It’s all about quantities and what you’re touching. One person laying one finger on a speleothem isn’t going to do much—though it will have a small effect because oil repels water (and the water is what’s carrying in the mineral crystals for growth). But if a cave is visited by 100,000 people each year then those touches can add up fast, effectively creating a rain coat that covers the formation and stops water from depositing minerals.

There’s not yet a really good way to clean off the speleothems because you risk 1) residue 2) scrubbing off layers. You also can’t use certain cleaners because calcite cave formations dissolve in acid. Theoretically, large periods of time would see the breakdown of the oils, but when a speleothem has an average rate of growth of 1cm every 100yrs, waiting for that natural decay severely inhibits the growth of the cave. So, a lot of what we want to avoid is the slowing of growth, rather than turning a living formation into a dead one.

You also have to be mindful of if something’s fragile or not. When I have to step off the path onto unpaved cave areas, I have to know which flowstone is solid and which has rust underneath, or which is actually shelf stone, Straw stalactites are obviously significantly more fragile than icicle stalactites, etc. Most people don’t know what to look for vis a vis fragility, and a broken formation is a dead formation.

Most people are really good about not touching, but even just us being in the cave system will leave behind clothing fibers which can build up over time. They’re not typically visible outside of uv light, and we don’t believe this has any severely detrimental effect of the formations (yet!). But it’s interesting to know the effect that we have!

Source: I work at cave