The worst Canadian wildfire season in recorded history spread an orange haze from Massachusetts to South Carolina and as far west as Missouri, threatening to disrupt flights and forcing millions to mask up or stay inside.

Philadelphia and Baltimore registered some of the worst air quality readings in the US with the level exceeding 300 on Thursday, a number considered hazardous, according to the Environmental Protection Agency’s AirNow map. Manhattan recorded in the 200 range — but New York’s Mayor Eric Adams warned the unprecedented conditions could again deteriorate and urged people to take precautions.

Inbound flights to New York’s LaGuardia Airport were grounded for a second day due to low visibility, while the Federal Aviation Administration warned that flights could also be halted at JFK, Newark and Teterboro.

The haze is expected to last into Monday or Tuesday before the larger weather pattern pumping smoke south from Canada starts to break up, according to Zack Taylor, a senior branch forecaster with the US Weather Prediction Center. The plume itself covers most of North America and in the long term, the only real solution will be if the fires in Canada are brought under control.

For now, dystopian scenes gripped those in cities unfamiliar with this type of pollution, which is far more prevalent in Asia and increasingly affects parts of North America. Historic wildfires have blanketed large parts of Northern California in smoke in recent years and turned San Francisco’s skies orange in 2020.

Canada is leading the world right now in the number of powerful blazes over the past 24 hours, data from NASA satellites show, with more than 400 forest fires active across the country and many of them raging out of control. Prime Minister Justin Trudeau blamed climate change for exacerbating the problem and said that hundreds of US firefighters have arrived in Canada.

President Joe Biden offered additional support to fight the blazes and New York state was distributing a million N95 masks on Thursday as officials across the region urged residents to stay inside.

--With assistance from Brian K. Sullivan.