on corn yields has been estimated from 1% to 8%, with 3–4% being the conclusion of one economics review. In another study looking at combined data from 236 university corn field trials from 1986 to 2005, atrazine treatments showed an average of 5.7 bushels more per acre (~400 kg per hectare) than alternative herbicide treatments. Effects on sorghum yields have been estimated to be as high as 20%, owing in part to the absence of alternative weed control products that can be used on sorghum. Chemistry and biochemistry Atrazine was invented in 1958 in the Geigy laboratories as the second of a series of 1,3,5-triazines. Atrazine is prepared from cyanuric chloride, which is treated sequentially with ethylamine and isopropylamine. Like other triazine herbicides, atrazine functions by binding to the plastoquinone-binding protein in photosystem II, which animals lack. Plant death results from starvation and oxidative damage caused by breakdown in the electron transport process. Oxidative damage is accelerated at high light intensity. Atrazine's effects in humans and animals primarily involve the endocrine system. Studies suggest that atrazine is an endocrine disruptor that can cause hormone imbalance. Atrazine has been found to act as an agonist of the G protein-coupled estrogen receptor 1. Atrazine has been shown to bind covalently to (chemically react with) a large number of mammalian proteins. Environment Levels Atrazine contamination of surface water (lakes, rivers, and streams) in the U.S. has been monitored by the EPA and has consistently exceeded levels of concern in two Missouri watersheds and one in Nebraska. Monitoring of atrazine levels in community water systems in 31 high-use states found that levels exceeded levels of concern for infant exposure during at least one year between 1993 and 2001 in 34 of 3670 community water systems using surface water, and in none of 14,500 community water systems using groundwater. Surface water monitoring data from 20 high atrazine use watersheds found peak atrazine levels up to 147 parts per billion, with daily averages in all cases below 10 parts per billion. Biodegradation Atrazine remains in soil for a matter of months (although in some soils can persist to at least four years) and can migrate from soil to groundwater; once in groundwater, it degrades slowly. It has been detected in groundwater at high levels in some regions of the U.S. where it is used on some crops and turf. The U.S. Environmental Protection Agency expresses concern