Wildfire smoke and air quality updates: Northeast flights disrupted

In Toronto, the poor air quality will last into the weekend, officials warned Wednesday.

"Air quality and visibility due to wildfire smoke can fluctuate over short distances and can vary considerably from hour to hour," officials said in a "Special Air Quality Statement." "Wildfire smoke can be harmful to everyone’s health even at low concentrations. Continue to take actions to protect your health and reduce exposure to smoke."

Washington, D.C., public schools have canceled all outdoor activities, including athletic games, for Wednesday due to the poor air quality.

A "Code RED Air Quality Alert" has been issued for the nation's capital.

New York City public schools also canceled outdoor activities.

"We urge everyone to reduce prolonged or heavy exertion outdoors," NYC Public Schools tweeted.

In Port Washington, New York, on Long Island, the superintendent announced Wednesday that all students will stay inside during physical education and recess.

"This decision is aimed at minimizing exposure to the poor air quality and ensuring a safe learning environment for everyone," the superintendent said.

Most of New England and much of the East Coast were under air quality alerts on Wednesday, with smoke from Canadian wildfires expected to reach as far south as South Carolina.

A new dose of very thick smoke was expected to drift into New York City and Philadelphia by Wednesday afternoon, lasting into the evening.

That smoke is then expected to move south into Washington, D.C., on Wednesday night and into Thursday morning.

Conditions around New York City and in the I-95 corridor could improve Thursday evening, but get worse in western New York, Pennsylvania and West Virginia, as the winds shift.

-ABC News’ Max Golembo