CNN — More than third of the US population is under air quality alerts, covering more than a dozen states from the Midwest to the East Coast, as smoke from Canadian wildfires sweeps across parts of the United States, prompting officials to call on the public to take safety precautions just weeks after similar wildfire smoke blanketed the Northeast.

Over 120 million people are under the alerts and some of the worst air quality, which is classified as “very unhealthy,” is centered over the Chicago, Cleveland, Detroit and Indianapolis metro areas. Some improvement in the air quality is expected on Thursday, particularly over the Great Lakes area, where rain and storms will help cleanse the air.

Canada is seeing its worst fire season on record as hundreds of blazes rage across the country – with more than 250 burning “out of control,” according to the Canadian Interagency Forest Fire Centre.

As massive clouds of smoke cross into the US, air quality alerts have been issued for the entire states of New York, Iowa, Wisconsin, Illinois, Indiana, Michigan, Delaware and Maryland as well are parts of Kansas, Missouri, Minnesota, Ohio, Pennsylvania, Virginia and North Carolina, the National Weather Service said.

New York Gov. Kathy Hochul expanded an air quality health advisory in New York to include the entire state on Wednesday. Air quality levels in parts of the state are now unhealthy for all people, while the New York City area remains unhealthy for sensitive groups. Hundreds of thousands of N95 masks are available to New Yorkers, with some being distributed at transit hubs and parks, Hochul said in a news release.

Live updates: US air quality impacted by wildfire smoke

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

Places from Pennsylvania to Iowa are experiencing very unhealthy air, which means the risk of detrimental health effects is increased for everyone, not just sensitive groups. A small area east and north of Detroit reached hazardous levels Wednesday morning – that’s considered a “health warning of emergency conditions,” according to the federal government’s Air Quality Index.

Chicago, Detroit, Indianapolis, Cleveland, Pittsburgh, Milwaukee and Davenport, Iowa, all saw readings in the very unhealthy range Wednesday morning. The smoke is causing unhealthy air quality readings stretching south as far as St. Louis, Cincinnati and Louisville, Kentucky.

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.

Locations under the Code Red warning include Chicago, Indianapolis, Pittsburgh and Erie in Pennsylvania, Columbus, Ohio, Fort Wayne, Indiana, and Madison and Milwaukee in Wisconsin.

The wildfire smoke has prompted closures and event cancellations in the Northeast and the Midwest. The zoo in Eerie, Pennsylvania, was closed for the day; the band Garbage canceled their show in Madison, Wisconsin; and in Ohio, concerts, some sports events and pools were closed. In Minneapolis, the Parks and Recreation Board canceled all Wednesday outdoor programs in the parks, according to a Facebook post.

Chicago and Detroit had the worst air quality in the world Tuesday night, with Detroit’s air at one point reaching a “very unhealthy” Air Quality Index of 205, according to IQAir. Chicago registered an Air Quality Index of 174 late Tuesday. The Air Quality Index runs from 0 to 500, where levels of 50 or below are considered healthy. The higher the index level, the more dangerous the air quality.

City skylines were blurred by smoke Tuesday and Wednesday, with a white haze lingering over roads and neighborhoods.

“I can feel like a burning in my throat,” Dalyca Khuder, who was visiting Detroit, told CNN affiliate WXYZ. “The air quality is just really bad and I don’t want that stuff in my lungs.”

Chicago asked all residents – especially those with heart or lung disease, older adults, pregnant people and young children – to avoid outdoor activities and protect themselves from exposure. Chicago Public Schools and camps are also moving activities indoors, city officials said in a news release.

“Within five steps of walking outside in the morning yesterday I could smell the smoke and see the haze—it looks like if you were looking through clingfilm,” Brent Rappaport, a Chicago resident, told CNN, referring to the air quality in the city on Tuesday and Wednesday.

“I usually bike to work in the summer months but with this air that’s really not possible.”

About 11 miles away, Evanston, Illinois, closed all swimming beaches and canceled a concert Tuesday due to the poor air quality, the city said on Facebook, asking residents to limit outdoor exposure through Wednesday.

In Michigan, the Mackinac Bridge was shrouded in smoke Tuesday. Motorists were asked to drive slowly and with caution due to the reduced visibility on the bridge.

High levels of fine particulate in the air in Michigan could become unhealthy or hazardous for all residents at some point – not just sensitive groups, the state’s health department warned.

“The most protective option when air is unhealthy for you is to stay indoors with air conditioning, reduce strenuous activities and limit outdoor activities. If you must be outside, N95 masks offer enhanced protection when used according to product instructions,” the Michigan Department of Health and Human Services said.

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.

American cities under air quality alerts are mostly in “Code Orange” – with the air deemed unhealthy for sensitive groups – or “Code Red,” which is when the air is believed to be unhealthy for the public.

Hazy skies could return to New York

Officials in New York warned that air quality levels could spike to hazardous levels this week – about three weeks after New York City topped the list of the world’s worst air pollution as smoke from the Canadian wildfires wafted south, turning skies orange.