From Maryland to the Canadian capital, a monstrous cloud of smoke spewed by Quebec’s wildfires has forced children to stay indoors, grounded flights in New York City and left millions of residents at risk of breathing unhealthy air.

More than 75 million people in the eastern US are under air quality alerts Wednesday due to the smoke, which made iconic skylines disappear behind wafting orange fumes.

The heaviest smoke Wednesday is expected to hit the Northeast through the mid-Atlantic and down to the Carolinas. Smoke conditions in those regions could last through at least Thursday.

As of Wednesday evening, major metro areas such as Philadelphia; Jersey City, New Jersey; and New Haven, Connecticut, had air quality indexes ranging from 200 to 300 – which is considered “very unhealthy,” according to government website AirNow.gov.