Haze obscures buildings along the Lake Michigan shoreline in Chicago on Wednesday.
CNN  — 

Dangerous air quality and hazy skies persist as smoke from Canada’s raging wildfires drifts south, leaving more than 100 million people under air quality alerts across a dozen states from Minnesota to New York and down to the Carolinas.

By 6 p.m. Thursday, Detroit topped the list of cities with the worst air quality in the world, followed by Washington, DC, according to IQAir. Chicago and New York City were among the top 10.

Air quality in Illinois improved Thursday afternoon as a massive thunderstorm complex raced across the Midwest, blowing through the harmful smoke from the wildfires and clearing the air in its wake.

The storm, referred to as a bow echo because of its arc-like appearance on radar, has produced wind gusts close to 90 mph. A wind gust of 88 mph was reported in western Illinois early Thursday afternoon, and a severe thunderstorm watch was in effect in Illinois and parts of Indiana ahead of the system.

The Storm Prediction Center was forecasting a level 4 out of 5 risk for damaging winds up to 90 mph, along with large hail and a few tornadoes for nearly 2 million people across Central Illinois. The straight-line winds in these weather phenomena can be just as devastating as tornadoes.

A storm in Effingham, Illinois on Thursday afternoon.

The storm could become a derecho, which is classified by a swath of wind damage extending more than 240 miles with wind gusts of 58 mph or greater along most of its length, according to the National Weather Service.

Thursday’s worst air pollution was centered over Detroit, Chicago, Indianapolis and Cleveland. Some improvement in air quality was expected Friday, and Thursday’s windy storms will help ease the pollution.

Readings for small particulate matter from the smoke dropped significantly behind the thunderstorm system, with the air quality index in Champaign, Illinois, falling from around 180, or “unhealthy,” on Thursday morning to just 1 after the storm blew through.

While DC and New York saw an increase in wildfire smoke on Thursday, levels are not expected to reach those seen a few weeks ago, when smoke from Canada’s wildfires also drifted over the US, tanking air quality levels and casting an orange haze over several cities.

In Canada – which is seeing its worst fire season on record – authorities have also issued air quality alerts across several provinces.

“With no end in sight to the Canadian wildfires and west to northwesterly winds expected to persist from south central Canada into the north central to northeast U.S., poor air quality conditions are likely to continue,” the National Weather Service warned.

A Code Red alert – warning of unhealthy air quality – was issued for much of the Midwest and Ohio Valley on Wednesday, according to AirNow.gov. Other affected American cities were mostly under Code Orange – with the air deemed unhealthy for sensitive groups.

Pennsylvania extended a partial Code Red air quality action day to include the entire state Thursday, urging young children, the elderly and those with respiratory problems to avoid outdoor activities.

Officials are distributing masks to residents in New York, where air quality is unhealthy “in every corner of the state,” Gov. Kathy Hochul said Thursday.

Pittsburgh Pirates play as smoke from wildfires in Canada descends on the downtown skyline in Pittsburgh on Wednesday.

New York deployed its third crew of firefighters to Canada on Wednesday, joining other first responders from Maine and New Hampshire to assist with firefighting efforts, Hochul said.

Several cities across Wisconsin, Pennsylvania and Iowa were experiencing “very unhealthy” air that poses health risks for everyone – not just sensitive groups, according to AirNow.gov.

Smoke was so thick it reduced visibility for up to three miles in some areas across the Great Lakes and the Ohio Valley.

Those areas could reach “unhealthy” or “very unhealthy” levels Thursday, before smoke begins to slowly disperse, forecasters said.

Residents are being advised to stay indoors with their air conditioning running, avoid going outside, and in some areas, wear N95 masks if they have to be outside.

Meanwhile, from Minnesota to Pennsylvania, events have been canceled due to the wildfire smoke.

Photos from across the country captured the eerie hazy or orange skies over iconic American skylines, with smoke lingering over freeways and neighborhoods.

Wildfire smoke carries particulate matter, or PM2.5 – a tiny but dangerous pollutant that, when inhaled, can travel deep into lung tissue and enter the bloodstream, according to the US Centers for Disease Control and Prevention. The particulate matter has been linked to a number of health problems including asthma, heart disease and other respiratory illnesses.

People with heart or breathing problems, children and older people may be particularly sensitive to PM2.5.

Exposure can cause short-term health effects like irritation to the eyes, nose, and throat, coughing, sneezing, runny nose, and shortness of breath. Exposure to high levels of fine particulate matter can also worsen medical conditions such as asthma and heart disease, New York health officials said.

How to limit exposure

People watch the sunset as the smoke from wildfires drifts into Toronto on Wednesday.

As smoke inundates cities, health departments across the country have been advising residents to limit their exposure to the pollutants. Many are calling for people to stay indoors and avoid activities that make them breathe faster or more deeply.

“The recommendation is to close your windows, turn on the air conditioners, turn on air filters,” Dr. Aida Capo, a pulmonologist with Hackensack Meridian Palisades Medical Center in New Jersey, told CNN.

Since particles can creep into homes, AirNow recommends those in areas with high levels of particle pollution consider purchasing an air cleaner.

For those who have to be outside, disposable respirators like N-95s or P-100s will help if worn correctly, AirNow says. Meanwhile, surgical masks, paper dust masks, scarves and bandanas won’t protect the lungs from PM2.5.

Residents in affected areas are also advised to reduce strenuous physical activity.

“If you have to walk, walk, but I wouldn’t go for a run or a jog,” said Dr. Shilpa Patel, medical director of Children’s National IMPACT DC Asthma Clinic in Washington.

“Just be prudent about your decision to be outside,” she added. “And keep in mind, even if you go outside and it doesn’t bother you, it could affect you later. Because these are small particulates … they go deep into your airways, and the response could be a little bit delayed.”

CNN Meteorologist Taylor Ward, CNN’s Dave Alsup, Angela Fritz and Joe Sutton contributed to this report.