An annual report on air quality ranked Bangor as one of the cleanest cities in the Northeast but flunked York County because of the number of “unhealthy air” days during recent summers.

The American Lung Association’s annual “State of the Air” report showed that overall air quality improved in Maine and nationally during the past three years. Yet areas of coastal and southern Maine reported worsening conditions for ground-level ozone or particle pollution, reflecting both tighter federal standards and ongoing concerns about pollution drifting into Maine from other states.

Three of Maine’s more rural counties – Androscoggin, Aroostook and Oxford – received an ozone grade of “A” based on the number of potentially unhealthy air days reported by federal authorities. Kennebec, Penobscot, Sagadahoc and Washington counties were all given “B” grades, with Sagadahoc dropping one grade since last year’s report. Hancock and Knox counties were once again given “C” grades.

Cumberland County dropped from a “C” to a “D” grade because of the nine days when ozone reached potentially unhealthy levels from 2012 to 2014 while York County held steady with an “F” grade because of its 14 days of unhealthy air. The remaining five counties – Franklin, Lincoln, Piscataquis, Somerset and Waldo – were not graded because ozone data was not collected in those areas.

Bangor was the only city in Maine – and one of four in the Northeast – to rank in the American Lung Association’s “Top 25 Cleanest U.S. Cities for Year-Round Particle Pollution,” coming in at No. 16. Burlington, Vermont, was the only New England city to rank higher on the list.

“Bangor is looking great,” said Dr. Marguerite Pennoyer, an allergist and immunologist in Scarborough who serves on the state leadership board of the American Lung Association in Maine. “They are doing well, but we are still seeing some problems in the southern and coastal areas.”

The association compiles its annual report based on ozone and particle pollution levels reported by the U.S. Environmental Protection Agency. But this year’s report differs from previous editions because, when it comes to ozone pollution, the grades are based on stricter air quality standards that have been adopted by the EPA but have yet to be implemented amid continuing efforts in Congress to gut the changes.

Last October, the EPA lowered the ambient air quality standards for ozone from 75 parts per billion to 70 parts per billion based on scientific evidence on the respiratory risks of ground-level ozone. Even the 70 parts per billion standard was weaker than health and environmental groups had sought, however.

A critical component of the upper atmosphere, ozone can cause asthma attacks and breathing problems when concentrations rise too high near the ground. Ground-level ozone is produced when pollution from vehicle or smokestack emissions, such as nitrogen oxide and volatile organic compounds, combines with intense sunlight and oxygen.

While children, the elderly and the infirm are especially vulnerable, officials warn that even healthy adults may experience more shortness of breath, irritation or other effects when exercising or exerting themselves outside.

Most of Maine will still be in compliance for air quality under the new federal standards, but York County or other areas in southern or coastal Maine could fall into “non-attainment.” That would potentially require the state to take steps to reduce in-state pollution sources, although experts say many of the components responsible for Maine’s ozone and smog problems blow up the East Coast from more populated or industrial states to the south.

Pennoyer, who described the effects of ozone as a “sunburn on the lungs,” said her practice sees a spike in calls on hot, summer days when air quality dips.

“It’s very noticeable in terms of the number of calls and the number of patients who are having airway problems,” Pennoyer said.

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