Generally - very.

To understand why, let’s take a look at how paper and cardboard recycling works.

The heat processes required for glass, metal or plastic recycling can burn off small amounts of food residue. So unless there is an excessive amount of food left on recyclables, your glass, metal and plastic materials will come out just fine.

But paper recycling is another story. Because paper doesn’t get heated during its recycling process, grease and oil combine with the pulp, which can ruin the batch.Once your paper and cardboard are sorted into grades and types, they move on to the paper mill. They may spend weeks in storage, where food particles and grease in a batch can turn rancid and attract insects or animals.

After that, they are washed with soapy water, and sometimes chemicals, to remove inks, plastic film, staples and glue.  The batch is then sent to a large receptacle where it’s mixed with water to create a slurry. 

That’s where the grease from your pizza box gunks up the works. Even though it’s been through a soapy bath, if you’ve ever washed something full of grease or oil, you know that it takes a quite bit of effort — and more than your average amount of soap — to remove it completely. 

Once in the slurry, the remaining oil floats to the top. It becomes impossible to separate the oil from the paper fibers at this point. Depending on what type of paper the mill was going to make and how much contamination is occurring, the entire batch may be ruined.

To avoid contaminating clean materials, when in doubt, cut off the clean tops of your pizza boxes and send the bottoms to the trash.

Aspirational Recycling

Many people have the best of intentions when they toss non-recyclables in to their curbside bin. Because they want an item to be recyclable, they “sneak” it in, even when they know that they are not supposed to.

When this happens, their unwanted item often causes problems. It may be caught early in the process, pulled out and thrown in the trash, where it should have gone in the first place. Or it may make it through and contaminate an entire batch of clean materials.

Bottom Line

Grease and oil are two of the worst contaminants in paper recycling. Check with your local recycling program to find out its policy on pizza boxes. Although most don’t allow them, some allow boxes with minimal grease to be recycled and others allow them to be put in with compost. When in doubt, just cut off the greasy parts, throw them in the trash and recycle the rest.