At least 10 states from the Midwest to the Northeast on Friday will again face smoky conditions created by raging wildfires in Canada, capping a week in which millions of people struggled to cope with poor air quality.

Smoke that has obscured skylines and made it difficult for some to breathe was expected to linger into the long Fourth of July weekend, according to the National Weather Service. However, the air quality was expected to slowly improve as thunderstorms help disperse the smoke.

An air quality health advisory was in effect for all of New York State on Friday.

As of 7 a.m. Eastern time, New York City had an Air Quality Index of 156, making it unhealthy. A reading of 301 or higher is considered hazardous. Similar air quality levels were reported Friday morning in Cleveland, Detroit, Milwaukee and Washington, D.C.

For much of the week, the Great Lakes Region, parts of the Midwest and the Northeast were choked with smoke, prompting residents to stay indoors to avoid unhealthy air. Those forced to leave home for work, errands or other activities masked up to stay safe.

By Friday morning, there were almost 500 wildfires burning across Canada, with nearly half of them burning out of control, according to the Canadian Interagency Forest Fire Centre. Canada’s wildfire season started several weeks early this year, which means the fires could impact air quality across North America for weeks.