Tens of millions of Americans face another day of smoky skies, irritated eyes and bad air quality, as Canadian wildfire smoke spread again over huge swathes of the US, affecting about 109 million people across the midwest, mid-Atlantic and north-east.

The pungent wildfire blanketed cities such as Chicago and Detroit, where residents on Friday were warned to stay indoors and reduce activity levels after the air quality index reached a “hazardous” 361, according to the government website AirNow.

The smoke drifted into Baltimore and Washington DC overnight, creating very unhealthy air quality with index values of 281 and 247, respectively, as of 6am eastern time. In New York City, where smoke has blanketed the city since Tuesday, air quality stands at an “unhealthy” 184.

Philadelphia and Cleveland had readings considered “very unhealthy” at about 260. Other parts of Michigan, Minnesota and Wisconsin also recorded readings in the “hazardous” range.

Overall, the global air quality index showed pollution in five major North American cities at higher levels than Kinshasa or Nairobi in Africa – the next cities on the list. Nasa’s Fire Information for Resource Management System, or Firms, shows pockets of Canadian wildfires reaching deep into the Northwest Territories.

In Michigan, the state’s department of environment, Great Lakes and energy recommended closing windows, minimizing the opening of doors and using HVAC systems rated Merv-13 or higher.

“If you must be outdoors for short periods of time, an N95 or P100 respirator marked with NIOSH is recommended,” that alert said.

But some cities in the north-east and New England are expected to get relief from smoky skies on Friday as stronger winds from Quebec – not from the wildfire zone in western Ontario – blow the smoke out.

“There has been a clear intensification in wildfire activity for Canada over the past few weeks. Smoke from major fires – particularly in Ontario – [is] already having severe air quality impacts across cities in the Great Lakes region and the north-eastern United States,” said Mark Parrington, senior scientist at the Copernicus Atmosphere Monitoring Service.

“Our forecasts show the smoke continuing to move eastwards across the North Atlantic, and potentially towards Europe, highlighting the scale of wildfire pollution and how it can travel thousands of kilometers across borders and impact air quality in places far beyond the fires themselves,” he added.

Organizers of the World Cup final in East Rutherford, New Jersey, will be watching the smoke patterns carefully: smoke currently over the mid-Atlantic is expected to blow back up into the north-east.

But an approaching storm system from the west, bringing severe thunderstorms, could interact with the smoke, creating worse conditions as rain drags smoky air from higher up in the atmosphere down to the ground.

When that storm passes late on Saturday, air quality is likely to improve in time for a 3pm Sunday kick-off, forecasters say.

Canada’s largest fire, near Ontario’s remote Wabakimi provincial park, is reported to be spread across 787,802 acres (318,812 hectares) by the Canadian Interagency Forest Fire Center. It is among 194 out-of-control and large fires that were burning as of Thursday.

Parrington said smoke plumes from the large-scale wildfires in Canada’s Northwest Territories had reached the Arctic Ocean as well as across other parts of the country.

Nearly 6m acres are estimated to have burned, less than a quarter of land consumed by blazes when Canadian wildfire smoke last blanketed the US in 2023. Fires in northern Minnesota have burned more than 63,000 acres.

In the west, wildfires in Oregon, Washington and Idaho have also been reported.

“The wildfire situation across North America just got worse. Tens of thousands of lightning strikes across the Pacific Northwest [have] ignited dozens of new wildfires across Oregon and Washington,” wrote forecaster Colin McCarthy on Thursday evening.