Look: Wildfires in Canada cover New York, other US cities in smoke

Hundreds of uncontrolled forest fires blazed across Canada on Wednesday, threatening critical infrastructure, forcing evacuations and sending a blanket of smoky air wafting over U.S. cities. Schools across the U.S.

A commuter wearing a face mask stands on the Staten Island Ferry while the Manhattan skyline is seen in the background covered by haze and smoke caused by wildfires in Canada, in New York, June 7.

East Coast canceled outdoor activities, airline traffic slowed and millions of Americans were urged to stay indoors as smoke from the wildfires drifted south, blanketing cities in thick, yellowish haze.

Canadian officials say this is shaping up to be the country's worst wildfire season ever. It started early on drier-than-usual ground and accelerated quickly. Smoke from the blazes has been lapping into the U.S. since last month but intensified with recent fires in Quebec, where about 100 were considered out of control Wednesday. [An aerial view of the wildfires in northern Quebec]

The skies above New York and many other North American cities grew progressively hazier through Wednesday, with an eerie yellowish tinge filtering through the smoky canopy. The air smelled like burning wood. Millions were directed to start wearing masks to protect themselves even as wildfires continue in neighbouring Canada.

And with weather systems expected to hardly budge, the smoky blanket billowing from wildfires in Quebec and Nova Scotia and sending plumes of fine particulate matter as far away as North Carolina should persist into Thursday and possibly the weekend.

New York's world-famous skyline, usually visible for miles, appeared to vanish underneath the otherworldly veil of smoke, which some residents said made them feel unwell.

Wildfire smoke has been linked with higher rates of heart attacks and strokes, increases in emergency room visits for asthma and other respiratory conditions, and eye irritation, itchy skin and rashes, among other problems.

The Manhattan skyline is covered by haze and smoke caused by wildfires in Canada, in New York, June 7

Hundreds of forest fires have scorched 9.4 million acres (3.8 million hectares) in Canada so far and forced 120,000 people from their homes in an unusually early and intense start to Canada's wildfire season.