Thousands of visitors were told to evacuate a remote Minnesota wilderness area accessible only by boat as wildfires send dangerously heavy smoke over the U.S. Midwest and Northeast this week.

More than 100 wildfires are burning in Canada and winds are carrying the smoke southeast. Warnings about unhealthy air conditions Wednesday extended from Minnesota through Toronto and into New York. Unusually hot summer temperatures were expected, too.

The best advice is to stay indoors to avoid both the smoke and the extreme heat, said Tyler Hasenstein, meteorologist with the National Weather Service in Chanhassen, Minnesota.

“Those two things coinciding with each other is not good from a health perspective,” he said.