Air quality in the Mother Lode and across the state is continuing to improve, a recent American Lung Association report showed.
The organization on Wednesday released its 2013 State of the Air report, compiled by averaging federally collected data from 2009 to 2011 to measure air pollution nationwide.
The report showed a decrease in high-pollution days in Tuolumne and Calaveras counties compared with the 2012 report, which consisted of 2008 to 2010 data.
Tuolumne County improved from an "F" to a "D" in the grading system for ground-level ozone. The county had an average of three days with unhealthy ozone levels per year.
High ozone levels are apparent in the form of smog and tend to be most common in summer months.
Calaveras County reported an average of 6.3 unhealthy ozone days, maintaining its failing grade.
However, Calaveras County showed less particle pollution compared with the previous report, climbing from a "B" to an "A."
Particle pollution is made up of a number of components, including acids, organic chemicals, metals, and soil or dust particles.
Will Barrett, policy manager for the American Lung Association in California, said fireplaces are a large contributor to particle pollution, making it more of a concern in winter months.
In the 2012 report, particle pollution was considered unhealthy for an average of 3.2 or fewer days, but more than zero, per year. That number dropped to zero in this year's report.
Tuolumne County does not have monitors for tracking particle pollution.
The American Lung Association attributed much of the improvement in ozone grades to seasonal factors. The 2012 report included data from 2008, when wildfires blanketed California and soiled the air.
The data reflects a long-term improvement for the Central Sierra.
Tuolumne County has seen a 94 percent reduction in smog since the Lung Association compiled its first State of the Air report in 2000, which included data from 1996 to 1998.
Calaveras County has seen an 84 percent reduction in smog.
"It's a really tremendous amount of improvement," Barrett said.
California is prone to poor air quality because of its geography, climate and large population, he said. The state's warm, sunny weathercontributes to pollution, and valleys trap it, he explained.
California cities dominate Top 10 lists for ozone and particle pollution in the U.S.
Although California still has much room for improvement, Barrett said the state's progress in alternative energy has largely helped improve air quality statewide.
"We have some of the most innovative clean air programs that are really helping to cut the levels of pollution tremendously," Barrett said.