Wildfires in the Pacific Northwest this summer gave rise to dangerous air quality throughout the region, and generated plumes of smoke that spread across vast swaths of North America.

All times in video are Mountain. | Source: National Oceanic and Atmospheric Administration; Cooperative Institute for Research in the Atmosphere

In July, a high-pressure system settled over the West, causing a widespread heat wave. It trapped smoke from wildfires like a lid, leaving communities with low visibility and ash-filled air. Smoke billowed into the atmosphere and traveled eastward, cutting a path across the Midwest.

The fires are burning hot enough to push smoke and ash miles into the air where winds are stronger, allowing the smoke to travel greater distances than what is produced by less powerful blazes.

Extent of wildfires and smoke particles over eight days Active fires as of Sept. 5 Air quality in parts of the Northwest reached hazardous levels. WASHINGTON Smoke in the atmosphere extended as far east as New York and Pennsylvania. NORTH DAKOTA MONTANA OREGON SOUTH DAKOTA IDAHO New YorK WYOMING Pennsylvania IOWA CALIFORNIA NEBRASKA OHIO INDIANA COLORADO ILLINOIS KANSAS MISSOURI OKLAHOMA Smoke and other particles in atmosphere Less More Air quality in parts of the Northwest reached hazardous levels. Smoke in the atmosphere extended as far east as New York and Pennsylvania. WASHINGTON NORTH DAKOTA MONTANA OREGON SOUTH DAKOTA IDAHO N.Y. WYOMING CALIFORNIA Pa. IOWA NEBRASKA OHIO Ind. Ill. COLORADO KANSAS Mo. Okla. Smoke and other particles in atmosphere Less More Air quality in parts of the Northwest reached hazardous levels. Smoke in the atmosphere extended as far east as New York and Pennsylvania. Wash. Mont. N.D. Ore. IDAHO S.D. N.Y. Wyo. Calif. Pa. IOWA Neb. OHIO Ill. Ind. Colo. Kan. Mo. Okla. Smoke and other particles in atmosphere Less More Smoke in the atmosphere extended as far east as New York and Pennsylvania. WA MT ND OR ID NY SD WY Ca IA Pa NE OH IN IL Co MO Ks OK Smoke and other particles in atmosphere Less More Atmospheric particle data is derived from an eight-day average (Aug. 29 to Sept. 5). The data is a measure of how much light the airborne particles prevent from traveling through the atmosphere. | Sources: NASA Earth Observations; U.S. Forest Service; Herwig G. Schutzler (relief map)

Colin Seftor, an atmospheric scientist at NASA, said he believed that the smoke beyond the Western states was high enough in the atmosphere that there were no immediate health concerns for the rest of the country. But he said that such large clouds of particulates in the air could have broader impact, like a reduction in temperatures on the ground.

In the heart of wildfire country, the effects of the smoke were felt on the ground. For more than 30 hours within a five-day period in early September, the air quality in the Pacific Northwest was worse than the air being observed anywhere else in the world, according to Berkeley Earth, a nonprofit group focused on climate science.

Both Washington and California declared states of emergency as skies turned brown and the air filled with smoke. In Portland, residents reported snowing ash.

Air quality measured on Sept. 5 Hazardous Very Unhealthy Unhealthy Unhealthy for sensitive groups Active fire CANADA WASHINGTON MONTANA OREGON IDAHO WYOMING CALIFORNIA NEVADA Sources: AirNow; Herwig G. Schutzler (relief map)

“Our entire state was completely blanketed with smoke,” said Camille St. Onge, Climate communication manager of the Washington State Department of Ecology. The Cheeka Peak air monitoring station on the Washington coast is typically one of the cleanest stations in the state. But in early September, it recorded an air quality condition of “hazardous,” the worst level as defined by the Environmental Protection Agency as it relates to the level of health concern.

Counties near the Rice Ridge fire in Montana, one of the worst fires this season, recorded more than twice as many hospital visits related to respiratory problems compared with the same period last year, according to the Montana Department of Health. Hazardous air quality levels were measured in Seeley Lake, Mont., for the first time since 2009, for a total of 18 days.

Even states without active wildfires have been affected by the spread of wildfire smoke. On Wednesday, the North Dakota Department of Health issued a press release urging caution about smoke traveling across the state’s border with Montana.

Sisters, Ore. Ogden, Utah. Seeley Lake, Mont. North Bend, Wash. Moscow, Idaho Columbia River Gorge, Wash.

Clockwise, from top left: Fedor Zarkhin/The Oregonian, via Associated Press; Benjamin Zack/Standard-Examiner, via Associated Press; Rion Sanders/The Great Falls Tribune, via Associated Press; Mark Graves/The Oregonian, via Associated Press; Geoff Crimmins/The Moscow-Pullman Daily News, via Associated Press; Bettina Hansen/The Seattle Times, via Associated Press