As hundreds of wildfires burn across Canada, smoke is traveling south and potentially affecting Americans’ health. Here's why it can be so dangerous.

Smoke from Canadian wildfires is engulfing much of the eastern and central United States and and is expected to continue for days.

Wildfire smoke was helping raise ozone to hazardous levels. No alerts have been issued, but the air quality may approach “code orange” levels again late Thursday afternoon in parts of the region, as it appears that the ozone factory will still be operating while the Canadian wildfire smoke persists in the skies over Philly.