The North Coast has some of the cleanest air in the state while Lake County rates among the top 10 counties nationwide, according to the American Lung Association's 2012 "State of the Air" report, released Wednesday.
The ratings were similar to those in the association's last report in 2008.
Sonoma, Marin, Mendocino and Lake counties received A-ratings for overall clean air. But there remains room for improvement, said association spokeswoman Jenny Bard.
"While we're very pleased there aren't general (problems), there are still areas we know can be very unhealthy at the neighborhood level," she said. There are a limited number of air monitoring devices, so some neighborhoods could have high levels that aren't reflected in the data, she said.
"There can be very high levels of pollution just from one person burning" wood in a stove or fireplace, she said.
The study examined ozone and particulate matter. Ozone, or smog, is created when emissions react with sunlight and heat. Particulates come from combustion sources, including vehicles, power plants and burning of wood, Bard said.
Wood burning is the primary cause of particulate pollution in the Bay Area, she said.
Lake County was the only California county rated among the top 10 cleanest in the nation when it comes to annual air particulate measurements, Bard said. "It tied with Maui" for 8th place, she said.
The most particulate-free air in the country was measured in Santa Fe.
While Lake County rated well for particulate measurements on an annual basis, its short term, 24-hour particulate measurements failed to make the grade. That's because the study included the summer of 2008, when wildfires raged in area forests, Bard said.
Still, Lake County's annual air quality was substantially better than that in Mendocino or Sonoma counties. Lake County's annual particulate pollution level was 4.6 micrograms per cubic meter compared with Sonoma County's 8 micrograms per cubic meter and Mendocino County's 8.7 micrograms per cubic meter, according to the study.
It's not surprising that the air quality in the greater North Bay area is superior to more congested areas to the south, said Bay Area Air Quality Management District spokesman Ralph Borrmann.
There's less industry, less population and less traffic, he said. Wind patters also tend to disperse pollution, Borrmann said.
"All those coalesce to make for better air quality," he said.
Sonoma County, with about 485,000 residents, is far more populous than Marin, Mendocino and Lake counties combines.