Buildings in lower Manhattan in New York are partially obscured by smoke from Canadian wildfires on Tuesday. Patrick Sison/Associated Press

Uncontrollable flames are ravaging swaths of Canadian forest in what authorities described as a “devastating” wildfire season that could become the worst the country has ever seen.

The United States’ northern neighbor is home to some of the world’s densest forests, and it experiences wildfires every year. But this year, the fires have been particularly widespread, numerous, and intense, burning through more than 3.7 million acres in Canada.

Canada’s government expects “higher-than-normal fire activity” to continue throughout the wildfire season – which typically lasts between April and September – due to a combination of ongoing drought conditions and hot temperature forecasts.

Smoke and haze from the Canadian wildfires have also affected the United States, leading authorities from New York to Minnesota to issue public health alerts and urge people to stay indoors and wear masks to protect themselves from potentially toxic fine particles in the air.

Here’s what you need to know.

Where are the wildfires still burning?

There are 423 active fires across Canada – 246 of them out of control, according to the Canadian Interagency Forest Fire Centre.

The worst-affected province is Quebec, where 154 fires have been recorded so far. There are also 70 active fires in Alberta, which is at the highest level of preparedness, and 68 in British Columbia. Twenty of the 45 new wildland fires recorded Tuesday were in Ontario, according to the center’s statistics.

The eastern provinces of Newfoundland and Labrador and New Brunswick, and the Yukon territory in the west, appear to have escaped most of the fires, however.

Current wildfires 🔥 across Canada via @NRCan https://t.co/YR85GAQAoQ.

Red – out of control
Yellow – being held
Blue – under control pic.twitter.com/PzqScZHEUo

— Dayna Vettese ❄️ (@daynavettese) June 5, 2023

But as in the United States, the impact of the fires has been felt beyond the worst-affected areas: Yellowknife, capital of the Northwest Territories, and the town of Fort Smith, which lies on that territory’s border with Alberta, were both forecasts to be classified “very high risk” due to the poor air quality overnight Tuesday – the highest level on the Canadian government’s index.

The Canadian capital, Ottawa, and the city of Montreal were also forecast to remain at “high risk” on Wednesday – though Quebec City was due to move down into the “moderate risk” category.

What caused the wildfires?

Wildfires typically begin either with lightning or through human activity – for instance, when someone throws a burning cigarette on the ground in a forest or starts an open fire nearby. They become more likely when the ground is dry and the air is hot, which is why wildfire season typically lasts throughout the summer Climate change also plays a role by causing more intense high-pressure zones, which bring prolonged periods of sunny, hot conditions and exacerbate drought.

This year, Alberta, Nova Scotia, and Quebec have experienced record heat. Some parts of Alberta recorded temperatures more than 7 degrees Celsius (12.6 F) higher last month than in a typical May. Canada’s Atlantic Region has been heavily affected by droughts since February, with parts of the region recording “their driest April on record,” according to authorities.

As The Washington Post has reported, the reach, frequency, and intensity of this year’s wildfires are very unusual. Wildfires are normal to an extent across Canada and the western United States in the summertime, but outbreaks as widespread and numerous as these are virtually unheard of in late May into June. The amount of smoke pouring into the U.S. Northeast is also exceptional.

Where is the air pollution worst?

Some of the worst air quality in Canada early Wednesday could be found in the capital, Ottawa, as well as in Toronto, Montreal, and Fort Smith, Northwest Territories, according to government figures. The air in Ottawa has an Air Quality Health Index of 10+, the maximum number, which implies a very high risk to human health. The AQHI measures the concentration in the air of tiny particles of air pollutants known to be harmful to people. The other three cities have an AQHI value of 7, which means a “high risk” to people.

As we continue to monitor the widespread smoke from wildfires in Canada, @NOAA's #GOESEast 🛰️ can see some of it being swept up by a large swirling low pressure system. Numerous #AirQuality Alerts are in effect across the central and eastern U.S.

More: https://t.co/wJGBXDcNu2 pic.twitter.com/sEXwa8CouK

— NOAA Satellites (@NOAASatellites) June 7, 2023

Eastern and northeastern U.S. states had the poorest air quality early Wednesday, according to data from the Environmental Protection Agency. Areas with particularly high concentrations of ozone and particle pollution in the air included Susquehanna Valley, Pa.; Greater New Castle County, Kent County, and Sussex County, Del.; and the Central New York region.

Sustained exposure to high levels of fine particulates contained in wildfire smoke is known to harm people. It can irritate the eyes, throat, and sinuses, causing people to cough and making it hard to breathe normally. The particles cause more risks in vulnerable groups such as older people and those who are pregnant. It can also aggravate people’s existing heart and lung conditions.

How to prepare for a wildfire

If you’re in one of the wildfire-affected areas in Canada, authorities recommend creating an emergency plan – covering issues including safe escape routes, where to meet family or roommates, and health insurance information – as well as preparing an emergency kit with supplies to last at least 72 hours.

The government recommends that people living in potential risk areas remove dried branches, leaves, and other fire hazards from around their homes, ensure they have access to a sprinkler, and check their fire alarms.

Before a fire, it’s also important to make sure you have enough fuel in your vehicle.

During an emergency, keep on top of wildfire news from local media and officials – and call emergency numbers to report a fire approaching your home or community, authorities say.

If it’s safe to do so, close all doors and windows and cover any vents or windows inside your house, and keep the lights on. Place your emergency kit and any valuables inside your car, which should be parked forward out of your driveway.

Do not attempt to drive through a wildfire.