After peddling his mountain bike along the circuitous spine of the Santa Monica mountains, self-described "fitness guru" Darrell Jacobs pauses to take in the view of downtown L.A. skyscrapers.
"The air is definitely clearer than when I was a kid here," says Mr. Jacobs, a computer consultant. "But in recent years we've begun to slide back a little. If we go back to the way things were then, I'm outta here."
His comment echoes the concerns of local officials and major US environmental groups who are applauding new federal guidelines for cleaner air that will impose far stricter requirements on the nation's dirtiest skies from Los Angeles to Denver to Houston.
But as 31 governors and their states' business associations grapple with news that their states are out of compliance, the same officials and environmentalists worry aloud that extended deadlines could erase hard-fought gains.
"The feds have pulled the rug out from under all current plans," says Mary Nichols, former secretary of the California Resource Agency and now director of the UCLA Institute of the Environment. "That leaves one of the most problematic regions in the country without any enforceable rules."
In southern California, one of the nation's worst smog offenders, the new rules extend the date when unhealthy air days must end completely by 11 years. But some say the state is already running out of new ideas to fight pollution.
To meet the previous standard by 2010, the Southern California Air Quality Management District introduced a hefty master plan last fall that takes aim at the use of everything from industrial rust sealers to household bug spray. The plan has yet to be formally approved by the EPA.
Local officials fear that the new guidelines will eliminate the need for that approval and so no enforceable rules will be in place for 17 more years.
"The new standard is good for public health and will mean cleaner air and fewer adverse health effects," says Sam Atwood, spokesman for the South Coast Air Quality Management District. "[But] we are concerned that this later deadline will take away the urgency for cleaning our air not only for industry but for the state and federal government."
The EPA announced April 15 that 474 counties nationwide fail to meet a new higher standard for limiting ground-level ozone, a key ingredient of smog. Old rules allowed an average of 120 parts per billion (p.p.b.) averaged over one hour. New guidelines say the average must reduced to 85 p.p.b. averaged over eight hours.
The new standards for smog is an important acknowledgment by the federal government that "the impacts of smog have long been worse than everyone thought," says Gail Ruderman Feuer, senior attorney for the Natural Resources Defense Council. "But we are concerned that this could significantly weaken air quality rules around the country in the short term," says Ms. Feuer.
Because of lawsuits by the American Lung Association and others in the mid-1990s, the Clinton administration adopted the higher standards alongside growing scientific evidence that health effects of smog were more detrimental to public health than first thought.
The health danger comes from ozone, a colorless gas that forms when car and smokestack emissions mix with paint fumes, other airborne chemicals, and sunshine.
The biggest ozone problems have come inland, where seaside breezes blow the emissions from tailpipes, smokestacks, off-road engines, and chemicals.
Once established, the new standards were challenged by industry groups all the way to the US Supreme Court. The high court upheld the newer standards which are just going into effect.
"It's just shocking that it has taken this long to get these designations and sad that it took a lawsuit and court-ordered deadline to do it," says Ms. Nichols.
The good news for advocates of clean air, say analysts, is that air quality will improve over the long run. The bad news for industries and individuals is that meeting the new standards will probably be more costly, and involve more sacrifice.
"Business will be very nervous about dealing with [these new standards]," says Jack Kyser, director of the Los Angeles Economic Development Corp. "Past experience was unpleasant, very expensive."
About 80 percent of pollution in the skies of Los Angeles and other urban areas is from such mobile sources as cars and trucks, to planes, trains, and ships. Local jurisdictions have no control over these sources.
Both local officials and industry representatives say the new guidelines will take longer to achieve because they are more ambitious.
Under the new standard, L.A. and three surrounding counties were out of compliance 120 days last year. The agricultural San Joaquin Valley which produces half the nation's fruits and vegetables was out of compliance 134 days.
Adding to the problem significantly by 2021 will be California's growing population, up by an average of 350,000 to 500,000 per year. "The population keeps growing and people are still not willing to use mass transit," says Mr. Kyser.
Some critics of the new federal guidelines also mention that a total of 506 counties were out of compliance before the final number dropped to 474.
"There were many counties left out of the revised final list for reasons that are not that clear," says Nichols.
Many counties already faced deadlines for reducing ozone pollution under the previous standards dating to 1979, but 100 for the first time face pressure.