Nadine Yousif
Reporting from Toronto

The US-Canada relationship was already tense, and the wildfires aren't helping.

The relationship between the two longtime allies has been strained since last year when Donald Trump imposed tariffs on Canada - a country that had enjoyed decades of free trade with the US.

Canadians have also taken offence at Trump’s musings that their nation should become the 51st state of the US. They even stopped travelling to their southern neighbour in protest, although the numbers of visits have since rebounded slightly.

Frustrating above all for Canada is its inability to reach a trade deal with the US that it deems fair.

Talks are still ongoing on the future of North American free trade, and there is pressure on Prime Minister Mark Carney to deliver a deal that will relieve key Canadian sectors like steel and autos from punishing US tariffs.

Now, Trump is threatening more tariffs - this time in response to wildfire smoke that is drifting from blazes burning in northern Ontario into the US.

Carney has not yet responded to the latest from Trump, but he did say yesterday that he believed fighting climate change is the responsibility of all countries, including the US.

Meanwhile, Ontario Premier Doug Ford noted earlier today that Canada has helped the US fight wildfires in California, and that it would welcome similar help in return.

You can read more about the threat of tariffs and the wildfire back and forth here.

We are ending our coverage of the fires and US air quality now. Thank you for joining us.

Ian Aikman
Live reporter

How many cigarettes would have the same impact on your health as breathing in smoke from wildfires?

You may have seen some attempts at an estimate. But an expert I spoke to says she doesn’t think the comparison is useful.

"It’s not that comparing it to cigarettes is not helpful at all, but it may lead people to think that it’s not as harmful as it potentially is," says Dr Gillian Goobie, a researcher in respiratory medicine at the University of British Columbia.

"Very, very robust data" shows that poor air quality, especially above AQI 100, is associated with risk of death and risk of adverse heart and lung outcomes, she says.

But it’s not clear whether wildfire smoke is more comparable to cigarette smoke or to other pollution from vehicles or industry.

There’s another key difference in how the smoke itself is inhaled. Smokers breathe in high concentrations of tobacco smoke multiple times a day, usually over many years. People exposed to wildfire smoke, on the other hand, can reduce their dosage by taking action, such as avoiding intensive outdoor exercise.

"The dose is going to be dependent on how long you’re outside and how heavily and deeply you’re breathing," Goobie says. "Whatever we can do to reduce those exposures during those very high intensity periods is likely to have positive benefits."

Rebecka Pieder
Reporting from Washington DC

Yesterday's air in Chicago was "immediately suffocating", Chicago residents say, but today is much better.

"I thought there was a fire in the neighbourhood," says student and Chicago resident Sarah Force about Thursday.

She put a mask on once she realised just how bad the air was, describing it as a gray smoky fog.

She wore a mask again today when she went out for errands but found it to nowhere near as bad.

Fellow resident StephanieJarosz says "you could feel it almost entering your nasal passages and your lungs".

Jarosz's lungs tightened and she found the air smelled like a "bonfire that had been going on for too long."

The daycare her daughter attends had informed her it was "not going to do any outdoor activities because of the quality of the air," both today and yesterday.

But Jarosz isn’t too worried about the pollution.

“I live in a city already," she says. "I'm assuming there's some level of pollution that I'm around all the time.”

Rebecka Pieder
Reporting from Washington DC

This is "probably one of the worst smoke pollution episodes on record for the area," Washington DC meteorologist Jason Samenow tells the BBC.

"We're under a Code Purple air quality alert," says Samenow, "which is, you know, signifying very unhealthy air."

Meteorologists seemingly follow the rainbow in rating air quality. Just one step beyond purple ("very unhealthy") is the highest level, maroon, signifying "hazardous". On the other end, green is good.

"The more pollutants in the air, the worse the air quality," Samenow, whose weather reports are followed closely by many throughout the region, says. "It's just based on the concentration of pollution" in the atmosphere.

The level of smoke and pollution shrouding the US capital area is due to the proximity and intensity of wildfires burning in Ontario.

And Code Purple "means that pretty much anybody who goes outside is going to feel it and should take precautions", he says.

For this reporter, stepping outdoors in Washington for even a few moments today brought on itchy eyes and a dry throat, and made me feel lightheaded.

But relief is on its way for Washington DC.

"There's a big cold front coming in tomorrow, and that's going to help push a lot of the smoke away," Samenow says. "I think we're going to see improving air quality by late Saturday and into Sunday."

