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Why is eastern Canada on fire — and when will the smoke clear?

4 basic questions about Canada’s wildfires, answered.

A smoky haze casts an orange glow on uplit  fountains in the foreground and a bridge in the distance.
A view of the Williamsburg Bridge from Domino Park in Brooklyn, New York, on June 7.
Courtesy of Natalie Kelapire
Benji Jones is a senior environmental reporter at Vox, covering biodiversity loss and climate change. Before joining Vox, he was a senior energy reporter at Insider. Benji previously worked as a wildlife researcher.

East Coasters finally understand what it’s like to live in California.

Earlier this week, a giant cloud of wildfire smoke from Canada wafted into New York City, Boston, and other eastern metropolises, engulfing skylines and putting millions of people at risk from air pollution. On Tuesday evening, Wednesday, and Thursday morning, NYC had the worst air quality of any major city in the world.

A time-lapse image of the New York City skyline being obscured by smoke. New York State Mesonet/University at Albany

It’s not only large northeastern cities that are smothered in smoke. States as far west as Minnesota and as far south as South Carolina have watched their air quality plummet, in some cases reaching record levels of pollution. The ongoing air quality crisis is likely one of North America’s worst wildfire smoke events in the last two decades.

Across the eastern seaboard, most of the smoke comes from recent forest fires in Quebec, a Canadian province in the far east that borders Maine. Roughly 140 fires were burning in the region as of Thursday morning, and many of them had not been contained.

This situation is both frightening and usual. While Canada is, on the whole, prone to wildfires, the fires usually aren’t this severe in the east, especially this early in the year. Plus, weather patterns have to be just right to carry the smoke hundreds of miles south into the US.

One big question now is whether these fires in Canada will become more common in the years ahead — and what that means for US cities that are not accustomed to smoke.

1) Why is eastern Canada burning?

The summer often brings severe wildfires to western Canada, especially as climate change continues to dry out vegetation and heat up the atmosphere. 2021 was a particularly devastating year, with blazes destroying entire towns.

Provinces in the east — including Quebec and Nova Scotia — are somewhat more safeguarded from fires, or at least devastating ones. Air coming off the North Atlantic Ocean typically keeps the region humid and cooler, making it less likely to burn, per Reuters.

The forests out east also tend to be less flammable, Reuters notes. Unlike western forests, which are dominated by fire-prone evergreens, eastern forests also have broadleaf deciduous trees, which are less flammable (their branches start higher off the ground and their leaves contain more moisture).

A satellite image shows smoke drifting south from wildfires burning in Quebec (on the right) and Ontario (on the left) on June 7.
CIRA/NOAA via AP

But under the right conditions, even eastern forests can burn.

This spring brought the right conditions across parts of the east — namely, low humidity and rainfall, and lots of heat. By the end of April, large parts of eastern Canada were abnormally dry, according to the country’s drought monitor. Some places, such as Sydney, Nova Scotia, recorded their driest April on record. When forests are dry, they ignite more easily.

“What’s unique about this year is that the forests are so dry that the fires are many times larger than they normally are,” Matthew Hurteau, a biology professor at the University of New Mexico, told Vox’s Rachel DuRose.

Still, there needs to be a source of ignition. And for the fires out east, it was likely a combination of lightning strikes, people (who might, say, toss a cigarette butt out their window), and human infrastructure (such as trains, which can create sparks).

2) When will the smoke disappear and the fires stop?

The reason there’s so much smoke spreading south into parts of the US is, in a word, weather. A large low-pressure system above Nova Scotia that’s swirling counter-clockwise has created winds that flow south from Quebec and then east toward New York and other coastal cities.

It will take a change in weather to clear smoke from eastern cities, though don’t expect relief in the near-term. On Thursday morning, air quality in New York, Philadelphia, Washington, DC, and other regions was hazardous. Conditions will likely remain poor in these regions through Friday morning, at which point the wind may shift directions and blow the smoke west toward Ohio, according to the National Weather Service.

The long-term forecast is more troubling.

The reality is that if fires continue to burn, smoke could continue to impact regions of the US. Forecasters predict Canada will face dry and, in some places, warmer-than-average conditions this summer, so the recipe for wildfires could persist for months. As long as there’s a risk of fire, there’s a risk of far-ranging smoke.

“As long as the fires continue, the smoke may simply be directed toward other areas of the US,” the National Weather Service said Wednesday.

3) Is wildfire smoke really that dangerous?

Yes, very much so, especially for people who already have lung or heart conditions, people who are pregnant, and children. Here’s how Vox’s resident physician and health reporter, Keren Landman, put it:

Breathing polluted air affects the body in a few different ways. Larger pieces of particulate matter — tiny particles of soot and dust — can irritate the linings of people’s airways in their noses, mouths, throats, and lungs. And smaller bits, along with toxic gases and molecules called volatile organic compounds, can sneak from the lungs into the bloodstream, where they can travel to other organs and cause a wide range of short- and long-term problems.

You can find her full story on the health risks of inhaling smoke here.

People who live in large cities like New York and Boston are already exposed to sources of dangerous air pollution including car exhaust. Research suggests, however, that wildfire smoke can be several times more harmful than these other sources.

Thankfully, there are pretty easy ways to avoid dangerous exposure, as my colleague Rebecca Leber writes: Stay indoors when you can, wear an N95 mask when you can’t, and pay attention to outdoor air quality measurements and forecasts the same way you do the weather.

4) Are smoky skies the new normal for East Coasters and the upper Midwest?

Climate change is heating up the planet, and warmer air sucks moisture out of trees and other plants, making them more flammable. That’s why warming is making fire seasons in Canada, the US, and elsewhere, longer and more severe. Research indicates that wildfires are now burning larger areas, compared to previous decades.

“As the atmosphere warms, the ability to suck moisture out of the fuel [trees and other vegetation] increases almost exponentially,” said Mike Flannigan, a wildland fire professor at the University of Alberta. “So unless we get more rain to compensate for that drying effect, our fields are going to be drier. Most of the models of future fire seasons for Canada look like no change in precipitation or even drier.”

Firefighters spray water on a forest fire in Shelburne County, Nova Scotia, on June 1, 2023.
Communications Nova Scotia /The Canadian Press via AP

That doesn’t mean that the eastern US will be engulfed in smoke every summer — again, the wind patterns have to be just so. Yet it does make such a frightening event more likely. What cities on the East Coast are seeing is very much a warning sign of what climate change can bring.

Rachel DuRose contributed reporting to this story.


Update, June 8, 10:25 am: This story was originally published on June 7 and has been updated with the recent status of the Canadian wildfires and projected wind shifts.

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