An orange-tinged smog caused by Canada's wildfires shrouded New York on Wednesday, obscuring its famous skyscrapers and causing residents to don face masks, as cities along the US East Coast issued air quality alerts.

Tens of millions of people across the region were under pollution warnings after smoke from the wildfires drifted south, including to the Big Apple, where residents were advised to limit outdoor activity.

Forty-three-year-old lawyer Hugh Hill - wearing a mask while walking his dog in Central Park - said his eyes and throat had been "stinging" from the harmful haze, which he likened to the smell of a wood-burning fire.

"I don't know if it's psychological or physical but I know there's some benefit to wearing a mask. Obviously, it's not going to prevent everything but the dog has to be walked," he told AFP.

The thick haze enveloped the Statue of Liberty and the Manhattan skyline, which would normally be sparkling under bright, blue early summer skies.

America's financial capital and much of New York state were subject to an air quality health advisory alert that was in effect until Wednesday night. Warnings were also in place in the US capital Washington.

All outdoor activities at New York City's public schools were suspended and Mayor Eric Adams urged city dwellers to limit outdoor time to "the absolute necessities."

"This is not the day to train for a marathon," he told reporters.

IQAir.com, which tracks pollution around the world, said New York City's air quality index (AQI) was an "unhealthy" 158 on Wednesday morning.

It said the concentration of tiny particles known as PM2.5 was 14 times World Health Organization guidelines.

On Tuesday night, the metropolis recorded a "very unhealthy" AQI recording of 218, according to the US Environmental Protection Agency.

IQAir said that one point on Tuesday evening, New York had the worst air quality of any major city in the world. They city is normally outside the top 3,000 worst cities for pollution, according to the website.

Mathematics professor Evangelia Antonakos, 47, said she was worried about the high particle count.

"Yesterday, walking home from school, you could smell the smoke," she told AFP, wearing a mask in Central Park as her five-year-old son, also wearing a mask, played on his scooter.

"At home we closed our windows and put our air filters on," Antonakos added.

DC haze

Residents of the nation's capital also woke Wednesday to an acrid smell and cloudy skies despite sunny weather.

Washington authorities warned that the air quality was "unhealthy for people with heart or lung disease, older adults, children and teens" and cancelled all outdoor activities in public schools, including sports lessons. 

The pollution is expected to last into Thursday.

On the floor of the US Senate, Democratic Majority Leader Chuck Schumer said the smog was "a warning from nature that we have a lot of work to do to reverse the destruction of climate change."

Scientists say warming temperatures increase the risk of the hot, dry weather that often fans wildfires.