More than 20,000 people in Yellowknife were evacuating the northern Canadian city as fire crews on Thursday battled to keep wildfires from reaching the city – the latest chapter in the country’s worst fire season ever.

Here are some questions and answers about Canada’s wildfires, which have charred millions of hectares and polluted the air in that nation and the U.S.

WHAT PARTS OF CANADA HAVE WILDFIRES HIT?

Wildfires are common in Canada’s western provinces, but this year the eastern provinces of Nova Scotia, Quebec and parts of Ontario have also reeled from out-of-control wildfires.

This week, the focus returned to the west, as wildfires burned about 16 km (10 miles) northwest of Yellowknife, the capital of the vast, sparsely populated Northwest Territories. The fires may reach the city’s outskirts by the weekend, the Territories’ fire information officer, Mike Westwick, told the Canadian Broadcasting Corporation on Thursday.

As of Wednesday, there were 1,054 active wildfires in Canada, including 230 in the Northwest Territories and 669 deemed out of control, according to the Canadian Interagency Forest Fire Center.

There have been 5,738 such fires so far this year, which have burned 13.7 million hectares (33.9 million acres). At least four firefighters have died in Canada.

The previous record wildfire season was in 1989, when 7.6 million hectares (18.8 million hectares) were torched.

This year’s fires started in late April in the western provinces of British Columbia and Alberta, displacing more than 30,000 people at their peak, and temporarily shutting down oil and gas production.