Air quality levels are forecast to drop to unhealthy levels across the U.S. Midwest and Northeast on Tuesday as smoke from Canadian wildfires intensifies.

AirNow’s Fire and Smoke Map showed the regions blanketed in yellow due to the drifting smoke. Winds from the northwest are pushing smoke into the region, with National Weather Service forecasters predicting the region’s worst air quality between the afternoon and overnight hours on Tuesday, Wednesday, and Thursday.

“This smoke may reach the ground in some areas, leading to potentially unhealthy air quality for several days,” Weather Channel meteorologists reported Tuesday. “Prolonged exposure to dense smoke can increase the risk of chronic lung issues and increase inflammation that could lead to heart attacks and strokes.”

Despite the wide-ranging smoke impacts, the Canadian wildfire season is on the calmer side compared with recent years. The Government of Canada’s national wildland fire summary listed 3,385 fires that have burned 1.81 million hectares as of Tuesday afternoon, far below the 5-year burned-hectare average of 3.83 million hectares and the 10-year average of 2.26 million hectares.

Officials recently upgraded Canada’s National Preparedness Level to Level 4, indicating significant wildland fire behavior in multiple jurisdictions and high demand for wildland fire resources. Agency Preparedness Levels differ region by region, the highest of which is Quebec at Level 4, followed by the Northwest Territories, Manitoba, Ontario, and Newfoundland and Labrador at Level 3.