A “frozen zone” was in place from 42nd Street to 43rd Street and from Second Avenue to Third Avenue in Manhattan on Tuesday afternoon, according to a Police Department spokesman, halting vehicle and pedestrian traffic in response to a building on the block that was unstable and at risk of partial collapse.

The zone was initially larger, from First Avenue to Third Avenue and from 40th Street to 45th Street. Mayor Zohran Mamdani said at a news conference earlier in the day that it had been established as a safety precaution near 235 East 42nd Street, the affected building.

Only emergency workers essential to shoring up the building were being allowed into the area, the police spokesman said.

Here is what else to know.

What is a frozen zone?

Although city officials did not say so, the frozen zone that was initially established appeared comparable to what the National Institute of Occupational Safety and Health refers to as a “collapse zone,” which the agency recommends creating “when there is potential for a structural collapse during firefighting operations.”

The institute defines such a zone as an area “around the perimeter of a structure that could contain debris if the building collapsed,” and it is often equal to one-and-a-half times the structure’s height.

How is public transportation affected?

The Metropolitan Transportation Authority announced that buses operating on or around Second Avenue were delayed or had been diverted because of a street closing near the damaged building.

As of 4 p.m., several bus routes, including the M42, M15, M15-SBS and northbound buses on First Avenue, continued to be affected. For the latest information, riders were advised to visit the M.T.A.’s website.

Subway service on the 7 train, the nearest subway line, was not affected.

How long will the zone remain frozen?

It was unclear how long the restrictions would be in effect. Emergency responders, structural engineers and project engineers were working to reinforce the affected parts of the building.

“Operations are ongoing,” the Fire Department said. “We have no estimate at this time.”

What if I live or work in the area?

Brad Weekes, the Police Department’s assistant commissioner of public information, said that people could leave the area but could not re-enter it while the restrictions were in place. Only emergency workers involved in stabilizing the building would be allowed in, he said.

Stefanos Chen and Davaughnia Wilson contributed reporting.