President Donald Trump said Friday he’s holding Canada responsible for wildfire smoke hovering over much of the Midwestern and eastern United States and vowed to call Prime Minister Mark Carney, suggesting that the “cost of this pollution” could be added to tariffs Canada is already paying.

“We are holding Canada responsible for the fact that they are not properly maintaining their Forests, and Brush therein, and the United States is being unnecessarily invaded by filthy, polluted, and unhealthy air, the quality of which is dangerous, and totally unacceptable!” Trump wrote in a Truth Social post.

More than 100 million people in 18 states and the District of Columbia are under air quality alerts. “Very unhealthy” and “hazardous” air quality stretches from northeast Minnesota to southeast Virginia.

“I will call the Prime Minister during the day to find out what they are going to do about it,” Trump continued.

“This is Willful Negligence, and becoming a yearly occurrence, costing the United States Billions of Dollars, which cost of this pollution must of necessity be added to the TARIFFS Canada is currently paying,” he added.

Tensions are already thick between the two countries.

After the Supreme Court ruled that Trump could not use emergency powers to levy tariffs, the president has resorted to using other statutes that require investigations and public comment periods. Although that has slowed the tariff process, the administration has committed to getting America’s effective tariff rate back to where it was earlier this year before the Supreme Court’s consequential ruling.

Canada, however, has long been a thorn in Trump’s side, and Trump has saved some of his highest tariffs for America’s northern neighbor. He has accused the country of unfair lumber trade and stealing US auto jobs. Despite Trump’s particular animosity toward Canada, the country has been largely insulated by the USMCA trade pact that Trump himself negotiated in his first term. That exempted thousands of goods that otherwise would have been subject to some of the highest tariffs the administration charged.

The Trump administration, however, declined to renew USMCA in its current form earlier this month.