Yellowing can have many causes such as BHT* off gassing from rebond pad, yarn lubricants, over-application of stain resistant treatments, changes in alkalinity, cleaning solutions, general soiling, and fume fading White vinegar applied to a clean white towel and held on the carpet will indicate if the yellowing can be removed If this works, a 10% solution of citric acid applied by a professional cleaner will usually remove the yellowing
Describes a transformation inherent to all paper fibers caused by aging A few hours in direct sunlight is enough to yellow paper