Biggest Disaster?

According to Homeland Security Secretary Michael Chertoff, ""Hurricane Katrina will go down as the largest natural disaster in American history." (CNN) I wonder where he got that from. According to CNN, 2.5 million people were displaced by the Dust Bowl series of droughts and dust storms (1930-1941), while only 1.3 million lived in metro New Orleans, and adding the others would probably bring it to no more than 2.0 million. If you have a more accurate number of how many people were "internally displaced" by the hurricane, I'd be interested in hearing it.