A steam-pipe explosion sent asbestos spewing from a crater in Manhattan’s Flatiron District, Mayor Bill de Blasio revealed Thursday afternoon.

Testing found the potentially deadly carcinogen near the scene of the blast, de Blasio said during a news conference nearby.

Nearly 50 buildings — including some with apartments — have been evacuated, and no one will be allowed to re-enter until they’re deemed safe, he said.

The buildings include 28 in what’s being called a “hot zone” at greatest risk of contamination.

Another 21 are located in a “warm zone” that’s farther from the site of the blast, which took place around 6:45 a.m. near Fifth Avenue and 21st Street.

Dr. Herminia Palacio, the city’s deputy mayor for health and human services, said there was only a “very, very low risk from any one-time, limited exposure” to asbestos.

But she said it was crucial to “reduce any long-term exposure,” and de Blasio said everyone who was nearby during the blast would have to throw out the clothes they were wearing — regardless of cost.

The mayor said officials were also concerned that asbestos fibers may have been sucked into the heating, ventilation and air conditioning systems of buildings in the area.

Crystal Johnson, who lives on East 21st Street with her two young children, said she is worried about their health due to potential ­asbestos exposure.

“I am very angry but I am just trying to keep my composure and be calm,” she told The Post at a school emergency shelter set up by the Red Cross and city’s Office of Emergency Management.

Asbestos “can give you tumors and block your airway . . . It is very scary. My head is just spinning around and I’m trying to grasp all of this,” she said, comforting daughter Melina, 10, and son Tristen, 6.

Police officers in protective suits entered the area of the heaviest potential contamination to retrieve pets and medication for evacuated residents.

City officials said that as of 6 p.m., 44 buildings were still being inspected for potential exposure to the cancer-causing substance.

Mayor de Blasio at an afternoon press conference said the closures, inspections and evacuations may last up to two days.

An estimated 50 ground-floor businesses and stores either couldn’t open or had to shut their doors Thursday. Impacted businesses ranged from popular and eclectic homeware store Fishs Eddy to furniture chain stores like Ethan Allen.

“We understand this is very much still an emergency-response situation,” said Jennifer Brown, the executive director of the Flatiron-23rd Street Partnership. “I know they’re assessing damage to the buildings and environmental issues.”