Hmmmm funny how boating or off-road don't make the list at all, but they seem to be up everyone's azzes anyway.
Data:
http://spreadsheets.google.com/pub?k...rXx_lDw20Je4Ng
Article:
http://blog.wired.com/wiredscience/2...al-data-o.html
Attempts have been made to study recreational boatings contribution to air polution. The problem is that it is too low to measure. The only thing they have been able to find out is that it is less (most likely much less) than 2%.