This is a pretty old article (Oct 2006), and likely superceded by more recent work: http://www.npr.org/templates/story/story.php?storyId=1222562...
which finds that autism "clusters" don't have anything to do with pollution or chemical exposure, but rather how educated parents are. One would guess that parents who work as engineers would probably count as "more educated" and likely as "older", too.
