.