A spike in pollution has some advocating more regulations to meet federal standards by 2010.
Brian and Lonnie Bishop had just unpacked their bags for four days of hiking in mountains near here when they glimpsed a front-page headline in the local newspaper: "Officials declare air unsafe, tell residents to 'stay inside.' "
"I couldn't believe it," says Brian, a self-employed computer consultant. "I thought L.A.'s problems with smog were long gone."
A half decade after announcing Los Angeles had its cleanest air in 50 years - and ceded the title of national smog capital to Houston - fresh concern is surfacing about the city's momentum in Windexing its skies.
A spate of hazy pollution this year culminated in the city announcing a Stage 1 smog alert last week. True, the designation lasted only 3 hours. But it was the first such alert - which brings with it an official caution about outdoor activities ranging from running to rollerblading - since 1998.
Officials are playing down the episode because of a rare confluence of extended hot weather and high-pressure systems, which cause air inversions, trapping ozone gases at lower altitudes. But independent experts say the moment has long been coming when population growth, industry expansion, and greater SUV use would overtake the area's hard-won gains of the 1990s.
And because L.A. led the nation in producing the harsher rules and tougher enforcement that led to those gains, these same officials wonder whether dramatic reductions have bottomed out nationwide.
"This may be the turning point where we as a nation may begin to lose the really impressive gains we made in the 1990s and go in the other direction," says Arthur Winer, professor of environmental-health sciences at the University of California. "If these changes are happening here, you can bet they are happening in other states and counties where there is less resolve and less stringent controls. The big issue is how to continue to compensate for industry and population growth."