Scrubber systems are used to make coal a "cleaner" fuel. Scrubbers remove sulfur from a power plant's exhaust. Scrubbers are of two types, wet scrubbers and dry scrubbers. Wet scrubbers use liquid chemicals to remove pollutants. Dry scrubbers typically use fabric filters. In many cases both are used together. As polluted air passes through a wet scrubber, chemicals in 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 products. 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. The use of coal in the United States about doubled during the last quarter (25 years) of the 20th century. However, sulfur in the air was reduced by 30% during that same period. This shows that scrubbers work.