By Tyler Clifford

NEW YORK A smoky yellow haze generated by hundreds of Canadian wildfires hovered over a large swath of the United States on Thursday, threatening to make breathing difficult and disrupting air travel for millions of people for a second full day.

The U.S. National Weather Service extended air quality alerts for another day for the East Coast from New England to South Carolina, as well as parts of the Midwest, including Ohio, Indiana and Michigan.

Advised to stay indoors, residents of New York, Washington and other big cities donned masks and scrambled to buy air purifiers to protect themselves from the yellow haze that arrived in the region on Tuesday afternoon.

The haze and low visibility led aviation officials to halt incoming flights to New York's LaGuardia Airport from the Northeast, Mid-Atlantic region and Ohio for a second day on Thursday morning.

Among major world cities, New York again had the worst air quality on Thursday morning with an unhealthy Air Quality Index reading of 185, according to a website operated by IQAir, a Swiss technology company. Detroit ranked third worst with a reading of 154.

The U.S. Air Quality Index (AQI) measures five major pollutants, including particulate matter produced by fires. Readings over 100 are classified as "unhealthy" and those exceeding 300 are "hazardous."

The air in many localities in Maryland, Pennsylvania and Delaware was even worse, with readings exceeding 300. Some suburbs of Baltimore and Philadelphia topped 400.

Conditions appeared to worsen in the Washington area on Thursday, with a smoky residue more visible and an acrid smell of burning wood more noticeable in the early morning.

"This problem is likely to continue or worsen through Friday," Washington Mayor Muriel Bowser said in a tweet. "We urge residents and visitors to follow precautions related to the 'Code Purple' air quality alert."

Large swaths of Michigan are also under red flag warnings due to dangerous fire weather conditions in both of the state's peninsulas, according to the weather agency.

Smoke crossed over the U.S. border from Canada, where hundreds of forest fires have scorched 9.4 million acres (3.8 million hectares) and forced 120,000 people from their homes in an unusually early and intense start to the wildfire season.

Health officials in more than a dozen states have warned that spending time outdoors and breathing the polluted air could cause respiratory issues due to high levels of fine particulates in the atmosphere.

The thick haze and soot stretching from high elevations to ground level marked the worst outbreak of wildfire smoke to blanket the U.S. Northeast in more than 20 years, according to the private forecasting service AccuWeather.

New York's skyline, which usually can be seen for miles in typical weather, was partially obscured by the haze on Thursday morning.

On Wednesday, schools in many cities and towns canceled outdoor activities, while some stores closed up shop early and major sports leagues postponed games.

(Reporting by Tyler Clifford; editing by Jonathan Oatis)