When Will It End? Sooner for the Northeast, Later for Elsewhere.

Noxious wildfire smoke that has engulfed New York City and other parts of the Northeast this week after drifting down from Canada was forecast to spread farther south on Thursday, enveloping millions more Americans in unusually dangerous pollution.

As about 150 fires burned in Quebec alone on Thursday, the pollution in the United States was expected to extend well beyond New York — to Boston, Philadelphia, Washington and other cities.

Even before dawn on Thursday, the pollution in parts of Pennsylvania, New York and New Jersey was officially “hazardous,” the highest tier of the Environmental Protection Agency’s six-tier Air Quality Index. The largest concentrations of fine particulate matter were in and around Philadelphia, where the authorities had issued a “code red” air quality alert. Washington’s air — officially “unhealthy” — was only slightly better.

Other pockets of pollution were lingering farther west, over Cleveland, Cincinnati, Detroit, Pittsburgh and regions in between, government data showed. Air quality alerts for Thursday extended west to Indiana and as far south as the Carolinas.

By Friday, the worst pollution is expected to move west, away from the Northeast, as a stagnant lower pressure system that has been sending the smoke southward this week changes direction, the National Weather Service said. But as long as the fires in Canada continue, it said, “the smoke may simply be directed towards other areas of the U.S.”

However, the further into the future you go, the less confident forecasters are in predicting the effects and density of the smoke.

That is partly because the high-resolution computer models get refreshed once an hour and are distributed a little less than a day in advance. Also, the models can’t predict how much smoke the wildfires will produce over the next several days.