Dr. Joel Michalek listens to a reporter's question during a press briefing on the Ranch Hand Study in the Pentagon on March 29, 2000. The Ranch Hand Study, titled after the Vietnam operation of the same name, is the result of the 1997 physical examination of 2,300 Vietnam veterans exposed to the defoliant Agent Orange during the Vietnam War. Michalek is the U.S. Air Force Health Study senior principle investigator for the study.