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

The air quality in New York City worsened Tuesday evening and is expected to "deteriorate further" Wednesday afternoon and evening, New York City Mayor Eric Adams said in a statement.

"At this point, the New York State Department of Environmental Conservation has issued an Air Quality Health Advisory for all five boroughs. While conditions are anticipated to temporarily improve later tonight through tomorrow morning, they are expected to deteriorate further tomorrow afternoon and evening," Adams said.

“Currently, we are taking precautions out of an abundance of caution to protect New Yorkers’ health until we are able to get a better sense of future air quality reports," he said.

Adams said students should still go to school on Wednesday, but New York City public schools won't have outdoor activities.

"These recommendations may change based on updated air quality conditions that come in, but, in the meantime, we recommend all New Yorkers to take the precautions they see fit to protect their health," he added.

The most effective way to protect yourself during wildfire emergencies is to stay indoors or limit time outdoors when there is smoke in the air, according to the Centers for Disease Control and Prevention. This is especially important for those with heart or lung conditions who are at higher risk for adverse health effects.

If you can, try to avoid exercising outdoors until the air quality improves, especially those with underlying lung disease.

"People with asthma and people who already have lung disease or underlying lung problems, it can exacerbate that, it can irritate that. And if the air quality is bad enough, it can even cause some symptoms of feeling unwell and respiratory symptoms in people who are healthy," said Dr. Stephanie Widmer, a member of ABC News' Medical Unit.

Pregnant people should also try to avoid spending time outdoors, especially those who are further along, Widmer said.

You should wear a mask that covers your nose and mouth, fits tightly to your face and can filter out smoke or ash particles before you breathe them in, according to the CDC. N95 or P100 masks can help protect your lungs from smoke or ash.

Click here for more tips on how to stay safe.

-ABC News' Nadine El-Bawab and Youri Benadjaoud