What causes carpet around the edges to turn grey in a stairwell?
The technical term, as used by the carpet cleaning industry, is Filter Dirt. This term refers to soil/dust that has passed through the furnace duct work, and the furnace air filter, and has now attached it self to the carpet fibers. When this dust is circulating through the house, from the cold air return to the heat/air ducts, it picks up an electric charge which makes it difficult to remove from the carpet fibers. When it shows up on the edge of each step, it is thought that the dirt/soil is coming up from the under side of the steps, through the small crack in the wood base holding up the steps. Some people plagued with this problem, have replaced the carpet ,and prior to this, caulked the seams between the steps and the wall. You need to look at getting your air ducts professionally cleaned. They should be cleaned every 5 years, anyway. also, you may need to consider replacing your furnace air filter with a paper pleated style. Do not purchase the 3 for a dollar fiberglass style. The cheap filters have the ability to only trap golf balls and pop cans. They allow most soil and dirt to pass right on through the system. Change your furnace air filter every 4 months religiously. Use a black marker to write on the duct work near the furnace of your change out dates. This gives you a written filter change out record in an appropriate location. The carpet cleaning industry does have a solvent based product to help clean the filtration soil from the carpet. One is called Filter Out. from Pro's Choice. Sometimes the stains are permanent, as the soil while traveling between the air ducts is picking up cooking oils from the kitchen, making for a difficult oil/dust, electrically charged stain to remove. These stains are also found around furniture setting right on the floor, and under doors that remain closed. Hope I've given you some guidance to help deal with future filtration soil problems in your home. (3M makes the best paper pleated air funace filters)
