Canada wildfires: How long does wildfire smoke last and is it dangerous?

Smoke from hundreds of wildfires burning in Canada has triggered air quality alerts for millions of people across the northern US and Canada as the smoke drifts south.

Tens of thousands of people have been evacuated in Canada due to the fires, which have so far burned more than 4 million hectares this year. The resulting smoke has created a grey haze over many cities, including New York, and in some places it has blotted out the sun.

The fires have also released a record amount of carbon into the atmosphere for May, which is still early in the North American fire season. Hotter and drier conditions due to climate change are expected to make such fires bigger and more intense.

Where is the smoke coming from?

As of 6 June, more than 300 fires were burning across Canada, more than 200 of which were out of control, according to the Canadian Interagency Forest Fire Centre. The eastern province of Quebec has had the most fires. Numerous fires were also burning in the western provinces of Alberta and British Columbia. Much of the smoke affecting the northern US was drifting south from the fires in the east.

Many of the fires started burning days ago, and the fires in western Canada have been burning for weeks. But large amounts of smoke only started reaching population centres in the US after a storm system formed on the Atlantic coast and started blowing smoke south.

Smoke from the fires has also reached Europe, says Albert Ansmann at the Leibniz Institute for Tropospheric Research in Germany. “We’ve seen the smoke since the fire season started” at nearly every layer of the atmosphere, including the lower stratosphere, he says. Wildfire smoke has been shown to deplete the ozone layer when it reaches that high.

Some smoke has also reached parts of the Arctic Ocean, where it can coat sea ice and accelerate melting.

How long will wildfire smoke last and when will it go away?

It could be weeks or more until the wildfires themselves are brought under control. But the weather system carrying the smoke into the northern US could change within the next several days. Large areas will probably remain smoky until then.

Is air pollution from wildfire smoke dangerous?

Air pollution from wildfire smoke largely has the same health effects as other types of air pollution, says Patrick Kinney at Boston University. These are mainly due to the fine particulate matter, or PM2.5, in the smoke, which can irritate the eyes, nose and throat, and can be particularly bad for people with asthma or heart disease, or for young children and elderly people.

Kinney says even a few days of such high levels of air pollution is enough to exacerbate existing respiratory problems and possibly cause new ones. “I think there will be observable spikes in people going to emergency rooms,” he says. “The longer that goes on the more likely that is.”

State health advisories recommend staying inside as much as possible, keeping windows shut and limiting the use of air conditioning. In smaller rooms, air purifiers can effectively reduce pollution. Wearing a high-quality mask if you do go outdoors can also help filter particles in the smoke. “I’d probably be wearing a mask most of the time,” says Kinney.

The New York Department of Environmental Conservation also advises people to avoid driving and to reduce energy use if possible in order to limit added air pollution for car exhaust and power generation, though this could only have a small effect overall.

What drives high levels of air pollution elsewhere?

At one point on 6 June, New York City was ranked as having the worst air quality in the world by the Swiss website IQAir. Detroit, Michigan, was also ranked in the top 10. The smoke from these fires will eventually blow over these cities and the rest of the northern US, but most of the other cities high up on the list deal with similar levels of air pollution nearly all the time.

At the top of the list, Delhi, India, has high levels of air pollution primarily driven by exhaust from inefficient vehicles and road dust from construction, as well as power plants burning fossil fuels and farmers burning plant waste. In Santiago, Chile, also among the top 10, burning wood to stay warm in winter is another source of air pollution.

Such chronic exposure to high levels of air pollution can have wide-ranging health effects, such as increased rates of respiratory disease.