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

The pull of the foul air to Northern California was significant enough that the Bay Area Air District issued an air quality advisory. According to AirNow, a federal air-quality measuring app that measures air quality in real time, the air quality at 2 p.m. through much of the region was only moderately healthy.

(Reuters) -Smoke from wildfires burning in three Canadian provinces covered about a third of the U.S. on Wednesday, forecasters said, but had little effect on air quality except in New England and parts of New York state and the Midwest.