The Environment and Engine Maintenance!
Engine emissions are the top issue with lawn equipment and are primarily caused by incomplete (in-efficient) burning of fuel and fuel additives. Two primary reasons for an efficient or in-efficient engine include; 1) engine design and 2) engine condition. Regarding condition; the second (likely bigger) pollution cause is from out-of-tune equipment. Clogged air filters, fouled spark plugs, old gas and dull blades all contribute to an in-efficiently running mower. The amount of unburned gas being spewed into the air can be substantial from something as simple as a dirty air filter. To go "green" try the following EPA suggestions:
Avoid Spilling Gasoline! - The overflow evaporates into greenhouse gases
Maintain Your Equipment - Keep your equipment tuned and sharp
Buy Mowers with Low "Air Index" Numbers or use Alternative Fuels like propane
Use Manual Tools - Try a manual reel mower for small lawns and a manual saw for small jobs instead of going straight for the chainsaw
Reduce Mowing Time - Try low-maintenance grasses and larger/faster mowers
Recycle Old Equipment - Do not use old, in-efficient equipment