The wildfires burning in Canada have sent the noxious smog billowing across the border, with residents in New York, the US capital Washington and the American Midwest urged to stay indoors due to dangerously low air quality.At one point on Saturday, the smoke made New York the most polluted city in the world, followed closely by Toronto and Washington, according to air tracker IQAir.The marquee World Cup match, pitting Argentina against Spain, will be played Sunday in an open-air stadium across the river from Manhattan, where the skyline was obscured by dense smoke and people wore masks outdoors.Tournament organizers are "monitoring closely," White House World Cup task force executive director Andrew Giuliani told a briefing.But a respite of sorts was expected to come in the form of heavy storms hitting the Big Apple on Saturday, which could wash some of the smoke away -- but also trigger flash flooding and dangerous winds.New York Mayor Zohran Mamdani asked residents to stay indoors and avoid travel. "Thunderstorms are expected to bring damaging winds strong enough to down trees and power lines, along with heavy rainfall that could cause flash flooding," Mamdani wrote on X."Smoke will continue to affect the area through this afternoon," the US National Weather Service said.