Lawn mowers have been getting attention off and on for being serious smog culprits.
What I wish some people would discuss in these is the possibility of using a manual, non-powered mower, especially for smaller yards. Alternatives to conventional grasses, like Pennsylvania sedge (or another sedge species) or buffalo grass, also need less mowing yet still provide a nice traditional yard. Depending on considerations like local codes and how picky your neighbors are, more radical solutions could also involve turning part of your yard into a naturalized woodland or meadow area. It provides more visual interest from both flowers and also various birds and butterflies that you may attract. Once established, mowing, watering and pesticides are minimized.