There was a great study a while back relating to emissions that I wish I could find.
I'm into dirt bikes, and there is a push away from the two stroke engine due to its high pollution level. I think it was a motorcycle site that did the study, and they found that a 747 taking off one time produces more pollution than all the 2 stroke motorcycles in the country. This was a few years ago when the two stroke was much more prevalant.
The jist of the article was to show how the government could focus on the wrong things, and go for an "easy" target. Two strokes migh be a high pollution engine, but their relative output compared to the huge volume of other polluting things like planes, factories and trucks was negligable.