Wildfire smoke and air quality updates: Northeast flights disrupted

The U.S. Environmental Protection Agency has specific guidance for vulnerable groups if the air quality in their area is deemed "unhealthy."

While the agency warns that all people will experience adverse side effects from exposure to the unhealthy air, it said those with heart or lung disease, pregnant people, children and the elderly are most sensitive.

Those groups should consider moving all of their activities indoors until the air quality alert is lifted, the EPA said.

Millions of people in New York City and other locations in the Northeast scrambled to keep themselves free of the smoky air throughout the day and evening Tuesday.

The city's skyline was barely visible for most parts of the day, and the smell of the smoke was strong as commuters hit the evening rush.

Large cities with the lowest air quality include New York City, Albany and Cincinnati, a map by Airnow, a website that publishes air quality data, shows.

Another large and dense plume of smoke will be moving down across parts of the Northeast on Wednesday, according to forecasters.

That batch of very dense smoke will push down across the Northeast throughout the day on Wednesday, giving some relief to New England and the Midwest.

-ABC News' Julia Jacobo, Max Golembo and Daniel Peck