Introduced in the 1970s, the herbicide atrazine is available in a variety of forms and is sold under an extensive array of trade names, such as Atranex, Atratol, Primatol, and Simazat. Due to its unusually long persistence in soil and the possibility of groundwater contamination, however, the chemical has been classified as a restricted use pesticide, and only those with the proper certification are able to legally purchase and apply the substance. Though quite effective against many broadleaf and grassy weeds, certain higher plant species are able to tolerate exposure to atrazine. Most scientists believe that this tolerance is a result of their ability to quickly breakdown the herbicide into non-toxic metabolites.