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THE air around Los Angeles is the cleanest it has been in the last half-century—but the city still has the worst air quality in the US.

LA is known as the world capital of photochemical smog, the build-up of low-level ozone and other noxious gases caused by the action of sunlight on vehicle and other fumes. Health officials estimate that the city’s dirty air accounts for about 1000 premature deaths each year.

But things are getting better. Figures compiled by the South Coast Air Quality Management District show that the number of stage I alerts—days when ozone levels exceed 0.2 parts per million and even healthy people are urged to stay indoors—have plummeted from 121 in 1977 to only seven last year.

“We’ve worked to reduce pollutants in everything from oil refineries to the local dry cleaner,” says Sam Atwood of the South Coast Air Quality Management District.