"The EPA's new proposal applies to engines under 25 hp which drive nearly all walk-behind and riding mowers as well as small generators and other devices. The rule would cut smog-forming emissions from the engines by 35%; the reductions would probably be accomplished by adding catalytic converters. The board has estimated that walk behind mowers will cost 18% more under the new regulations while the price of comercial turf care mowers will go up 3%." Not an exact quote but this is what I read in a couple of papers this week. Suggested date for regulations to go into effect, 2011.