Soil found on a carpet can be classified as spots and stains; surface litter (paper, pet hair, lint, and so forth); gritty unattached particles; and that which is adhering to the fibers.
Surface litter can be picked up with a vacuum. Although unsightly, generally this material does not soil or harm the carpet.
Most gritty, unattached soil is tracked in on the feet. The longer it remains on the carpet, the more damage it causes. Gritty soil scratches and produces pits on fibers, dulling them and making them appear to be more soiled than they are. Grit also produces a cutting action that removes fibers and shortens the life of the carpet. Remove this soil by daily vacuuming of traffic areas and overall vacuuming at least once a week.
Soil that gives the carpet its dirty look is composed of sticky oils and greases containing tiny pieces of soil materials. Thorough professional cleaning can remove most of this type of soil. The longer oily soil remains on the fiber, the more difficult it is to remove.
Some oily soils change chemically and produce a yellowish film on the fiber that is impossible to remove. Other oils actually dissolve into some synthetic fibers, becoming part of the fibers themselves. These cannot be removed without damage to the fiber.
For proper carpet maintenance, remove spots immediately, vacuum traffic areas daily, vacuum thoroughly once a week and have a professional cleaning when traffic areas begin to show soil.