A common question that conscientious recyclers ask is whether or not pizza boxes are recyclable. They are, in and of themselves, recyclable. They are made of corrugated cardboard and can be easily recycled with other cardboard or mixed paper. The problem is their contents. In reality, most pizza boxes are not recyclable because of the grease and cheese left on the boxes.
Pizza boxes and any other paper product stained with food or grease are not recyclable unless the tainted part is removed. So you can cut or tear out the soiled portions of your pizza boxes and trash them. Usually, you can tear the top of the box off, recycle that and throw away the bottom part containing the grease. If the entire box is grease-free, the whole box can be recycled with a guilt-free conscience.
Sneaking your dirty pizza box into the recycling bin can do more harm than good. In fact, contamination in the recycling business is a big problem and food is one of the worst contaminants in the paper recycling process. Grease and oil are not as big of a problem for plastic, metal and glass, as those materials are recycled using a heat process. But when paper products, like cardboard, are recycled, they are mixed with water and turned into a slurry. Since we all know water and oil don’t mix, the issue is clear.
Grease from pizza boxes causes oil to form at the top of the slurry, and paper fibers cannot separate from oils during the pulping process. Essentially, this contaminant causes the entire batch to be ruined. This also is the reason that other food-related items are non-recyclable (paper plates, napkins, paper towels etc).
Some estimates put the costs of irresponsible contamination in the neighborhood of $700 million per year industry-wide. The City of Phoenix, for example, estimates that contamination costs them around $1 million annually, because of damage to machinery, disposal costs for the non-recyclable material and wasted time, materials and efficiency. So be sure to RECYCLE IT RIGHT!
[thanks to Earth 911 for the information]