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

Detroit and Chicago posted air quality index readings in the "hazardous" range, with tracker IQAir saying they were the top polluted cities in the world.

Winds drifting south meant the capital Washington was also hard-hit, coming in at the second-highest "very unhealthy" ranking on the index, when authorities urge all people to avoid unnecessary outdoor activity.

World Cup organizers said Friday they were "monitoring closely" the smoke gripping swaths of the United States ahead of the tournament's weekend final in the New York - New Jersey final which has been affected by dangerously unhealthy air.

"There's been discussion about it, and we have somebody with the National Weather Service that sits in FIFA headquarters there, so we're monitoring closely," Andrew Giuliani, White House World Cup task force executive director, told a briefing.

Subscribe to beIN SPORTS!