Dense wildfire smoke billowing down from Canada set off unhealthy air quality alerts across the United States again Friday, fomenting concern over the weekend's World Cup final outside New York.

New York City has been dealing with a "Code Red" due to the heat and air quality advisories from Canadian wildfire smoke days before the Spain-Argentina World Cup final.

As thick smoke settles in over major cities, Canada recorded the worst air quality in the world.

Authorities issue health warnings after smoke from wildfires in Canada envelops north-eastern US