Wildfire smoke will drive poor air quality through Saturday as new plumes arrive from Canada

Smoke from wildfires in Canada is pouring into the US and will usher in unhealthy air quality for millions of people in Northeast and Midwest.

Massive plumes of Canadian wildfire smoke are pouring over the border into the United States, bringing dangerous air quality to more than 120 million people in the Midwest and Northeast. The polluted air will last through Saturday as new waves of smoke waft south. Smoke shrouded skylines and caused air quality alerts to go into effect Thursday from Boston to Minneapolis.

Elevated levels of fine particulates from the smoke drove air quality indexes well above the "hazardous" level in cities closest to the fires, including Chicago, Detroit and Toronto. Smoke has already been an issue this summer in parts of the West, Plains and Midwest as stateside fires have burned more than 3.6 million acres, mostly in the western half of the country. But the thicker smoke that has moved over the Great Lakes and Northeast is from wildfires to the north.

In Canada, 3,500 fires have burned more than 6 million acres this summer, with a dozen blazes flaring up in Ontario in recent weeks, filling the skies with smoke drifting south, like it did in an extreme way in 2023. unknown content item - Canadian wildfire activity this year is nowhere near the hyperactivity of 2023 but the combination of wildfires in Ontario and a heat dome in the central US spells smoky trouble for millions. Planet-heating fossil fuel pollution is increasing the chance of prolonged smoke seasons as it tips the odds that extreme wildfire seasons like 2023's — Canada's worst season on record — won't remain an outlier for long.

Unhealthy smoke heads for Great Lakes and Northeast unknown content item - The latest round of smoke has sunk to the surface due to a weather pattern change, unlike Tuesday's smoke plume, which largely stayed higher in the atmosphere. As a result, air quality has deteriorated across the Northeast, Great Lakes and Midwest. Poor air quality is likely to be an issue through Saturday as the smoke lingers and new plumes arrive from the north.

Wildfire smoke contains dangerous, tiny pollutants called PM2.5 that can travel deep into the lungs or enter the bloodstream when inhaled. The minuscule particles can lead to breathing problems like bronchitis and cause inflammation that aggravates diabetes, heart disease and other health conditions. People with lung or heart disease, children and older adults are especially at risk for smoke-related illness.

The NWS advises people avoid smoke by limiting outdoor activities and keeping windows closed overnight. The only silver lining the smoke brings is that it could dampen some of the heat. Smoke blocks sunlight, which would be welcome news for many parts of the Northeast expected to see high temperatures in the coming days.

The smoke is now streaming into the US thanks to a record-breaking heat dome parked over the central part of the country. So far there have been several heat domes — large, slow-moving high pressure systems — this year from the West Coast to the Northeast. Air flows clockwise around these domes, but until now, none of them have been in quite the right spot to drag Canadian smoke south.

The northern edge of this week's heat dome is perfectly placed over northern Minnesota and southern Ontario, where wildfires rage. This placement means smoke will flow east and south, right into parts of the Midwest and Northeast. With several months left in wildfire season, the door will remain open for more Canadian smoke plumes to migrate south.

How does this smoke compare to 2023? This time three years ago, records were being broken as 4,300 fires had already burned 25 million acres across Canada. In June 2023, smoke consumed the New York City skyline as northerly winds pushed a wall of wildfire smoke from Quebec into the Big Apple.

The culprits were a dominant high pressure system in the Hudson Bay and a storm system over Atlantic Canada. Together, they funneled smoke into the Midwest and Northeast. Both systems were stagnant, keeping the smoke in place for about four days.

2023's smoke event was so far-reaching, a study from last year estimates more than 350 million people were exposed to daily wildfire smoke-induced air pollution. Fortunately, wildfire activity in Canada started later compared to that record-setting year, making it unlikely this year's smoke will be as pervasive. Smoke seasons are getting worse In the US, the window for outbreaks of unhealthy wildfire smoke is getting bigger, as fire seasons in the West have become longer and more extreme.

Climate change was found to be responsible for the majority of the increase of surface wildfire smoke. This smoke has eroded decades of air quality improvements in parts of the US, particularly in the West. Planet-warming pollution caused approximately 15,000 more deaths in the US from wildfire particulate matter from 2006 to 2020 than would have otherwise occurred in a cooler world, a study published last year found.

For more CNN news and newsletters create an account at CNN.com