Image source, Getty Images

Zaineb Sharif
BBC News

Image source, Getty Images

I am expected to fly out from Washington, DC to Chicago in the evening.

I just received this message from the airline: "Your flight from Washington to Chicago is delayed due to weather conditions that affected a plane assigned to your flight."

Poor air quality can often create low-visibility conditions at airports and can cause flight delays.

Chicago and Washington, DC topped the list for the worst air quality in the world today - so I'm not entirely surprised.

The air quality has shifted slightly in North America this afternoon.

At 222, the air in Detroit is still the most polluted in the world right now, according to IQAir.

Washington DC ties with Kinshasa, Democratic Republic of Congo for second place, at 175.

Chicago, which was higher up the list this morning, has seen air pollution decrease throughout the day. Now, the city sits at 8th place with 136. Chicago now has a Code Orange designation, meaning that the air is "unhealthy for sensitive groups"

New York City has dropped way down the list as the smoke has cleared, and is now in the Code Yellow category: "moderate" air quality, at 88.

Auckland, New Zealand, continues to be the city with the least air pollution in the world. Air quality 8.

New York City's famous landmarks, including the Empire State Building and Statue of Liberty, are obscured by smoke brought south from Canada's wildfires.

Those braving the outdoors are donning masks, with air quality warnings advising people to mostly remain indoors.

Image source, Getty Images

Image source, Getty Images

Image source, Getty Images

Wildfires are not only a problem in eastern Canada. Over on the country's Pacific coast, more blazes have been reported, according to the British Columbia Wildfire Service.

There are currently 59 active fires in British Columbia, the agency says, adding that 39 of the fires started in the last 24 hours. More than half (31 fires) are out of control, officials say, meaning firefighters are using more aggressive means to control them.

Thunderstorms in the forecast this week may also bring lightning strike-related blazes, the agency says.

US President Donald Trump is hitting out at Canada for the wildfire smoke wafting south into the United States.

"We are holding Canada responsible for the fact that they are not properly maintaining their Forests, and Brush therein, and the United States is being unnecessarily invaded by filthy, polluted, and unhealthy air, the quality of which is dangerous, and totally unacceptable!" he wrote on his Truth Social platform.

Trump says he will call Canadian Prime Minister Mark Carney to demand an explanation over his country's "willful negligence" that has imposed an "incalculable cost" on the US via the smoke.

The "cost of this pollution must of necessity be added to the TARIFFS Canada is currently paying", Trump threatens.

Katie Williams
Live reporter

Image source, Francia Garcia Hernandez

Conditions in Chicago have improved slightly compared to yesterday, when you could see smoke "filling entire blocks" as you drove along the highway, says Francia Garcia Hernandez, a 33-year-old journalist who works in the city.

But things are still "very smoky and hazy", she tells me.

Garcia Hernandez has lived in Chicago for about seven years, and she says yesterday's smog was the worst she's ever seen. High-rise buildings were barely visible and smoke was "filling the air".

The smoke has impacted day-to-day life in the city, she says, with some local events cancelled, people wearing masks and fewer people exercising outdoors.

She adds that the conditions raise health concerns for residents, noting she had a slight headache yesterday after spending some time outdoors. While she can't be sure it was related, some health experts have warned headaches can be one potential smog-related symptom.

Blaming Canada for wildfire smoke misses the bigger picture, experts saypublished at 20:14 BST 17 July

20:14 BST 17 July

Image source, Getty Images

by Marco Silva

Some Republican politicians in the US have blamed Canada for failing to properly manage its forests and prevent the wildfires causing smoke to drift across the border, but scientists BBC Verify has spoken to say the picture is more complicated.

“Weather doesn’t care about international borders,” says Patrick James from the University of Toronto.

Once smoke reaches the atmosphere, it travels wherever the winds take it - and smoke from major US wildfires has also affected Canada in recent years.

Experts also say many of the current fires are burning in Canada's vast, remote forests, where fires can be difficult to detect or contain before they become too large.

While better forest management can reduce wildfire risk in some areas, particularly near communities, it cannot prevent fires across an ecosystem of this scale.

Scientists also say increasingly severe wildfire seasons are being driven in part by climate change, which is creating hotter, drier conditions that allow fires to spread more easily.

“Climate change is a global issue, and it would be inaccurate to suggest that Canada alone caused or could have prevented these wildfires,” says Anabela Bonada from the University of Waterloo.

