The smoke is forecast to dissipate over the weekend.
Millions of Americans are on alert for unhealthy air quality as smoke from wildfires in neighboring Canada drifts to the United States.
Wildfires have burned a record of 20 million acres across Canada so far this year, with no end in sight. There are currently 500 active wildfires throughout the country and more than 250 have been deemed out of control, according to the Canadian Interagency Forest Fire Center. The smoke has been making its way to the U.S. for over a month.
Columbus, Ohio, hit an Air Quality Index of 244 on Thursday morning, the highest reading ever for the city.
The AQI has since lowered to 169. Any number over 100 is considered unhealthy for sensitive groups.
-ABC News' Jeremy Edwards
The Canadian wildfire smoke drifting into the United States is expected to largely dissipate by the weekend.
By Friday morning, the smoke is forecast to linger in the eastern Great Lakes from Detroit to Ohio, Pennsylvania, western New York state, down to Virginia and the Carolinas.
New York City is not expected to see much smoke, but skies over the Big Apple could still be a bit hazy due to the nearby plumes.
By Friday evening, the smoke will really begin to diminish in the East with lingering effects from New York to the Carolinas.
As of Thursday morning, 23 U.S. states are under air quality alerts from the Midwest to parts of the South and into the Northeast where wildfire smoke from neighboring Canada is creating hazy skies.
The worst-affected areas are currently from Wisconsin down to Tennessee and up to Virginia and even parts of Pennsylvania.
As the day develops, plumes of smoke are expected to linger over the Great Lakes, the Ohio Valley and into the Appalachian Mountains. Some smoke might even make its way into the Mid-Atlantic and parts of New Jersey.
New York City will be largely spared from the heaviest smoke. An air quality alert was issued there only as a precaution. Most of the day will be sunny with perhaps a touch of haze due to the smoke nearby.
Videos verified by ABC News show smoke from Canada's wildfires casting a haze over several U.S. cities on Wednesday.
One video, taken by a driver and posted on Twitter, shows the wildfire smoke hanging over a highway near Lawrenceburg, Indiana, as the state and much of the Midwest were under air quality alerts.
Lawrenceburg is located some 100 miles southeast of Indianapolis, near the state border with Ohio and Kentucky. The city is about 25 miles west of Cincinnati, Ohio.
-ABC News' Matthew Holroyd
