Millions of people across North America face another day of hazardous air pollution as nearly 1,000 active wildfires continue to rage in Canada. “Hazardous” air quality alerts are once again in effect today for major cities including Chicago, Detroit, Columbus and Minneapolis. Here in New York, air quality has improved — at least for now — following what officials are calling the second-worst smoke event in the city’s modern history. On Friday, President Trump threatened to impose new tariffs on Canada over the record levels of hazardous air.