The level of smog has decreased faster in Ventura County than in several other of the nation's most polluted areas because of strict local and state regulations, air pollution officials said Monday.

The county's air, once rated fourth-worst in the nation for highest concentrations of ozone pollution, and recently ranked eighth-worst, has fallen to 13th place in the pollution rankings. And regulators say new rankings due out this fall should show the county holding the line in contrast to other areas nationwide.

The favorable rankings come as the county passes a quarter-century of rapid development and population growth and its Air Pollution Control District marks its 25th year in existence.

"We've had a very serious problem, but we are cleaning up our air better than other areas in the country," said Richard Baldwin, the county's Air Pollution Control officer. "It's been steady progress over the years in spite of growth."

But despite the progress, Baldwin said the county's air still failed to meet federal health standards for air quality 10 days last year, the best year on record. And the county's air failed to meet more stringent state standards on 65 days last year.

So far this year, the county has failed to meet the federal health standards on only one day. Figures on the state standards were not available.

"We are still a long ways from having healthy air for everyone," Baldwin said.

The rankings come from the U. S. Environmental Protection Agency, which produces an annual report on nationwide pollution from ozone, the primary component of smog. Ozone is formed when chemicals from vehicle and industry exhaust react in sunlight.

The most recent EPA report, which used 1991 figures, showed that heaviest concentrations of ozone levels were higher in 12 other areas of the nation. In 1990, only seven other areas of the nation had higher concentrations of ozone.

Ozone pollution can damage lungs in the elderly and infirm and can inhibit lung development in children.

"Medical studies show now that even healthy people are affected by ozone," Baldwin said.

Ventura County has implemented strict new controls on local pollution sources, such as Southern California Edison, the oil industry and smaller businesses, including dry cleaners, paint vendors and automotive paint shops.

At the state level, California has the strictest automotive exhaust standards in the nation, Bill Sessa, spokesman for the state Air Resources Board, said.