Lightning, which is more common in hotter temperatures, is usually the cause of wildfires that burn the most land. In 2023, scientists at Canada’s natural resources department found that lightning sparked fires that burned 93 percent of the total wildfire area, and the remaining seven percent of the area burned from human causes.

(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.

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