Leigh, There is a pressure differential, but it is not in and of itself lethal. The lethal part of a tornado is flying debris. This seems to be one of the myths of tornadoes like opening the windows on your house to allow the pressure to equalize quickly. When opening the windows on a house would allow high speed winds into the structure and help blow the house apart.

http://en.wikipedia.org/wiki/Tornado

Tornado myths are about 3/4s of the way down the page.