Something mystified me about this article. Apparently, preservation experts are concerned that microorganisms could wipe out Stone Age cave paintings, as if this were an urgent threat. Has something altered the caves in which these paintings appear that has invited intensified bacterial growth?

Don McMillan
Modesto, Calif.

What threatens the paintings is a modern scourge: tourists. People’s exhalations and body heat affect a cave’s temperature and humidity, and, as mentioned in the article, cells and hair that people shed can nourish microbes .—S. Perkins