Madeline Halpert
Reporting from Detroit, Michigan

Image source, Madeline Halpert / BBC

I’m here in Detroit, Michigan, where the air has been awful for the past two days - the worst in the world right now. The smoke in the air burns your eyes and throat when stepping outside even for a few minutes.

Very few people are outside, and many of those who are are wearing N95 masks. The smoke is supposed to improve tomorrow, and is better than the even thicker haze from yesterday, but most are still hunkering down indoors.

Image source, Madeline Halpert / BBC

Bernd Debusmann Jr
White House reporter

The inclement weather has sparked concerns for Sunday's World Cup final between Argentina and Spain at MetLife Stadium in New Jersey - and the White House is watching.

The BBC understands that White House officials - including Andrew Giuliani, the head of the White House's World Cup task force - is holding informal meetings with FIFA officials and others monitoring the wildfires. No larger, formal meeting has so far been planned, though.

Forecasters believe that rains over the weekend will improve conditions before the two teams take the field at 1500e/2000b.

Later today, President Donald Trump is expected to attend a FIFA reception, Press Secretary Karoline Leavitt told press yesterday.

Trump will also be in attendance at the game on Sunday.

Katie Williams
Live reporter

I've just spoken with Matthew Hoppe, incident commander for the Namaygoosisagagun First Nation - a community devastated by the wildfires in northern Ontario.

He says the community had to "self-evacuate" via small boats on Monday afternoon with the fire rapidly approaching.

Thankfully, there were no deaths or direct injuries, he says, but the community has been "completely levelled" and people are "distraught, overwhelmed, and at a complete loss".

Hoppe says the fire tore through with very little advance warning. After "repeated pleas for help", residents eventually had to flee.

While there have been other forest fires in the region in the past, the community has not been directly impacted in this way before, he says.

In terms of what's next, Hoppe says the community is speaking with the government to "firm up resources". The longer-term plan is ultimately to "rebuild and return".

Image source, First Nation via Matthew Hoppe

Stav Danaos
BBC Weather

It's difficult to say how long these Canadian wildfires will burn for, but many of the current wildfires are likely to go on for weeks. Some could even be burning for months, especially in remote boreal forests.

Very often, large boreal forest fires aren't fully extinguished by firefighters at all. Instead, fire crews focus on protecting communities and critical infrastructure while allowing fires in remote areas to burn until weather changes. Heavy widespread rain or early autumn snowfall are often weather events that finally stop them.

Canada is still in the middle of its wildfire season, with July and August typically the most active months. Officials expect fire danger to remain elevated throughout much of the rest of the summer in many regions.

There are more than 190 wildfires burning in Ontario at the moment.

They are the source of most of the smoke that swept through Toronto on Wednesday and Thursday on its way to major US cities like New York and Washington.

On Wednesday, Toronto had the worst air quality in the world, according to Swiss air quality monitor IQAir (Detroit, Michigan, took that spot on Friday).

Here are some scenes from the past two days.

Image source, Getty Images

Image source, Reuters

Image source, EPA

Image source, Getty Images

Image source, Getty Images

Image source, Getty Images

New York City is handing out free N95 masks as authorities urge residents to protect themselves from the poor air quality being brought on by the Canadian wildfires.

The masks are being distributed from all libraries and all police precincts, where the city advises masks are limited to two per person.

Firehouses in the Bronx, Brooklyn, Manhattan, Queens and Staten Island are also giving them out.

(Masks have also been handed out to staff here at the BBC's Washington DC office.)

If you’re in an area impacted by wildfire smoke today, here are some tips on how to reduce health risks:

• Stay inside as much as possible, with windows closed

• Avoid strenuous outdoor activities

• Wear a well-fitting N95 mask when outside and keep windows closed in cars

• Monitor existing health conditions closely and talk to your doctor if symptoms worsen

This list was compiled from tips shared by health organisations like the American Thoracic Society, the Centers for Disease Control and Prevention (CDC) and the Environmental Protection Agency (EPA).

You can find specific government advice for your area via the EPA’s AirNow air quality checker, external in the US and the Air Quality Health Index, external in Canada.

Bill McKenna
Picture editor

There are so many days here in Washington DC where the air is clear and I can see the National Cathedral up on the hill from our rooftop - but not today.

These comparison photos give a sense of the conditions currently being experienced, as wildfire smoke blankets parts of the US and Canada.

Image source, Bill McKenna / BBC

Image source, Bill McKenna / BBC