DSLR lenses - do 'sealed' lenses not suck in dust?

Thanks, I've been pondering the Canon 17-40 as it says it is sealed. I am not so worried about getting dust inside a lens, I am more concerned about dust on the sensor. And I do not understand if static electrical attraction is the main culprit or if lens action moving in-out helps pull dust into the body through card door, battery door, etc.