A warning sign stands in a field contaminated with dioxin near Danang airport, during a ceremony marking the start of a project to clean up dioxin left over from the Vietnam War, at a former U.S. military base in Danang, Vietnam Thursday Aug. 9, 2012. The sign reads; "Dioxin contamination zone - livestock, poultry and fishery operations not permitted". The U.S. and Vietnam on Thursday launched a four-year joint effort to clean up dioxin leftover from Agent Orange that was mixed, stored and loaded onto planes at the former U.S. military base, which is now part of Danang’s airport. Dioxin can linger in soils and at the bottom of lakes and rivers for generations, entering the food supply through the fat of fish and other animals. (AP Photo/Maika Elan)