Third is where yellowing occurs under furniture or area rugs in geometric shapes. Often, straight lines of non-yellowed carpet are found along a wall over the tack strip or along seams where seaming tape prevents migration of BHT through the carpet backing. While the intensity of the yellow color may vary from faint to very bright, the color is always a pure lemon yellow - this is distinctive of BHTs breakdown products. Most rebonded polyurethane cushion contains some BHT, as well as many prime cushions, and together these types comprise about 90 percent of the market. Despite this fact, BHT yellowing is still very rare.
Statistics indicate that less than one claim in a thousand involved yellowing of any kind.
A very special combination of atmospheric contaminants, moisture, light and other conditions must come together for this phenomenon to occur. Most complaints occur where the heating seasons are long and atmospheric contaminants from fossil fuels are present. Carpet cleaners are often blamed for causing yellowing. In BHT yellowing cases the cleaning process only triggers or accelerates the color change. The cleaning provides the moisture and raises the pH of the fiber.