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Chlorine dioxide is an excellent bleaching chemical because it is very selective for lignin. However, it is relatively expensive, so chlorine dioxide was originally used in later bleaching stages where there is low lignin content to produce pulp with a high, stable brightness without compromising strength. Over the last few decades, the use of chlorine dioxide has expanded. At first, chlorine dioxide was used to protect pulp strength by replacing 5% to 10% of the chlorine. It was later found that substituting up to 50% chlorine dioxide improved the delignification efficiency. To reduce the environmental impact of using elemental chlorine, chlorine dioxide has essentially replaced chlorine and become the foundation of elemental chlorine-free, or ECF bleaching. Because chlorine dioxide reacts much differently with pulp than chlorine, the production of harmful dioxins is less than 2% of that created by chlorine.