Over time, carpet fibers become abraded or distorted to the degree that they no longer retain their original texture, shape or ability to reflect light like they once did. This is a physical change, also known as Traffic Lane Graying, where the fibers sustain scratches, thus creating a dull/faded appearance.
Most carpets today are made of synthetic fibers such as: nylon, olefin, polyester, and some acrylic. These synthetic fibers rarely wear out (except on stairs), instead they ugly out. What that means is, the fibers crush, mat down, or flatten out. They also abrade or become scratched. Your carpets are made up of thousands upon thousands of very small thread-like fibers that are twisted together. These fibers are basically translucent pieces of plastic that light passes through, like tubing or colored Plexiglas. Once the fibers are scratched, they reflect light differently, appearing dull and dirty.
Improperly cared for carpet can look the same. What scratches carpet is gritty soil and sand that is left in the carpet and when it is walked on it rubs against the fibers and scratches them. After so much of this scratching, you can clean the soil and sand out, but the damage already done is irreversible and you get what we call in the carpet cleaning industry …Abrasion Dullness. This is where the fibers are clean, but since they are so scratched up they do not reflect the light back to the eyes and now take on a dull appearance, which is commonly mistaken for soiled carpets. This usually happens first in the heavily walked on areas or traffic lanes.