(CNN) — As an orange cloud of wildfire smoke from Canada smothers parts of the eastern United States, authorities are warning residents of both countries of unhealthy or hazardous air on Wednesday.

More than 55 million people in the eastern United States are on air quality alerts due to smoke. The heaviest smoke is expected to affect the northeast, across the mid-Atlantic and into the Carolinas, and smoky conditions in those areas could last at least through Thursday.

Major metropolitan areas in Pennsylvania, New York, New Jersey and Connecticut now have air quality indices (AQIs) above 150, which is considered ‘unhealthy’, according to government website AirNow .gov.

Philadelphia had an AQI of 205 Wednesday morning, which is classified as “very unhealthy.”

New York, Jersey City (NJ) and New Haven (Connecticut) had “unhealthy” AQIs ranging from 155 to 171 as of Wednesday morning.

New York City recorded the world’s worst air pollution for a few hours on Tuesday due to heavy smoke from more than 100 wildfires burning north of the Canadian border.

The smoke also triggered air quality alerts in parts of Massachusetts, New Hampshire, Pennsylvania, Maryland, Virginia and the Carolinas, according to the National Weather Service.

Alarming air quality prompted New York City Mayor Eric Adams to ask residents to limit outdoor activities and state environmental authorities to issue an air quality health advisory in the city until Wednesday.

“Active children, adults and people with lung conditions such as asthma should reduce prolonged or vigorous exertion outdoors,” the authorities advised.

New York Public Schools canceled all outdoor activities this Wednesday, but will remain open. At least 10 school districts in central New York state canceled outdoor activities and events on Tuesday.

Smoke from the Canadian fires has steadily blanketed the northeast and mid-Atlantic for more than a week, raising concerns about the effects of persistently poor air quality. New York’s air quality index topped 150 on Tuesday, according to IQairan “unhealthy” level of pollution for sensitive groups such as the elderly, children and people with respiratory problems.

New York City was still among the top five with the most polluted air in the world on Tuesday afternoon, IQair reported, along with Dhaka in Bangladesh; Jakarta, Indonesia; and New Delhi, the capital of India.

Wildfire smoke contains very small particles, or PM2.5, the smallest but also the most dangerous pollutant. When inhaled, it can penetrate deep into lung tissue and enter the bloodstream. It comes from sources such as fossil burning, dust storms and wildfires, and has been linked to a host of health problems including asthma, heart disease and respiratory disorders.

Millions of people die every year from health problems related to air pollution. In 2016, approximately 4.2 million premature deaths have been associated with these fine particlesaccording to the World Health Organization (WHO).

On Tuesday, the concentration of PM2.5 in the air of New York exceeded the guideline established by the WHO by more than 10 times.

“If you can see or smell the smoke, you should know you’re exposed,” said William Barrett, senior national director of clean air advocacy for the American Lung Association. “And it’s important that you do as much as possible to stay indoors during these episodes of heavy pollution, it’s really important to be mindful of your health and any development of symptoms.”

Barrett said those particularly vulnerable to wildfire smoke “are children, the elderly, pregnant women or people with respiratory or cardiovascular problems”, whose symptoms may worsen or develop again. when exposed to smoke.

“Be sure to take appropriate action and consult with health care providers about any concerning symptoms that occur during these times,” Barrett said.

There have been more than 150 active wildfires in Quebec this week, according to the Canadian Interagency Wildfire Center; more than double the number of fires in any other Canadian province.

So far in 2024, there have been more than 400 wildfires across Quebec, double the average for this time of year. Nearly 4 million hectares have been charred by forest fires in Canada so far this year, including more than 200,000 hectares burned in Quebec alone.

Air quality advisories were in effect Tuesday in parts of the Northeast and Midwest of the United States as smoke from the wildfires poured west toward Detroit and Chicago.

“Weather conditions are such that widespread levels of ozone or particulates are expected to be above the unhealthy category for sensitive groups on the Air Quality Index,” the National Weather Service in Chicago said. “Active children and adults, especially those with lung or respiratory conditions such as asthma, should limit prolonged outdoor activity.”

Detroit was on IQair’s list of the 10 worst places for air pollution on Tuesday afternoon. Air quality in Chicago was moderate Tuesday afternoon and is expected to remain moderate for the next few days.

Meanwhile, Pittsburgh’s air quality was at unhealthy levels, with forecasts showing a slight change to an unhealthy level for sensitive groups on Tuesday, before moderating again on Wednesday. The city is currently under an air quality alert that was in effect until Tuesday.

Parts of New York and New England will also remain under air quality alerts on Tuesday, including most of New York state and all of Massachusetts, Connecticut, Rhode Island and from Vermont. Cities like Baltimore, Boston, Hartford, Providence and Montpelier in Vermont are expected to have unhealthy air quality for sensitive groups on Tuesday.

A cold front is expected to move south over the next few days carrying smoke south and east throughout the week.

Human-caused climate change has exacerbated the hot, dry conditions that allow wildfires to start and increase in number. Scientists recently reported that wildfires in the western United States and Canada that have scorched millions of acres, an area roughly the size of South Carolina, could be attributed to pollution by the carbon of the largest cement and fossil fuel companies.

And when they burn, the smoke can travel thousands of miles downstream, endangering millions more.

“Forest fires are really a global warming problem,” Glory Dolphin Hammes once told CNN, CEO of IQAir North America. “They have a lot to do with climate change, which is creating essentially dangerous conditions.”

CNN’s Robert Shackelford, Jennifer Gray and Monica Garrett contributed to this report.