The U.S. Environmental Protection Agency warns that wildfire smoke contains dangerous pollutants in the form of fine particles known as PM 2.5 that can penetrate deep into the lungs and enter the bloodstream. Short-term exposure can lead to bronchitis, worsen asthma and create other health issues.

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