The photos and videos out of the Northeast look like scenes from "Mad Max," as a monstrous cloud of smoke spewed by Quebec's wildfires engulfed communities.
The air is an eerie shade of orange and the visibility is low. Distant buildings that you would otherwise be able to see on a clear day are blotted out by the murky haze.
But why is it orange -- and not white, gray or some other color?
Wildfire smoke turns the air orange for the same reason clear air makes the sky look blue -- it has to do with what kind of tiny particles are in the air, how many there are and what wavelength color they block.
Think back to the days in school when you learned about ROYGBIV, all the colors of the rainbow. Sunlight contains all of those colors. As it passes through the Earth's atmosphere, the sun's light hits all of the molecules and particles in the air.
The colors we ultimately see are whatever wavelengths are left over after they've interacted with those particles. Wildfire smoke blocks the shorter wavelengths -- like yellow, green and blue -- leaving just the red and orange to pass through.
This effect is even more pronounced in the morning and evening, when the sun is low in the sky. The light has more atmosphere to pass through before it gets to our eyes, which amplifies the colors and how thick the smoke looks in the air.
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Major metro areas had air quality indexes ranging from 200 to 300 -- which is considered "very unhealthy," according to government website AirNow.gov.
The enormous cloud of pollution could cause long-term health effects, depending on the individual and their amount of exposure, experts have warned. And officials have urged many residents to stay indoors. Smoke conditions could last through at least Thursday.
