Air quality has taken a serious hit across the United States as a two-fold blow of wildfires in the West and in Canada have sent smoke pouring across large sections of the country — a problem that’s set to linger for days.

While the smoke is worst along the Western Slope where the fires are burning, levels are also fluctuating on the Front Range. Widespread moderate to heavy smoke was evident in Rio Blanco, Garfield, Routt, Jackson, Grand and Summit counties on Thursday morning.

Researchers found that wildfire smoke can raise ozone to dangerous levels, posing hidden health risks even in remote areas.