EPA Statistics: 5% of Nations Air Pollution

"Each weekend, about 54 million Americans mow their lawns, using 800 million gallons of gas per year and producing tons of air pollutants. Garden equipment engines, which have had unregulated emmissions until very recently, emit high levels of carbon monoxide, volatile organic compounds and nitrogen oxides, producing up to 5% of the nation's air pollution and a good deal more in metropolitan areas.
A conventional lawn mower pollutes as much in an hour as 40 late model cars (or as much as as much air pollution as driving a car for 100 miles)."

In a single day, Southern California's lawn tools spew out more pollution than all the aircraft in the Los Angeles area. A single mower puts out more pollution than 73 new cars. CNN Onine