(CNN) — More than 100 million people were under air quality alert early Friday from Wisconsin to Vermont and all the way to North Carolina as smoke from Canadian wildfires continues to drift into the southern United States, although conditions are expected to slowly improve over the holiday weekend.

THE thunderstorms this Thursday They eased some smoke in parts of the Midwest, and rains forecast for Friday should bring more relief. But the smoke may clear less quickly in the Ohio Valley and mid-Atlantic, where the storms will be more isolated.

“Air quality is expected to improve in the near term as the combination of thunderstorm activity and dispersal of smoke will cause air quality conditions to improve across much of the country before the end of of the week”, said the National Weather Service.

The more than 500 active wildfires ravaging Canada have deteriorated air quality in parts of the country and in the United States. On Friday morning, Toronto topped a list of the best cities in the world with the worst air quality, followed by Washington, according to IQ Air.

Detroit, which topped the same list Thursday night, placed fourth early Friday, while New York was sixth worst in the world, the website showed.

This Friday, the worst air quality in the United States is expected to spread from Michigan to Ohio and the mid-Atlantic, including Washington.

It is hoped that the area in these areas is classified as “no saludable for sensitive groups”, a level 3 of 6, or “poco saludable”, a level 4 of 6, in the Índice de Calidad del Aire de EE.UU. desarrollado for the United States Environmental Protection Agency Current air quality ratings can be viewed at airnow.govan association of agencies that includes the EPA.

The category “Unsanitary for sensitive groups” generally includes the elderly, young children, people with certain chronic diseases and outdoor workers.

Separately, New York Governor Kathy Hochul announced Thursday that a statewide air quality health advisory will remain in effect until Friday, urging residents to take the necessary preventive measures, such as the use of masks, to stay safe outdoors.

“With more visible ground-level smoke and air quality continuing to reach unhealthy levels in many parts of the state, we encourage New Yorkers to take precautions to protect their health,” Hochul said.

Western and central New York, as well as the eastern Lake Ontario region, are expected to reach the “unhealthy” air quality threshold as early as Friday, the governor’s office said. Other areas of the state could see the tune listed in the less severe category of “unhealthy for sensitive groups.”

Air quality in New York was at unhealthy levels due to smoke from wildfires in Canada on Thursday. (Credit: Fatih Aktas/Anadolu Agency/Getty Images)

Canada Day fireworks in Montreal canceled

The Canadian provinces of Ontario, Quebec and British Columbia had some of the worst air quality in the country on Thursday evening, according to IQ Air.

The impacts of the fires have led Canadian authorities to cancel Canada Day fireworks in Montreal scheduled for this Saturday, although other festivities are planned.

While forecasters worried about air quality this Saturday, the decision to cancel was “solidarity”, said Stéphane Guertin, president of Tandem Communications, which organized the event.

“I’m just sympathetic about what’s going on right now. That’s why we canceled it. It’s about what’s happening in the fires and what’s being broadcast right now,” Guertin told CNN.

“Something bad is happening right now in northern Quebec, so we’re all in this together. So it was cancelled. We’re not going to air anything else. »

The Toronto skyline is seen covered in smoke Wednesday from the CN Tower. (Credit: Arif Balkan/NurPhoto/Getty Images)

Canada is experiencing its worst wildfire season on record, and scientists have warned that such intense fires are becoming more common as the planet warms.

The smoke from these flames can travel thousands of miles, endangering millions more.

Smoke from wildfires is particularly dangerous because it contains particulate matter, or PM2.5, a tiny but dangerous pollutant. When inhaled, it can penetrate deep into lung tissue and enter the bloodstream, according to the US Centers for Disease Control and Prevention (CDC).

It comes from sources such as burning fossil fuels, dust storms and wildfires. Particulate matter has been linked to health problems including asthma, heart disease and other respiratory diseases.

— CNN’s Michelle Watson and Raja Razek contributed to this report.