The American Lung Association's newly released 2017 State of the Air Report ranks San Luis Obispo County in the Top 10. San Luis Obispo County ranked tenth for the most polluted county in the nation under the annual particle pollution category. County Air Officials say For 90 percent of the county, we have really good air quality, but according to data collected during the three year study SLO County received an “F” grade for ozone. The biggest problems come from the Nipomo Mesa and particulates from the Oceano Dunes and the eastern part of the county near the San Juaquin Valley. Some air officials say people can do their part to help by walking, riding or taking the bus. They also recommend replacing old cars with new ones that don't just rely on gasoline.