Massive plumes of Canadian wildfire smoke are set to pour over the border into the United States, bringing dangerous air quality to more than 100 million people in the Midwest and Northeast. In Canada, more than 3,000 fires have burned nearly 4.5 million acres this summer, with a dozen blazes flaring up in Ontario in recent weeks.

A first round of smoke is already floating over a large area from Chicago to Boston. But a weather pattern change will cause lingering smoke and additional plumes to sink to the surface Wednesday and Thursday bringing unhealthy air quality from the Upper Midwest to New England. Wildfire smoke contains dangerous, tiny pollutants called PM2.5 that can travel deep into the lungs or enter the bloodstream when inhaled.

Much of Michigan, Minnesota and Wisconsin are under air quality alerts due to smoke over the next few days. The smoke is now streaming into the US thanks to a record-breaking heat dome parked over the central part of the country. With several months left in wildfire season, the door will remain open for more Canadian smoke plumes to migrate south.

Climate change was found to be responsible for the majority of the increase of surface wildfire smoke. This smoke has eroded decades of air quality improvements in parts of the US. Planet-warming pollution caused approximately 15,000 more deaths in the US from wildfire particulate matter from 2006 to 2020 than would have otherwise occurred in a cooler world.