NEW YORK >> Fire officials say that the lethal smoke that killed 17 people in a Bronx high-rise fire Sunday probably spread so rapidly because one simple tool for containing just such a blaze failed amid the panic to escape.

Mamadou Wague, who lived with his family in the third-floor duplex where the fire broke out, told The New York Post that in his haste, he forgot to close the apartment door behind him. A self-closing mechanism then malfunctioned, clearing a path for smoke to begin filling the 19-story tower.

Here is why fire officials say that closing the door can be one of the most critical actions people can take to stop the spread and save lives when confronting a fire.

Why does closing the door make a difference?

Fires feed on oxygen. Daniel Madrzykowski, a director of research for the Underwriters Laboratories’ Fire Safety Research Institute, said that when a door is left open it provides a source of air that “essentially acts as a pump” fueling the flames.

Closing doors can cut off the pump, slowly starving a fire of much of that fuel. It can also provide one of the most effective barriers to temporarily inhibit the spread of flames and smoke, giving firefighters crucial time to respond.

“Closing the door limits smoke spread and limits the oxygen that is available for combustion,” Madrzykowski said.

Those benefits are why New York requires that apartment doors in any building with three or more units be outfitted with special hinges to close on their own, and why the city encourages residents to close the doors to bedrooms while they sleep.

A public service announcement produced by the Fire Department and NY1, urging residents to “close the door” in case of fire, won an Emmy in 2000, the year after a pair of deadly fires in city towers.

Still, problems with open doors have persisted.

The city passed legislation increasing oversight over the doors in 2018, in the aftermath of another residential fire in the Bronx that killed 13 people the previous year. During the fiscal year that ended in June, the Department of Housing Preservation and Development issued 22,000 violations for self-closing doors, 18,000 of which have since been closed as corrected.

So what happened in the Bronx?

City officials said that the fire in the Bronx started when an electrical space heater in a third-floor duplex burst into flames Sunday morning. Residents in the unit rushed out, but the self-closing door to the apartment failed to shut behind them.

The open door allowed oxygen to flow in, feeding the growing fire, and allowed thick, heavy smoke to escape into the rest of the building.

Fire officials said they found that the fire had actually barely escaped the apartment before it was put out. (The tower did not have buildingwide sprinklers.) But when a door to the stairwell was left open on the 15th floor, it created “a flue effect, like a chimney,” a Fire Department spokesperson said, rapidly pulling smoke upward.

Residents on higher floors trying to escape grew sick from smoke inhalation, some of them fatally. Others frantically struggled to stop smoke from seeping through the cracks under their doors.

Jeff Kimble, an assistant professor of fire safety at the University of North Carolina Charlotte, said smoke “behaves like a fluid would,” finding its way through small openings and into crevices. But because smoke is hot, lighter than the surrounding air, he added, “it has a propensity to want to rise.”

Newer high-rise buildings often have features to reduce the spread of smoke, like stairwells with higher air pressure and dampers that shut down parts of heating and air-conditioning systems. But most older buildings, like the nearly 50-year-old Twin Parks North West, do not.

City officials have not yet said precisely why the apartment door failed to close. Kelly Magee, a spokesperson for the property owners, said the door had been working properly as of last July, when maintenance workers who came to fix a lock inspected it, and there were no outstanding complaints.

Investigators said the door had not been obstructed, but one official did note that some of the doors to other apartments did not automatically close when marshals tested them after the fire.

There could be several reasons.

Springs or pistons in the closing mechanism can malfunction if they become dirty, worn out or disabled, said Rick Chandler, who was the city buildings commissioner from 2015 to 2019.

Some of the mechanisms are designed to function up to a certain angle, after which the doors may fail to close on their own. For example, a sample of hinges sold online are designed to close when doors are opened up to 94 degrees for one model and 120 for another.

How do I decide whether to flee or stay put?

Deciding whether to stay or go comes down to two critical factors: how close the fire is to your apartment, and whether your building is fireproof.

If the fire is in your apartment, the fire safety rules are unambiguous, regardless of what type of building you live in: Get out of the building, fast, and make sure to close all doors behind you. The New York Fire Department suggests you knock on the doors of your neighbors as you leave, warning them of the blaze.

If the fire isn’t in your apartment, deciding whether to stay or go comes down to what type of building you live in. In more modern and legally fireproofed buildings, fires are more likely to be contained, thanks to fireproof building materials and doors that automatically close, sealing off the fire in smaller spaces.

If you live in a fireproof building and the blaze is not in your unit, the Fire Department almost always recommends staying put, sealing off your apartment and calling for help. Depending on where the fire is in your building, trying to escape could be dangerous. Particularly if the blaze is on a lower floor, you risk being caught in smoke filled halls or stairways, given the propensity of smoke and heat to rise.

“We do recommend in high-rise, fireproof buildings that people should shelter in place,” the city’s fire commissioner, Daniel A. Nigro, said Monday. “It’s safer to be in your apartment than to venture out and try to get down the stairs, and sometimes into a much more dangerous situation.”

If you live in a nonfireproof building, regardless of where the fire is, the Fire Department recommends leaving the facility immediately.

What else should I do?

If you’ve determined the best course of action is to stay in your unit, the Fire Department recommends keeping your doors closed and sealing doorways with duct tape or wet sheets and towels.

As long as the fire is not directly below you, you should crack open a window and let fresh air into your space. Call firefighters, let them know where you are and describe the conditions.

Before any of that, though, the Fire Department strongly encourages residents to familiarize themselves with their building’s fire safety plans. Ask your management company what type of building you live in, and what the legally-required fire safety exit plans are for the complex.

Every owner of a building with three or more units is legally required to have fire safety plans posted inside every apartment door and to redistribute the plan annually.

“Prevention and planning is the key to protecting your family and neighborhoods,” the department’s brief on apartment fire safety says.

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This article originally appeared in The New York Times.

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