Steam pipe blast spews asbestos and creates bus-size crater in the middle of New York City There's "concern whether the debris entered buildings and air conditioners."

An 86-year-old underground steam pipe exploded in New York City Thursday morning, spewing dangerous asbestos-tainted debris throughout the area and leaving a crater in a street the size of a city bus, officials said.

The blast occurred about 6:39 a.m. in Manhattan's Flatiron District, just blocks from the iconic Flatiron Building, according to the New York City Fire Department.

While only five people suffered minor injuries, asbestos contamination was the bigger concern, New York City Mayor Bill de Blasio said.

"There was asbestos in the steam line casing. That's obviously a real concern to us," de Blasio said at a Thursday afternoon news conference near the site of the blast at Fifth Avenue and 21st Street.

The steam pipe was installed in 1932, officials said.

The mayor spoke shortly after getting the results of tests conducted by Con Edison and the city Department of Environmental Protection.

A total of 49 buildings were evacuated, many of them containing residential units, and will require decontamination that could take a couple of days, the mayor said.

"There is real concern whether the debris entered buildings and air conditioners," de Blasio said.

He added: "We have also tested the air since the time of the incident. The air cleared fairly quickly after the incident. So the air in this area now is safe. There is no meaningful presence of asbestos in the air at this point."

De Blasio called it fortunate that the explosion occurred in the early-morning hours before the area was teeming with people and commuters headed to work.

"Thank God no one was seriously injured at that time," he said.

Aerial footage showed a vehicle covered in mud stopped just inches from the crater, which measured roughly 32 feet in diameter, or about three traffic lanes wide.

"It was a loud boom and the building just shook," Byron Chavers, who works in the area, told ABC New York City station WABC-TV.

Five people suffered minor injuries but did not have to go to a hospital, FDNY Commissioner Daniel Nigro said.

"That's the good news for today," Nigro said at a news conference near the scene of the blast.

Twenty-eight of the 49 evacuated buildings were in what officials called "the hot zone" and will likely remain evacuated the longest, de Blasio said, adding that the buildings need to be scrubbed and closely examined for contamination.

The mayor also advised anyone who was in the area and got debris on their clothing to take the garments to a Con Ed center. The utility will compensate people for their clothing, he said.

At least 100 firefighters were being decontaminated and several civilians in the area at the time of the blast may also need to be decontaminated, Nigro said.

The blast sent steam swirling into the air and rattled windows and nerves.

The cause of the explosion is under investigation but the age of the 20-inch steam pipe could be the reason for the rupture.

The explosion also disrupted a gas line, a water main and caused some electrical outages, Nigro said.

A similar steam pipe explosion occurred in midtown Manhattan almost 11 years to the date of Thursday's blast. The steam pipe rupture July 18, 2007, on Lexington Avenue at 41st Street, near Grand Central Terminal, killing a woman, injuring 20 people and creating a 25-foot crater that swallowed a tow truck.