The drone of lawn mowers is a classic sign of summer in the suburbs. But these gas guzzlers contribute heavily to another summer phenomenon: smog. The yearly pollution from one gas mower is equivalent to "43 new cars driving 12,000 miles each," says Sam Atwood, a spokesman for the South Coast Air Quality Management District.

Atwood’s district can’t regulate gas mowers; its jurisdiction covers "non-mobile" sources, such as businesses and industries, which produce only 20 percent of smog-causing chemicals. That’s why the district has created the "Mow Down Air Pollution" program, which allows residents of Riverside, San Bernardino, Orange and L.A. counties to trade in gas mowers for $300 rebates on electric mowers. The rebate money comes from fines collected from businesses that don’t participate in ride-sharing programs. In two Saturdays this spring, over 1,400 mowers were exchanged in Riverside and San Bernardino alone.

The California Air Resource Board also sponsors mower trade-ins throughout California. "Small engines are not well-regulated and end up being much greater polluters than cars," says board spokesman Jerry Martin.

The nation’s most successful trade-in program is run and funded by central Arizona’s power and water utility, the Salt River Project. In a partnership with Home Depot, the SRP has recycled over 15,000 gas mowers since 1996.