It was only after the two superpowers agreed on a Partial Test Ban Treaty in 1963, which allowed only underground nuclear weapons testing, that the level of radioactivity in food began to decline. Shields Warren (1896-1980), one of the authors of a report on the effects of the atomic bombs dropped on Japan, was criticized for downplaying the effects of residual radiation in Hiroshima and Nagasaki, but later warned of the dangers of fallout. Fallout refers to the spread of radioactivity in the context of a given meteorological situation. A model experiment was conducted in 2008.