MANCHESTER, N.H. — A report shows that progress on air quality has stalled in New Hampshire.

The American Lung Association’s State of the Air 2014 notes that new Hampshire’s air quality has improved overall since the reports started 15 years ago.

But board member Rick Rumba says there’s still a concern with regard to the increasing impact of small air pollution sources such as residential wood stoves and fireplaces. He says homeowners should be encouraged to burn wood more cleanly and efficiently, using lower-emitting, EPA-certified devices.

The report says Hillsborough County saw its grade for ozone drop from a C to a D – experiencing three additional days when the air was unhealthy. The county has the worst ozone pollution in New Hampshire. All other counties with monitors saw their ozone levels remain the same.

On the other hand, Hillsborough County was the only one to see its annual level of particle pollution get slightly better. Levels in Belknap, Cheshire and Merrimack got slightly worse. Neither Grafton nor Rockingham had sufficient data to be graded in last year’s report, but both counties received passing grades in 2014.

The Boston-Worcester-Providence metro area, which includes Manchester, New Hampshire, ranked tied for 96th on the list of most polluted cities for annual particle pollution. This was the metro area’s best annual level since the American Lung Association began analyzing air quality 15 years ago.