New York City topped the list of the world's worst air pollution Tuesday morning as harmful smoke wafted south from more than a hundred wildfires burning in Quebec.

Smoke from Canada's fires has periodically engulfed the Northeast and Mid-Atlantic for more than a week, raising concerns over the harms of persistent poor air quality. New York City's air quality index peaked above 150 on Tuesday, according to IQair -- a level of pollution that is "unhealthy" for sensitive groups like the elderly, young children and those with respiratory issues.

New York City was still among the top five cities with the worst air pollution in the world Tuesday afternoon, IQair reported, alongside Dhaka, Bangladesh; Jakarta, Indonesia; and New Delhi, India.

Wildfire smoke contains very tiny particulate matter, or PM2.5 -- the tiniest pollutant yet also the most dangerous. When inhaled, it can travel deep into lung tissue and enter the bloodstream. It comes from sources like the combustion of fossil fuels, dust storms and wildfires, and has been linked to a number of health problems including asthma, heart disease and other respiratory illnesses.

Millions of people die each year from air pollution-related health issues. In 2016, around 4.2 million premature deaths were associated with fine particulate matter, according to the World Health Organization.

On Tuesday, the concentration of PM2.5 in New York City's air was more than 10 times the guideline set by the World Health Organization.

There are more than 150 active wildfires burning in Quebec this week, according to the Canadian Interagency Forest Fire Center -- more than double the number of fires burning in any other Canadian province.

More than 400 wildfires have ignited across Quebec so far in 2023, twice the average for this time of year. Nearly 9 million acres have been charred by wildfires in Canada so far this year, with nearly half a million acres burned across Quebec alone.

Air quality alerts were in effect across parts of the Northeast and the Midwest on Tuesday as wildfire smoke spread west into Detroit and Chicago.

"Weather conditions are such that widespread ozone and or particulate levels are expected to be at or above the unhealthy for sensitive groups category of the air quality index," the National Weather Service in Chicago said. "Active children and adults especially people with pulmonary or respiratory disease such as asthma should limit prolonged outdoor activity."

Detroit was listed in IQair's top 10 worst locations for air pollution on Tuesday afternoon. Chicago's air quality was moderate on Tuesday afternoon and is expected to remain moderate for the next couple of days.

Meanwhile, Pittsburgh's air quality was at unhealthy levels, with forecasts showing a slight shift to unhealthy level for sensitive groups on Tuesday before returning to moderate by Wednesday. The city is currently under an air quality alert all of Tuesday.

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

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

Human-caused climate change has exacerbated the hot and dry conditions that allow wildfires to ignite and grow. Scientists recently reported that millions of acres scorched by wildfires in the Western US and Canada — an area roughly the size of South Carolina — could be traced back to carbon pollution from the world's largest fossil fuel and cement companies, scientists reported Tuesday.

And when they burn, the smoke can travel thousands of miles downstream, putting millions more people in harm's way.

"Wildfires is very much so a global warming issue," Glory Dolphin Hammes, CEO of IQAir North America, previously told CNN. "It has very much to do with climate change, which is creating essentially unsafe conditions."