A report by the American Lung Association has found that New Hampshire's Rockingham County is the only one in the state that's received a failing grade for high ozone days.
New Hampshire has received passing grades for year-round and short-term particle pollution in the association's "State of the Air" report.
Officials say ozone pollution has decreased nationwide due to cleanup of major sources of emissions, such as coal-fired power plants and vehicles.
In New Hampshire, Belknap and Cheshire counties received a grade of A on high ozone days. Hillsborough County received a D.
While Rockingham County received the lowest grade on ozone, it got an A on short-term particle pollution and it passed for annual particle pollution.