Scrubber systems are used to make coal a "cleaner," fuel. Scrubbers remove sulfur from a power plant's exhaust. As polluted air passes through a scrubber, chemicals from within the scrubber react with the air pollution causing it to precipitate out. Modern scrubbers can remove up to 98% of the sulfur and 99% of the precipitate matter from within the smokestacks. The sulfur and metals that are removed from the scrubbers can be sold as a marketable product by resource recovery. Sludge from lime scrubbers, can be sold to wallboard manufacturers, and fly ash from chimney flues, can be sold to make lightweight concrete that can substitute for wood.