What to know about the Canadian and US wildfires and their impact

Wildfire smoke is expected to continue affecting air quality in Toronto and US cities throughout the week

Cities across north-eastern Canada and the US are suffering from intense smoke brought on by wildfires burning across Ontario and Minnesota.

Residents in New York, Boston and Toronto have been encouraged to avoid "strenuous activity" over potential health impacts caused by the pollution.

Canada wildfires leave train 'encased in flames' as smoke drifts towards US

There are currently 858 wildfires actively burning across Canada – nearly 200 of those in Ontario - according to the Canadian Interagency Forest Fire Centre.

Along the northern edge of Minnesota there are 17 fires that are still burning and an emergency declaration is in place to help mobilise suppression efforts.

In Ontario, the fires are north of Lake Superior in more remote parts of the province, but within or close to national parks and First Nation communities.

So far, six communities are under evacuation orders with reports of damage to properties.

Grand Council Chief Linda Debassige said in a statement on Wednesday that the remote Namaygoosisagagun First Nation in northern Ontario was "devastated by an unexpected and fast-moving wildfire" that severely damaged homes and community buildings.

Wildfires are very common in Canada, but the number of outbreaks has rapidly increased in recent weeks.

The National Interagency Fire Centre in the US and Natural Resources Canada predicted this was likely, external because of sustained hot weather at the end of June across northern Ontario, and below average rainfall, external.

This hot weather is being caused by a heat dome - an area of high pressure that gets stuck trapping warm air and the smoke underneath.

Extreme and long-lasting heat draws more moisture out of soils and plants – this dry vegetation acts as fuel for fires.

But this hot air can also become unstable causing the risk of storms which can produce strong winds, external – causing the fires and smoke to spread.

Satellite images over the past few days show the wind carrying smoke primarily south-east from Ontario across to Toronto, New York State and reaching Boston.

IQAir, a company which tracks global air quality, ranked Detroit, Toronto and Minneapolis as the cities currently with the worst air quality in the world.

Wildfire smoke is hazardous for humans as it contains a mixture of very small polluting particles like PM2.5 and nitrogen dioxide.

"These particles when they get into our system get all the way down into our lungs, all the way down to our tiny breathing