The Sistine Chapel’s 1993 air system was built for 700 people. Its visitors were the threat.

In October 2014 the Vatican Museums switched on a new climate system in the Sistine Chapel, the first major overhaul since the early 1990s. The reason was not the age of the equipment but the size of the crowd. The air-conditioning installed in 1993 had been designed for a maximum of 700 people in the chapel at once. By 2014 the chapel was receiving close to 20,000 visitors a day in peak season, and annual attendance had exceeded five million since 2011. The replacement was sized for up to 2,000 people inside at the same time, roughly three times the capacity of the system it replaced.

The concern was explicitly the visitors. Antonio Paolucci, then director of the Vatican Museums, described the crowd as something that "emanates sweat, breath, carbon dioxide, all sorts of dust", and warned that better tools for controlling temperature and humidity were needed to avoid serious damage to Michelangelo’s work. Reuters reported in June 2014 that he had said a cap on visitor numbers might become necessary; at the time there was no daily limit, only a queuing system. Trade coverage of the project put it the same way: up to 20,000 tourists a day bringing humidity and dirt into a room whose painted surfaces are especially vulnerable to modern pollutants.

The mechanism is ordinary physics applied to an extraordinary room. Every person is a small, steady source of heat and water vapour, and carries dust in on clothes and shoes. Warm air rises, so a dense crowd sets up its own circulation: plumes climb from the floor, spread along the underside of the vault, run down the walls and are drawn back in over the heads of the next group. In a tall, enclosed space the painted vault sits at the top of every lap of that loop. Warm, damp, particle-laden air reaching the plaster again and again is exactly the condition conservators try to avoid: moisture cycling in the surface and fine dust settling onto it.

Nothing about this shows up on a normal comfort check. A thermostat at head height can read 22 °C while the conditions at the ceiling, twenty metres up, are set by the crowd’s plumes rather than the supply air. A system can also meet its design numbers perfectly and still fail, because the design number, 700 occupants, stopped describing the room. The published design intent of the 2014 system addresses both points. Targets of 20–25 °C and 50–60% relative humidity are held, sensors and two people-counting cameras let the air handling follow the actual occupancy, and the airflow is arranged so that, in the words of the International Institute of Refrigeration’s report, the ceilings and walls are bathed in temperate air while "visitors, meanwhile, are showered with a higher velocity air flow that keeps dirt and humidity at floor level." Carrier states that its team used computer simulations of the chapel’s airflow in the design.

Zephyr’s simulation of a simplified chapel cross-section shows both states. With 2,000 people under an air system that only stirs the room, the crowd’s plumes rise in two large rolls, sweep along the full width of the vault and return down the walls; warm air pools under the painted ceiling and the particles it carries reach the fresco band on every circuit. With conditioned air supplied gently from just below the windows and extracted at floor level at the foot of each wall, the same plumes are turned over above the visitors’ heads and carried down and out at the floor. The air under the vault becomes almost still, and new deposit on the frescoes stops; dust that has already settled stays until it is cleaned.

The lesson reaches well beyond one chapel. Museums, churches, libraries and archives are routinely run at visitor numbers their original ventilation never assumed, and the first sign is often not discomfort but damage to the surfaces furthest from the people. The questions are the same everywhere: how many people the air was designed for, where their heat and moisture actually go, and whether the airflow takes it out low or lifts it to the ceiling. All of them can be answered before the next season’s crowd arrives.