Although air quality in Ventura County is improving, the county remained one the smoggiest in the nation from 2004 to 2006, according to a report released today by the American Lung Association.

With an average of 15 days per year with unhealthy ozone levels in the three-year period, Ventura County ranked 15th in the nation for smog and earned an F grade for its ozone levels, according to the association's State of the Air report for 2008.

That's an improvement from 2003 to 2005, when the county ranked 12th in ozone pollution, with an average 19.8 days with unhealthy smog levels each year.

The improvement is part of a continued downturn in ozone pollution that began in the late 1980s, said Michael Villegas, director of the Ventura County Air Pollution Control District.

"We used to actually have smog alerts, and one of those hasn't occurred in more than a decade," Villegas said, referring to notices sent out when ozone levels are unhealthy for everyone.

When local pollution peaked in the late 1970s, more than 100 dangerously smoggy days were reported in Ventura County.

The amount of short-term particulate pollution from 2004 to 2006 also dropped from previous years in Ventura County. The county recorded five days with unhealthy levels of soot during that period, one less than from 2003 to 2005, according to the report.

That was good enough to bump the county's particulate-pollution grade up to a C from the D it received in last year's State of the Air report.

The 45 smoggy days recorded in Ventura County from 2004 to 2006 were all "orange," or unhealthy for sensitive people — including people with breathing disorders, children, athletes and people working outside, Villegas said.

People should pay attention to air quality reports and stay indoors when pollution levels are high, said Robert Levin, health officer for the Ventura County Department of Public Health.

All of Ventura County's smog events from 2004 to 2006 were in Simi Valley and Ojai. Prevailing winds blow pollution from freeways, industry and oil fields throughout the county into the two valleys, Villegas said.

The ozone collects in the valleys and turns to smog when it reacts with sunlight and high temperatures during summer months.

Villegas said state emission controls on cars and local emissions restrictions contributed to the drop in ozone pollution in the county.

The Air Pollution Control District imposes emission standards on everything from power plants to paint solvents, which are among the compounds that react with sunlight to form smog, he said.

The agency also has programs to replace older diesel engines with cleaner engines, or retrofit them with emissions control devices.

Though pollution has decreased in some parts of California, American Lung Association officials aren't ready to celebrate.

"We see improvements in some areas of the state, but the levels of ozone and particle pollution in California remain dangerously high," said board Chairwoman Gwendolyn Young of the American Lung Association of California.

California has some of the most polluted places in the country as well as some of the cleanest.

Despite a marked improvement in air quality over the past decade, Los Angeles still has some of the most polluted air in the nation, according to the report. Bakersfield and Fresno are also among the cities with the worst air in the nation.

Salinas has some of the nation's cleanest air. Santa Barbara was among only 19 counties to earn an "A" grade for ozone or soot levels.

Across the nation, about 42 percent of people live in counties with unhealthy levels of ozone and particulate pollution, according to the report, which drew on data for 700 counties compiled by the U.S. Environmental Protection Agency.

High ozone levels can decrease lung function, aggravate respiratory illness and increase the risk of respiratory infection, the report said. Soot can increase the risk of heart attacks, asthma attacks and strokes.

During a telephone news conference on Wednesday, American Lung Association officials cited old diesel trucks and buses as the source of much of California's air pollution and called for them to be modified to reduce emissions or replaced with cleaner vehicles.