Paraquat is the trade name for N,N'-Dimethyl-4,4'-bipyridinium dichloride, a viologen and is today among the most commonly used herbicides. Paraquat is used as a quaternary ammonium herbicide. It is toxic to humans if ingested. Other members of this class include diquat, cyperquat, diethamquat, difenzoquat, and morfamquat. All of these are easily reduced to the radical ion, which generates superoxide radical that reacts with unsaturated membrane lipids. As a herbicide the compound is quick-acting, non-selective, and kills green plant tissue on contact. It is redistributed within the plant but does not harm mature bark. Paraquat protects crops by controlling a wide range of annual and certain perennial weeds (unwanted plants) that reduce crop yield and quality by competing with the crop for water, nutrients, and light. The key characteristics that distinguish the non-selective contact herbicide paraquat from other active ingredients used in plant protection products are: It is non-selective, which means it kills a wide range of annual grasses and broad-leaved weeds and the tops of established perennial weeds. It is very fast-acting. It is rain-fast within minutes of application. It becomes biologically inactive upon contact with soil.