Bleach refers to a number of chemicals which remove colour, whiten or disinfect, often by oxidation. The bleaching process has been known for millennia, but the chemicals currently used for bleaching resulted from the work of several 18th century scientists. Chlorine is the basis for the most commonly used bleaches, for example, the solution of sodium hypochlorite, which is so ubiquitous that most simply call it "bleach", and calcium hypochlorite, the active compound in "bleaching powder". Oxidizing bleaching agents that do not contain chlorine most often are based on peroxides, such as hydrogen peroxide, sodium percarbonate and sodium perborate. While most bleaches are oxidizing agents, some are reducing agents such as sodium dithionite and sodium borohydride. Bleaches are used as household chemicals to whiten clothes and remove stains and as disinfectants, primarily in the bathroom and kitchen. Many bleaches have strong bactericidal properties, and are used for disinfecting and sterilizing and thus are used in swimming pool sanitation to control bacteria, viruses and algae and in any institution where sterile conditions are needed. They are also used in many industrial processes, notably in the bleaching of wood pulp. Bleach is also used for removing mildew, killing weeds and increasing the longevity of flowers.