A significant number of consumers use products to curl or straighten their hair. These products function by chemically breaking and reforming keratin disulfide bonds. The most popular waving products contain salts of thioglycolic acid to break the bonds and hydrogen peroxide to reform them. Straighteners generally rely on high alkalinity to break the bonds and reform them with acid. Problems with permanent waving are usually due to overtreatment or undertreatment of the hair rather than allergy or irritation. Straighteners can be irritating because of their high alkalinity and can seriously damage hair if used excessively.