These are the US cities, states with air quality being affected by Canadian fires

(NEW YORK) — As Canada continues to battle its worst wildfire season on record, toxic smoke has traveled south and is once again blanketing states across the U.S.

A total of 20 states are under air quality alerts Monday, stretching from Montana to New York and as far south as North Carolina and Tennessee as of Monday morning, according to AirNow, an air quality website run by a partnership between the federal government and state and local air quality agencies.

Cities including Billings, Montana, Cleveland and Pittsburgh had Air Quality Index ratings of above 150, which is considered “unhealthy.”

Meanwhile, residents in Chicago woke up to the fifth worst air quality among major cities worldwide, according to an ABC News analysis.

“Haze from wildfire smoke will persist in our region into Monday,” National Weather Service Chicago tweeted Sunday. “Resulting poor air quality may be unhealthy for sensitive groups. Limit prolonged outdoor activity if you have a chronic respiratory issue.”

Mayor Brandon Johnson recommended over the weekend that vulnerable groups, such as children, teenagers, senior citizens, pregnant people and those with heart or lung disease, stay indoors and that people who travel outdoors consider wearing masks,

This is because wildfire smoke is made up of several toxins, including fine particulate matter — known as PM2.5 — which is 30 times smaller in diameter than a human hair.

These particles are too small to be seen with the naked eye and can be breathed deep into the body, entering the nose and throat and traveling to the lungs.

PM2.5 can cause short-term health effects, even for healthy people, including irritation of the eyes, nose and throat; coughing, sneezing; and shortness of breath. It can also cause long-term effects such as asthma and heart disease.

By Monday evening, the heaviest smoke in the U.S. is expected to be concentrated over upstate New York around Tupper Lake, which is about 80 miles from the Canadian border, and Burlington, Vermont.

“Air Quality Health Advisories are in effect for the entire state today,” New York Gov. Kathy Hochul tweeted Monday. “New Yorkers should continue to monitor the latest information from [the New York State Department of Environmental Conservation], http://airnow.gov, or their weather app, and take necessary steps to stay safe.”

Port Authority of New York and New Jersey announced it would be handing out masks all day Monday at the bus terminal in Manhattan.

The forecast shows on Tuesday morning, much of the surface smoke will start dissipating and by the evening, it is expected to be light to medium.

However, Canada may not be the only country in North America that has to battle wildfires. Red flag warnings went into effect Monday for much of eastern Washington and parts of Oregon and Colorado due to dry air, dry ground, and breezy conditions.

Combined with low humidity and strong winds, the conditions are prime for fires to be sparked and to grow rather quickly.

ABC News’ Kenton Gewecke contributed to this report.