A few things to add:
Got an interesting phone call today. I work for a woman every other Saturday - mow for an hour and do whatever else (weeding, cleanup, etc) for another hour. She calls to say that we can't mow today, so don't bring the mower. Why is that? Because today is an "ozone alert day" and they said on TV not to mow or fill up your car at the gas station. So I did a couple of hours of cleanup and will mow in a couple of days. I've been hearing stuff like this on the radio recently. I don't think you're not allowed to mow, I think they are just suggesting not to. But she didn't want to........
Saw an interesting business card at the health food store a few weeks ago - Clean Air Lawn Care - this guy is mowing with a manual push mower which he is carrying in a trailer that he pulls on a bicycle! One day I saw him going down the street with this setup. That's a little more radical than I would aspire to, but I have to admire it.
A friend of mine was doing lawn care a few years ago using all electric tools. He had corded and cordless mowers and several people working for him. Advertised eco-friendly, clean, quiet lawn care and had lots of business. Later decided he didn't want to work in the sun, but lasted a few years. I tried an electric weedeater once and took it back - seemed like a toy. Used a customer's electric mower once - also seemed like a toy but can do a good job if used regularly on a well-kept lawn. In my opinion, electirc tools suitable for commercial use are a long ways off - but I think they are working on them. I'm sure they will be way expensive at first and may involve a backpack battery.
People used to mow with scythes!
Probably my Honda commercial mowers (and definitely the Honda 4-cycle trimmer) are lots less poluting than the average homeowner's mower.