http://www.scientificamerican.com/podcast/episode.cfm?id=la-needs-to-stop-being-such-a-cow-t-12-04-29

Early inhabitants of what’s now Los Angeles called the region the Valley of Smoke. But it was the car that really made Los Angeles's smog get out of hand.

Or was it?

You might not think of Los Angeles as a cow town. But there are nearly 300,000 cattle in the region, according to the U.S. Department of Agriculture. And now flights to measure local air pollution have revealed that the cows may be a bigger factor in dangerous air pollution than the cars are.

Ground-level ozone is bad enough, but smog gets really nasty when it also includes tiny particles of pollution. And the bacteria eating all that dairy cow waste turn out to be as big a source of this type of pollution as L.A.'s nearly 10 million cars and trucks.

The good news: it's a lot easier to control emissions from a few big dairy farms than 10 million vehicles. So an effort to reduce the pollution from cows and their bacteria could have a bigger impact on Los Angelenos air than further emission cuts from cars. So says a study in the journal Geophysical Research Letters. Let's mooo-ve on that, shall we?