In much of the Northeast and Mid-Atlantic, the skies in recent days have been milky, hazy and dense; the sunsets have been vivid. That’s because smoke from wildfires scorching several western states and parts of Canada are pouring into the skies, drifting eastward and triggering unhealthy air conditions thousands of miles away.

There are 78 large fires actively burning in the United States, mostly in Western states. The largest of the U.S. fires, the Bootleg Fire, merged this week with a nearby blaze, and the combined fire has ballooned to more than 394,000 acres.

Dave Lawrence, a meteorologist and emergency response specialist with the National Weather Service, said the smoke is stretching from its origin point in the West, through the Northern Plains, across the Great Lakes and into much of the Northeast and Mid-Atlantic. The Weather Service noted smoky skies in parts of Maryland and Washington; in New York and surrounding states; in parts of North Carolina.

“It’s a very wide swath of smoke coming from those Western fires,” he said. “I wouldn’t say it’s a surprise — these are very large wildfires, and they’re producing a tremendous amount of smoke.”

Scientists said depending on weather patterns, it can take a few or several days from the time the smoke billows up from a fire in the West to when the smoke reaches the eastern part of the country.

Given the number of fires in Canada and in the West, “you’re mixing smoke from multiple fires over the eastern U.S.,” said Joel Thornton, a professor of atmospheric sciences at the University of Washington.

More than 2,200 firefighters continue to battle what’s the largest of these wildfires burning in the United States. As of Wednesday, the Bootleg Fire was 32 percent contained — officials said earlier this week that even as crews continue to fight the flames, it will take a “season-ending weather event,” such as major rain or snow, to totally extinguish the fire.

In an update Tuesday, officials said “extreme fire behavior” had forced firefighters to move to safety zones for 10 straight days. On Wednesday, fire officials said crews were able to take advantage of more favorable weather as they continue working on the fire.

“Fighting this fire is a marathon, not a sprint,” Rob Allen, an incident commander for firefighting efforts, said in a statement. “We’re in this for as long as it takes to safely confine this monster.”

Meanwhile, plumes from the Bootleg Fire, which ignited July 6, and other blazes are traveling across the county.

The result has been a hazy sky, tinged with an orange-red or yellowish hue. The haze is so dense in some places that it’s led to reduced visibility. In some areas, people have been able to smell the smoke.

Thornton, who has researched wildfire smoke transport, said while it’s common for smoke from western fires to travel east, it’s “less common that it's impacting the surface air quality so much so far away.”

But it’s not unprecedented.

“It can happen, it’s more a matter of which way the winds are blowing and where the fires are relative to those winds,” he said.

As the smoke has spilled into the skies, several cities have been under air-quality alerts this week. In New York, air quality reached a code-red level that signals air that can be unhealthy for everyone. In Washington and Baltimore, as well as in much of North Carolina, code orange air-quality alerts were issued for Wednesday, meaning the air was unhealthy for sensitive groups. Officials warned the wildfire smoke would increase the level of fine particulates in the air. Thornton said there is a “well established correlation between exposure to fine particulates” — which measure 2.5 micrometers in diameter or smaller in size and are small enough to penetrate deep into the lungs — and negative health effects.

The hazy skies will eventually dissipate. The Weather Service in New York, for example, said a cold front was expected to clear out the skies Wednesday night.

But the wildfire season is just beginning, with its peak in the western United States still months away.

Lawrence said it depends on wind patterns, and how many more fires erupt, but additional stretches of hazy skies may be looming.

“The long-term drought in the West, coupled with very warm temperatures and not a lot of precipitation so far this summer, it kind of will lead toward increased risk for additional wildfires out here,” he said. “If the wind conditions are right, it would not surprise any of us to have additional periods of hazy skies due to smoke again.”

One thing people can do to protect themselves from the smoky air is “do something that we’re all used to now: wear a mask,” Thornton said, adding that a well-fitting N95 mask is ideal.

“If you’re going to be spending a long time outdoors, with the levels of particulate matter, a mask is not a bad idea,” he said.

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