3 cities among worst; when will it recede?

Smoke from wildfires raging in Canada engulfed more U.S. cities on Thursday − from the Midwest to the Northeast to the mid-Atlantic to the Southeast − leading to perilous air quality for millions and prompting warnings to limit outdoor time.

The unhealthy air shifted slightly south since Wednesday, and more parts of the country are being affected, National Weather Service meteorologist Bryan Jackson told USA TODAY. The federal government tracks and publishes searchable realtime results.

“Some places are having a repeat of yesterday, some are worse than yesterday,” Jackson said, adding that the smoke moved into cities like Pittsburgh and Washington, D.C. starting Wednesday afternoon. The Canadian wildfires have burned a larger area than the state of Maine.

Washington, Chicago, Detroit among cities with worst air quality in the world

As of Thursday morning, Washington, Chicago and Detroit were among the cities with the worst air quality in the world, according to IQ Air’s Air Quality Index. In some areas, authorities discouraged people from exercising outside over concerns the pollution could damage their breathing, especially if they have asthma.

Cities in over a dozen states have issued air quality alerts

Cities in over a dozen states from Duluth, Minnesota, to Portland, Maine, and as far south as Atlanta issued Code Orange air quality alerts or worse Thursday, according to the weather service.

Jackson said Madison, Wisconsin, as well as Pittsburgh and Columbus, Ohio, are experiencing dangerous levels of air pollution Thursday, and numbers on the Air Quality Index scale are hovering around 200, or “very unhealthy.”

Air quality index basics:How it works, what each color code means and how to stay safe

Entire states, including New York, New Jersey, Maryland and Vermont were blanketed by the warnings Thursday, according to the weather service.

In West Virginia, a “special weather statement” has been issued for most of the state warning motorists of low visibility on roads and “hazy conditions” caused by a combination of the Canadian wildfire smoke and river valley fog.

When will conditions improve?

Some parts of the Midwest could see improvements in the air quality on Friday, Jackson said. A southerly wind that’s spreading west across the upper Midwest could clear some smoke, he said.

“That is supposed to push the smoke farther north and allow it to disperse more,” he said.

But in the eastern U.S., a swath of states stretching from Georgia to New York will likely have more hazy conditions Friday, he said.

In Illinois, Indiana, Iowa and northern Missouri, rain forecast for Friday could also help lessen poor air quality, Jackson said.

Overall, “that will improve the western side” of the smoky conditions that have descended on the eastern half of the U.S., Jackson said.

What are the Air Quality Index levels?

Governments around the world track and rank their air quality daily – in some cases in real time – using a color-coded scale that goes from “good” to “hazardous.” The AQI is a measure of multiple types of pollution, from wildfire smoke to power-plant emissions, dust, ozone, pollen and several chemicals, including sulfur dioxide, which comes from burning coal or oil.

The Air Quality Index levels are:

Green/Good: 0 to 50, air quality is satisfactory, and air pollution poses little or no risk.
Yellow/Moderate: 51 to 100, air quality is acceptable. However, there may be a risk for some people, particularly those who are unusually sensitive to air pollution.
Orange/Unhealthy for sensitive groups: 101 to 150, members of sensitive groups may experience health effects. The general public is less likely to be affected.
Red/Unhealthy: 151 to 200, some members of the general public may experience health effects; members of sensitive groups may experience more serious health effects.
Purple/Very unhealthy: 201 to 300, health alert: The risk of health effects is increased for everyone.
Maroon/Hazardous: 301 and higher, health warning of emergency conditions: everyone is more likely to be affected.

Where are the Canadian wildfires?

Air quality is unhealthy because of the level of particulates in the air from Canadian wildfires, which have drifted into the U.S. at various points in the past several weeks. Normally the jet stream pushes from west to east, but this summer, it has flowed more often from the north and east, carrying Canada’s wildfire smoke with it.

“I just don’t remember the last time we saw smoke drift from the Canadian Maritimes back to the west into the Great Lakes region — from a meteorology, climatology standpoint, that’s pretty hard to do. But we saw it this year,” said Jeffrey Andresen, a meteorologist and climatologist with Michigan State University’s Department of Geography.

There were 500 active wildfires throughout Canada as of Thursday, according to the Canadian Interagency Forest Fire Centre. Of those, 255 were considered out of control. Nearly a quarter of the fires burning in Canada were in Quebec.

This year has seen a record-breaking fire season for Canada, and it’s still early. More than 31,000 square miles have burned this year, exceeding the record set in 1989 of 29,187 square miles, according to the National Forestry Database. That previous record was set over the full fire season of April-October, meaning smoke could linger over the U.S. for several more months.