Wildfires scorching several Canadian provinces have driven at least 33,400 people from their homes, with smoke now reaching all the way to Europe.

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.

While the smoke may cloud your vision of the sky or ruin a day at the beach on this week's warmest days of the season so far, it is more than just a nuisance. All across the U.S., the smoke is causing poor air quality, which can cause health effects from mild symptoms in the healthy to dangerous risks for sensitive groups.