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.

Dr. Ulep recommends wearing an N-95 mask when spending time outdoors if the air is polluted to that level. To stay healthy indoors, Dr. Ulep recommends keeping windows and doors closed and holding back from activities like smoking.