The Vatican Museums are considering putting a cap on visitor numbers amid fears among tour guides that overcrowding could provoke a stampede unless security policy is changed.

The museums, a sprawling structure of 54 galleries containing a vast collection of treasures gathered by pontiffs over the centuries, draw in more than 6 million people a year. But unlike other major Rome landmarks, such as the Colosseum, they currently have no daily limit on visitors.

Speaking to the Guardian on condition of anonymity, tour guides claimed that at least 10 visitors fainted each day as slow-moving crowds filed through the long and narrow corridor that leads to the most popular attraction, the Sistine Chapel, while others have suffered injuries and panic attacks. One visitor was saved by a guard after having a heart attack in February.

The tour guides, who each pay €250 a year for a licence to work there, say conditions are most perilous during peak summer season, when visitor numbers can swell to more than 30,000 a day and temperatures reach as high as 40C. Only the Sistine Chapel has air conditioning.

More than 6 million people visit the Vatican Museums each year. Photograph: Supplied

The building contains a number of emergency exits, but there are only two along the mile-long Sistine Chapel passageway – one at either end.

“The situation is slowly getting back to normal as the high season is over, but from March to October, it is hellish,” said one of the guides.

“Safety is the main problem, because when inside you feel completely trapped, you can barely see your feet. Crowds are one of the most dangerous things; look at what happened in Mecca. It might never happen at the Vatican, but then again it might. That’s what scares me.”

About 3,000 guides work at the museums. Another said she fainted last year and broke her foot. “It’s dangerous for the tourists, for the patrimony and for us,” she added. The same guide looked into suing the Vatican, but lawyers told her it was impossible due to the state having its own set of rules surrounding building security.

In response to the claims, Barbara Jatta, who Pope Francis appointed as director of the museums in late 2016, told the Guardian: “Together with the Vatican governorate, we are strongly working towards fixing the right number from 2019.”

Jatta added that a Spanish firm had been hired to improve the ticketing system and security measures, while work on extending air conditioning to the Raphael Rooms and Borgia Apartments would begin in November. There are also plans to open a second entrance to ease congestion and to extend opening hours on the busiest days of the year. An extra 20 custodians have been hired, enabling previously closed-off galleries to open, while tour guides receive training, with the emphasis on them steering their customers towards lesser-known parts of the complex, such as the Gregorian Etruscan Museum, which will be reachable by elevator from Christmas.

“There are seven miles of corridors – I would advise the guides to take people somewhere instead of the Sistine Chapel,” said Jatta.

But this is the first hint of the Vatican governorate showing serious signs of limiting the number of visitors to its biggest earner. The museums bring in an estimated €100m (£88m) a year, of which half goes to the Vatican state.

The museums also represent the Catholic faith, and the church has in the past been reluctant to close the door to pilgrims who may have travelled to Rome from poorer countries on a once-in-a-lifetime trip, but without booking tickets in advance online. A regular ticket costs €17-€20, while a guided tour that lasts three to four hours is around €70. Private tours can be taken an hour before the museum opens for around €400 per person.

Museum custodians have also complained about safety conditions, writing a letter expressing their concerns to the Vatican’s governorate in 2016.

“They were reprimanded,” said a source at a tourism association in Rome. “At the Vatican Museums the problem is scandalous, but it’s not like situations in Italy where one can protest. We have asked several times for things to change, but they say they have their rules, and so they decide.”

Visitor numbers rose rapidly from 2011 alongside the growth in mass tourism. Antonio Paolucci, the previous director, determined 6 million a year as the upper limit and suggested a cap if the situation became intolerable.

Jatta recognises the huge challenge of ensuring a safe and enjoyable experience for all and protecting the art works from the dust, humidity and perspiration that comes with millions of visitors a year, but also the difficulty of improving conditions in a centuries-old building.

“I understand how difficult it is for tour guides but all our efforts are focused on giving them the best [conditions] in the museums, as it’s also in my interest that they work well and securely,” she said. “We are strongly working towards this.”