Scores of wildfires burning across Canada and Minnesota on Thursday blanketed thick smoke across the Midwest and New England. The noxious plumes prompted dangerous air quality readings, but the smoke could begin to dissipate in some areas by Friday.

Air quality in Toledo, Milwaukee and Detroit slid to dangerous levels on Thursday.

Here’s what you need to know about the smoke, the air and the forecast.

What caused all this smoke?

Dense smoke wafted across Canada and the United States from more than 100 wildfires burning in northwestern Ontario, near Thunder Bay, a city about an hour’s drive north from Minnesota.

About a dozen fires raged in Minnesota over the past week, according to the governor’s office, leading to evacuation orders.

A heat dome has trapped high temperatures along with the smoke close to the ground from the Midwest into New England.

When will the smoke end?

Soon, for many people. Not soon enough for many more.

Generally, the farther away you are from the fires in northern Minnesota and southern Ontario, the sooner this will ease.

The Northeast is expected to be smoky at times on Friday, especially later in the day. But forecasters expect another weather system eventually to push the hottest air away, taking the smoke with it.

But in the Upper Midwest, closer to the fires, the heat and smoke are likely to linger through the weekend. It could still be quite heavy at times across the Great Lakes, especially on Saturday.

How is air quality measured?

The Air Quality Index measures the density of five pollutants in the air: ground-level ozone, particulates, carbon monoxide, nitrogen dioxide and sulfur dioxide.

The index is measured from 0 to 500 or even higher in extreme cases. As the numbers tick up on the scale, the more dangerous the air quality.

The air quality in some Midwest cities reached readings of 500 or above. The A.Q.I. soared above 800 in Toledo, Ohio, at about 5:30 p.m. Thursday.

Should I be wearing a mask?

As smoke lingers in the air, donning a mask can filter out dangerous particles.

You may want to cover your face when the A.Q.I. climbs above 200, a pollution level dangerous for anyone.

A cloth or a surgical mask does not block out the dangerous particles as well as a N95 mask.

Who’s most at risk?

The people most at risk are children, pregnant women and people with respiratory conditions, such as asthma, according to the Centers for Disease Control and Prevention.

For these groups, the C.D.C. recommends staying inside, keeping windows shut and using an air filtration system.

How can I protect my pets?

The dangerous particles lingering in the air can be dangerous for pets, too.

To protect your pets, shorten the amount of time the animals spend outside. Toweling down your pets after they come from outside can help reduce pollutants stuck on it.