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.

An air quality map shows conditions across the United States on Wednesday as Canadian wildfire smoke is forecast to continue to spread across the country, prompting air quality alerts and advisories in multiple states.