Smoke from wildfires in the western United States and southern Canada is spilling into the eastern United States and is thick enough and low enough to foul the air.

       Washington and Baltimore are among several major cities from the Mid-Atlantic to the Northeast under air-quality alerts Wednesday. Philadelphia, New York City and Boston are among the others.

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       Washington and Baltimore have code orange air-quality alerts for Wednesday, which means the air will be unhealthful for sensitive groups, including children, older adults and people with respiratory conditions, including asthma. “The effects of air pollution can be minimized by avoiding strenuous activity or exercise outdoors,” states the National Weather Service alert.

       Extreme fire behavior has erupted in the West. Here’s what that means.

       In its air-quality discussion, the Maryland Department of the Environment said the smoke will increase fine particulates in the air, which can be harmful to breathe in high concentrations, particularly for sensitive groups. The highest concentrations are predicted during the morning and early afternoon Wednesday before a front arrives that helps clear the air.

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       Behind the front, winds from the north will push the smoke away Thursday and Friday. “A much cleaner air mass will move into the region, although some diffuse smoke may still linger for a period of time,” the Maryland Department of Environment wrote.

       Wildfire smoke from the western United States, transported along the jet stream, is not uncommon in the eastern United States, although it is more frequently seen later in the summer and the fall. Last year, it occurred in the Washington region in mid-September. Ordinarily, it’s not this thick and usually remains suspended at high altitudes, where it doesn’t affect air quality at ground level.

       But the combination of the proximity of some of the fires in eastern Canada and the prevailing weather pattern, which features a zone of high pressure aloft that is causing the air to sink, the smoke is reaching close to the surface. You might even be able to smell it.

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       On Tuesday, some of the highest smoke concentrations on the East Coast were focused north of the Mason-Dixon Line between Philadelphia and New York City, where air quality plummeted.

       New York observed its worst air quality in 14 years, with the particle pollution reaching code red levels, signaling unhealthful air for everyone. The smoky haze was so dense it reduced visibility.

       “I really can’t think of any recent particle pollution event of this extent and magnitude in the Eastern US,” tweeted Ryan Stauffer, an air-quality researcher at NASA.

       Although the smoke is cutting visibility and compromising air quality, it is also lowering high temperatures a degree or two by filtering sunlight. In addition, by scattering sunlight, the smoke has also helped the sun take on a vivid red glow at sunrise and sunset.

       Here are several images from our readers on Twitter showing the wildfire-tinted sky at sunset Tuesday …