Well, I remember reading an article some time after 9/11 where they studied the incidence of PTSD in New York following that event.

Even in areas that were some distance from the towers there was a relatively high incidence. They put it down to a background level of stress including sources like violence on TV.

This was in comparison to other parts of the world that were less modern and less busy than experienced more traumatic events but had less PTSD. The conclusion was that if you generally have a more boring/ quiet life you will be better able to absorb trauma if it occurs.