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, where a train crew in northern Ontario filmed themselves surrounded by flames before being safely evacuated. 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.

In far northeastern Minnesota, rangers were trying to warn people that the Boundary Waters Canoe Area Wilderness was closed Tuesday because about 17 fires caused by lightning more than a week ago were spreading through the vast wilderness accessible primarily by canoe.

Rangers estimated anywhere from 6,000 to 10,000 people were inside the 1.1-million-acre wilderness, Superior National Forest spokesperson Joy VanDrie said. “It’s an arduous job,” VanDrie said of rangers and campers having to canoe for hours or even carry their boats over land to evacuate. No injuries or deaths have been reported.