With billions of dollars in planned commercial development projects and billions more in federal aid hanging in the balance, it's a race that LA hopes to lose. With only a few weeks left in the nation's smoggiest season of the year, 100-degree heat and high humidity in Houston over the Labor Day weekend pushed that city's air-quality levels down to the federal government's "unhealthy" level. But in Los Angeles, 80-degree weather and mild breezes made breathing in the LA Basin much easier.

So far this year, Houston has had 36 days when its air-quality levels didn't meet national health limits for ozone. LA has had 34.

Though the two cities have recently been poking fun at each other over the dirty-air race, it's really no laughing matter. In addition to endangering public health, cities that don't meet federal clean air standards risk losing billions of dollars in government aid. State and federal authorities can even ban new real estate projects in a city if they feel additional construction would worsen air quality.