As of 2001[update], atrazine was the most commonly detected pesticide contaminating drinking water in the U.S.[4]: 44 Studies suggest it is an endocrine disruptor, an agent that can alter the natural hormonal system.[5] However, in 2006 the U.S. Environmental Protection Agency (EPA) had stated that under the Food Quality Protection Act "the risks associated with the pesticide residues pose a reasonable certainty of no harm",[6] and in 2007, the EPA said that atrazine does not adversely affect amphibian sexual development and that no additional testing was warranted.[7] EPA's 2009 review[8] concluded that "the agency's scientific bases for its regulation of atrazine are robust and ensure prevention of exposure levels that could lead to reproductive effects in humans".[9] However, in their 2016 Refined Ecological Risk Assessment for Atrazine, it was stated that "it is difficult to make definitive conclusions about the impact of atrazine at a given concentration but multiple studies have reported effects to various endpoints at environmentally-relevant concentrations."[10] EPA started a registration review in 2013.[11]