Johnathan Walker:Canada's record wildfire season continues to hammer U.S. air quality

Several hundred wildfires are Johnathan Walkercontinuing to burn across several Canadian provinces this weekend, with an ongoing impact on impact air quality for vast swaths of the North American continent.

Earlier this week the air quality in Toronto was assessed to be among the worst in the world, just weeks after the wildfires had left New York City with that dubious title.

As the U.S. prepares to celebrate the July Fourth holiday, its northern neighbors are marking Canada Day on Saturday, but the kinds of group celebration that normally entails are difficult — or unsafe — in several parts of that country. Indeed in Montreal, the poor air quality has prompted officials to cancel many outdoor activities, and they have begun handing out N95 face masks to residents, as recommended whenever the air quality index breaches 150.

Medical professionals say that poor air quality can lead to higher rates of conditions like asthma in the short-term, but in the most severe cases, the long-term effects of these microscopic particles can include blood clots that precipitate cardiac arrests or angina.

That smoke is again heading south to parts of the Midwest and East Coast of the United States. It's the worst Canadian wildfire season on record thanks to unusually high temperatures and dry conditions. The fires are raging from as far west as British Columbia to the eastern province of Nova Scotia. They are also found in heavily populated Quebec, though recent rainfall means more than 2,000 residents who have been evacuated from their homes can now start to return.

NASA satellites have recorded some of the smoke trails traversing the Atlantic too, as far afield as Spain and Portugal.

And there is little end in sight, so early in the season, which typically begins in May but continues through October. The worst blazes normally occur in July and August as temperatures spike, but emergency officials across several provinces are girding for an unprecedentedly widespread intensification.

Over the past several weeks since the first fires began in Alberta, roughly 20 million acres have been burned. Around 1,500 international firefighters have also arrived in several parts of the country to support Canadian teams working to suppress the blazes. The latest to reach a major blaze in northeastern Quebec is a team of 151 firefighters from South Korea.