Michelangelo's Sistine Chapel ceiling frescoes turned 500 on Wednesday, Oct 31, with the Vatican warning it may eventually limit visitors to protect one of the wonders of Western civilization, Reuters reported.

The Sistine Chapel is arguably the most visited room in the world.

With mass tourism growing, every year some five million people, as many as 20,000 a day in summer, enter the chapel, crane their necks upwards. Most are left awestruck.

Earlier this month, Italian literary critic Pietro Citati sparked a storm by writing an open letter in a major Italian newspaper denouncing the behavior of crowds visiting what is technically a sacred place.

Tourists, he said, "resemble drunken herds" as they unwittingly risked damaging the frescoes with their breath, their perspiration, the dust on their shoes and their body heat.

The atmosphere, Citati wrote, was anything but contemplative as the tourists ignored the Vatican's requests for silence, composure and a ban on taking photographs.

Under the current system, visitors to the Vatican museums can either book times to enter or wait in long queues outside, but there is no cap on the total daily number.

In 1994, at the end of a 14-year restoration project, technicians installed an elaborate system of dehumidifiers, air conditioning, filters and micro-climate controls in the chapel.

But the number of visitors has grown in the past 18 years, putting the system under stress.