As the Sistine Chapel celebrates its 500th anniversary, the Vatican said Wednesday it may have to limit the huge numbers of visitors because pollution is damaging Michelangelo's frescoes, one of the artistic wonders of the world.

The scenes, including the moment in which God reaches out and gives life to Adam with the touch of his finger, are being damaged by the breath, sweat and heat of the 10,000 to 30,000 tourists who walk through the chapel every day.

The Vatican had an air-conditioning system fitted during restorations in the chapel in the 1990s, but critics have warned that it is no longer sufficient to deal with the amount of dust and dirt being dragged in by today's visitors.

Museums director Antonio Paolucci said a specialist company has been asked to design a new air-purifying system - but that if a solution is not found by next year the Vatican will be forced to begin reducing the number of tourists.

Michelangelo spent much of four years on his back painting the chapel's ceiling frescoes, which Paolucci said most tourists could not bear to miss on a tour of Rome, describing them as a "fatal attraction, an object of desire".

The chapel was commissioned by Pope Julius II, who said an evening vespers service on October 31, 1512, to inaugurate the room, and current Pope Benedict XVI will say his own vespers there later on Wednesday to mark the 500th anniversary.

The chapel - which is used by cardinals when they meet to elect a new pope - is one of the most visited sites in the world and guards struggle to enforce an atmosphere of prayer among the hordes of photograph-snapping tourists.

Tourists and pilgrims often queue for hours for a chance to see first hand the 2,500 square metres (27,000 square feet) of colourful frescoes which Michelangelo began in 1508 after designing scaffolding to allow him to reach the curved ceiling.

The Vatican had previously ruled out the idea of timed visits because the chapel is a place of prayer, but the issue flared up again in September, when one of Italy's leading literary critics publicly denounced the situation.

In a protest letter to leading newspaper Corriere della Sera, Pietro Citati said the chapel was an "unimaginable disaster" where tourists behave like "drunken herds" and "any form of contemplation was impossible... in the universal confusion."

"The church needs money for its various activities, but these monstrous conditions are impossible," said Citati.

Paolucci retorted that "the days when only Russian grand dukes and English lords... could gain access to the great masterpieces are definitely over," but on Wednesday admitted the Vatican may not have any choice in the long term.