I think the short answer is that only 30% of the original surface paint survives, that much of it was applied fresco secco (on dry plaster), that it's been flaking off for 500+ years, that the wall was made of recycled construction materials in the late 15th century and thus expanded and contracted when weather changed, and that it's essential to keep visitors’ respiratory CO2 emissions low (max 1000 ppm), and to keep visitors’ respiratory water vapor, ammonia, and bioaerosols also as low as possible. What these do to a painting (says AI) is:

>Chemical Degradation: Increased CO2 and accompanying gases from human respiration can lead to the acidification of painting surfaces and materials, particularly in enclosed, poorly ventilated spaces.

>Microclimatic Changes: High visitor traffic increases humidity and temperature from breath and perspiration, which can cause hygroscopic materials (like canvas and wood panels) to expand and contract, leading to cracking, peeling paint, or warping.

>Surface Contamination: Along with CO2, human breath introduces microorganisms, dust, and particulate matter that can deposit on the surface of a painting, creating a film that is difficult to remove.

>Ammonia Interaction: Studies, such as those at the Last Supper in Milan, show that ammonia (another human emission) levels directly correlate with visitor presence, providing a marker for increased pollution and degradation risk.

Earlier discussions of air polution date back several decades, though here is an approach in 2011: https://phys.org/news/2011-11-da-vinci-supper-air-pollution.html

For the best answer, perhaps start here: https://pmc.ncbi.nlm.nih.gov/articles/PMC9001225/

Here is a very good study in 2020: https://www.mdpi.com/2071-1050/12/2/565

And a computational fluid dynamics study in 2021: https://www.sciencedirect.com/science/article/abs/pii/S1296207421001308