AFTER a 25-year struggle that holds lessons for smog-choked cities around the world, Los Angeles is celebrating its cleanest skies in four decades.
This year the ignominious mantra once synonymous with everyday life here - ''the smog alert'' - is being sounded far less often, and the city's skies, though still taking on a skim-milk sheen in summer, contain far fewer pollutants than they used to.
Officials attribute much of the improvement to some of the world's most stringent smog controls, ones that govern everything from paint vapors to lawn-mower emissions. Yet, even with the progress, the city's skies remain the nation's dirtiest, and debate continues over how best to curb smog levels here.
Air-pollution regulators and environmentalists are using the latest statistics - a 43 percent drop in smog alerts, for instance - to highlight the need for vigilance. They worry that hard-fought gains will be undermined by complacent public attitudes or budget-cutting legislators who relax regulations or cut agency funding.
''These new statistics show that our ideas are working at a time when there is a tendency to throw in the towel on environmental improvement,'' says James Lents, chief officer of the South Coast Air Quality Management District (AQMD).
Of concern to Mr. Lents and others: Congressional bills now on target to lower standards of the Clean Air Act - the federal law that forces states to stay within healthy limits - and proposals that would cut the federal Environmental Protection Agency budget by 30 percent.
''That steep a cut would make [the EPA] inoperable by our view,'' says Linda Waade, director of the Coalition for Clean Air in Santa Monica, Calif. ''It would obliterate staffing and programs to the point of undermining their mandate.''
The gains announced this month - only 13 ''stage 1'' alerts for ozone this year compared with 83 sounded a decade ago, and 25 to 30 percent less smog than in the 1980s - were achieved primarily by forcing long-term changes. Reforms range from alternative fuels and catalytic converters, to vapor recovery hoods and ''pollution bartering'' between companies.
California took the lead in 1969 by being the first state to enforce emission standards on autos. The state has followed suit by maintaining the nation's strictest emissions standards and pioneering major programs for industry in 1989 and 1991.
''Other cities should be paying attention to how so much progress has been made in the worst place for smog in the country,'' says Ward Elliott, president of Group Against Smog Pollution (GASP), a Claremont-based antipollution organization.
The AQMD's 1989 clean-air plan targeted an exhaustive list of highly reactive chemicals routinely used in everything from paints to backyard lighter fluid. A second program, added in 1991, known as ''smog exchange,'' is showing the ability of market incentives to curb pollution.
Instead of mandating a specific technology or process to meet certain established benchmarks of allowable pollution, firms are allowed to choose how to meet limits and given financial incentives to accumulate shares by falling below prescribed limits. They can then sell those shares to other firms who have exceeded their allotments.
''If the smog-exchange program continues along the route we are on now, we are on target to attain federal clean-air standards by 2010,'' says Joe Cassmassi, senior AQMD meteorologist. ''We need to acknowledge our progress on all these fronts and recognize it would be a mistake to depart from our course now.''
Three primary agencies are involved in the war on smog. The AQMD focuses primarily on stationary industries; the California Air Resources Board (CARB) on cars, trucks, and buses; and the Federal Environmental Protection Agency on regulations pertaining to interstate commerce, incoming ships, and military bases.
''It's a constant battle to plan, coordinate, and carry out varying ways of attacking these problems,'' Mr. Cassmassi says. ''We've had to be pioneers, because we've got the worse combination of topography, weather, and freeways in the universe.''
AQMD's Lents says the California success model is translatable to any American city and most foreign cities except some like Mexico City and in India where fuels such as dung are used. The cost of these plans annually to business is estimated at $5 billion, Lents says. But the savings in health costs and days lost at work is $8 billion to $10 billion, he says.
Environmental groups here are applauding the just-released Los Angeles statistics, which they admit show steady and substantial progress over two decades. But they underline how far there still is to go. The four counties that make up the Los Angeles basin still lay claim to the nation's worst air pollution, with no other metropolitan region even close.
Gladys Meade, former health director of the American Lung Association of California, says cars, trucks, buses, and industry generated enough ozone pollution to violate federal standards 91 times last year.
''Much is being made of the improved vistas this city has been able to achieve,'' Ms. Meade says. ''But much of what is harmful are gases that no one can see. When it's bad outside for 1 of every 3 days, you've got a long way to go.''
Others say federal guidelines on gaseous emissions should be stricter to begin with and should include tighter limits on particulate pollution.
''An average health guideline doesn't always protect those who are more sensitive than most,'' Waade says. ''Children and the elderly suffer a much greater percentage of ozone-related problems, even on days the federal guidelines say are OK.''