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

(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.

The smoky scene calls to mind the record-shattering Canadian wildfire summer of 2023, with smoke that choked Michigan and much of the U.S. for days at a time. Researchers say wildfire smoke is becoming a more regular part of Michigan summers.