Wool fibers and fabrics sometimes revert to a natural off-white color that may appear yellow. Yellowness is caused by exposure to air, and is accelerated by sunlight, artificial light, and the heat used in cleaning and finishing. Even wool that has been bleached or finished with fluorescent brighteners is susceptible to yellowing over time. Frequently, restoration is not possible. The color may be slightly improved by bleaching in three percent hydrogen peroxide.