Great question. I think because plastic and rubber are both petrolium byproducts, the 'shreds' of both are bad. I googled, found: > Current tires consist of 19

Great question. I think because plastic and rubber are both petrolium byproducts, the 'shreds' of both are bad. I googled, found: > Current tires consist of 19% natural rubber and 24% synthetic polymers (plastics), as well as metal and other materials. So tires have both rubber and plastics. both are bad, and both get ejected into the environment at a horrific rate. :(