NEW YORK (AP) — A set of 354 narrow steps spirals all the way up to the Statue of Liberty’s crown, and it’s the only escape route for tourists in an emergency.

On a recent summer day, one tourist put his hands on his knees and gasped for air as a few others funneled down the tightly twisting staircase to the statue’s pedestal. They were covered in sweat.

‘‘It was hot up there,’’ said Lucie Munier, visiting from France. ‘‘I think I would be scared in an emergency, but it is already pretty scary even when it is calm.’’

When a smoke alarm tripped inside the statue last month, hundreds of tourists were rushed down the equivalent of about 15 flights of stairs — the same ones that firefighters would need to trudge up if the 125-year-old landmark catches fire.

National Park Service officials have closed the statue in recent years for a $20 million security upgrade, and kept the crown shut since the Sept. 11, 2001, terror attacks until last year to improve its fire safety. Next year, the statue will close again, for nine months to a year, so workers can build a second stairwell at Lady Liberty’s pedestal, said National Parks Service spokesman Darren Boch.

‘‘Given its age and the fact that it is a historic structure and there’s not much we can do to change it, it’s just not going to be 100 percent in line’’ with the most up-to-date safety standards, Boch said.

Most tourists, 3,000 maximum per day, ascend the first 186 steps from the ground up to the pedestal. No more than 10 people at a time are allowed all the way up to the crown, in part so they can be quickly evacuated if necessary.

Though the statue was built in the 1800s before the days of modern fire codes, there isn’t much flammable material inside. Standing 22 stories high, it’s made from steel and copper as thick as two pennies put together. The staircases are metal. The massive pedestal is stone and concrete.

Elevators and electrical equipment that go up to the pedestal were built much later and are equipped with fire sensors like the faulty one that went off July 21; a faulty smoke head was blamed. There are sprinklers throughout the structure. A standpipe carries water to the top.

‘‘For a structure of that age, it’s actually quite well equipped to deal with an emergency,’’ said Steve Ritea, a Fire Department of New York spokesman.

The fire department keeps a tool shed at Liberty Island. But if a fire alarm sounds, firefighters travel about 20 minutes by boat from a downtown Manhattan firehouse, bringing a hose line and other equipment.

The boat, Marine 1, can pump 50,000 gallons of water per minute, can park alongside the statue, and is equipped with a crane. It can also evacuate tourists if necessary. Other watercraft can respond, and can arrive faster, depending on the extent of the emergency.

The statue is scheduled to close in October 2011, after its 125th anniversary, to create a secondary stairwell down from the pedestal. Right now, tourists go up one side and down another. One elevator is installed for tourists who can’t or don’t want to walk up; firefighters don’t use elevators in emergencies.

The statue shut down after the Sept. 11 terror attacks and reopened in 2004 after the $20 million upgrade. The crown remained off-limits, mostly because the narrow, double-helix staircase could not be safely evacuated in an emergency and didn’t comply with fire and building codes.

A stairwell to the observation area at the pedestal is forgiving, and air conditioned, with handrails and landings where people can rest of they need to.

From there, those with access to the crown ascend into the statue’s body, where it’s considerably warmer, aided by handrails. The shallow steps are 19 inches wide and taper at one end. Head clearance is just a little more than 6 feet.

The parks service is upfront about the difficulty of the climb to the top, warning that only people who can walk unassisted should even attempt it and should drink water at least 30 minutes beforehand on hot days. Tourists often suffer heat exhaustion, shortness of breath, panic attacks, claustrophobia and fear of heights.

The crown remains extremely popular, however: No tickets are available until November.

Neil and Wendy Smith, visiting from England with their children Laura, Dan and George, took their time climbing up to the crown.

‘‘It was nice, but in an emergency, it would be frightening,’’ Wendy Smith said.