According to the U.S. Environmental Protection Agency (EPA):

Most people do not associate air pollution with mowing the lawn. Yet emissions from lawn mowers, snow blowers, chain saws, leaf vacuums, and similar outdoor power equipment are a significant source of pollution. Today’s small engines emit high levels of carbon monoxide, a colorless, odorless, poisonous gas. They also emit hydrocarbons and nitrogen oxides, pollutants that contribute to the formation of ozone. While ozone occurs naturally in the upper atmosphere and shields the earth from harmful radiation, ozone at ground level is a noxious pollutant. Ground-level ozone impairs lung function, inhibits plant growth, and is a key ingredient of smog.

Electric equipment is cleaner than equipment powered by gasoline engines. Electrically-powered lawn and garden tools produce essentially no pollution from exhaust emissions or through fuel evaporation.

http://www.epa.gov/otaq/consumer/19-yard.pdf

The exchange of 1,000 gasoline-powered lawn mowers for electric mowers has the potential of reducing volatile organic compound (VOC) emissions by 9.8 tons per year, which is equivalent to removing 230 cars from the highways.

http://www.epa.gov/oaqps001/community/details/yardequip_addl_info.html