Jessica Leighton, a deputy commissioner at the Department of Health and Mental Hygiene, also cautioned people inside buildings near the blast to close windows and set air-conditioners to recirculate the air inside instead of drawing in air from outside.

As people on the sidewalks scrambled to flee the blast and office workers in the buildings above looked down in horror, debris from the geyser pelted nearby skyscrapers. Then it rained on the streets like a sudden hailstorm. Some witnesses said the jet of steam roared like Niagara Falls.

Some people ran so fast their shoes came off. Others dropped their briefcases and purses. Men in tailored suits were caught in a lapel-singeing cloud. At a health club high up in the Grand Hyatt hotel next to Grand Central, people working out on the treadmills said the explosion was so powerful they worried the building would collapse. The steam shot up from a crater that looked like that of a volcano, with orange flames and bubbling mud around the edges. The explosion packed enough force to flip over a tow truck that ended up in the crater, which was about 35 by 40 feet. Several hours after the blast, officials said the crater could grow even larger because pavement at the edges was in danger of collapse.

Image The injured were treated on 42nd Street between Fifth and Madison Avenues. Credit... Hiroko Masuike for The New York Times

The cloud of steam — and the hail of debris that followed — lasted more than two hours and immediately raised concerns about asbestos, which was used when the pipe was laid in the 1920s. Officials advised people who had been in the neighborhood to discard their clothes and bathe carefully.

The mayor said the explosion appeared to have been caused by cold water that reached the pipe, which measured more than a foot and half in diameter and dated to 1924. “Cold water apparently causes these to explode,” he said.

Con Edison, which maintains the steam pipes beneath the city’s streets, said the pipe ruptured at 5:56 p.m. Kevin Burke, the chairman of Con Edison, said crews had checked the pipe after the thunderstorm that soaked the city in the morning. He said a heavy rain can cause a “vapor condition” if rainwater seeps onto a steam pipe, causing the steam to condense. He said the inspection earlier in the day had given no indication that anything was amiss.