HELENA — Montana is called Big Sky Country due to its wide open spaces and clear sky, but there are times when wildfires, geography and weather patterns combine to make the air in the western part of the state unsafe for people with asthma or chronic lung disease.

The American Lung Association’s annual “State of the Air” report lists the city of Missoula along with Ravalli and Lincoln counties among the worst areas in the country for the number of days with small particle pollution that makes breathing the air unhealthy for at least some residents.

This year’s report includes data from 2013-2015 — and 2015 was an especially bad fire year.

Ravalli County had 30 days over those three years that the air was rated “unhealthy for sensitive people,” ‘’unhealthy” or “very unhealthy.” Lincoln County had 22 and Missoula had 20, the American Lung Association said.

Sarah Coefield, an air quality specialist with the Missoula City-County Health Department, says the areas that received failing grades all sit in valleys where high pressure ridges in the atmosphere trap the smoke from western wildfires.

“We’re all cast under the same pall from the wildfire smoke,” Coefield said Tuesday.

The Missoula area has made great strides in cleaning up its air since the 1980s, when the area was “full of wood stoves and fireplaces and the street lights would come on during the day because the smoke was so thick,” she said.

Beginning in 1994, the county started phasing out fireplaces and wood stoves in a defined “air stagnation zone,” and allowing only pellet stoves certified by the Environmental Protection Agency to have lower emissions.

“Our primary source of particulate pollution is the wildfires, which unfortunately you can’t regulate away,” Coefield said.

In extreme northwestern Montana, there was a program from 2005 to 2008 in which residents of Lincoln County could trade their older woodstoves for cleaner-burning woodstoves, said environmental health specialist Jacob Mertes.

“It has done a great job of lowering the particulate levels here in the Libby area,” he said, “so much so in fact that if we don’t count wildfire events ... we haven’t exceeded the national ambient air quality standard since 2006.”

The report indicated Libby had one day in 2015 where the air quality was rated “very unhealthy.” The 2015 wildfire season included a large fire about 5 miles from Libby, Mertes said.

The American Lung Association supports continued efforts to reduce greenhouse gas emissions to mitigate climate change and fire danger.

Eight of 11 Montana counties that have particulate monitoring equipment had failing grades for the number of high particle pollution days from 2013-2015, including Flathead, Lewis and Clark and Silver Bow, the report said. Lewis and Clark County also had a failing grade for 2012-2014.