Thermal Paper and Acetone Vapor
Acetone vapor, as an organic solvent, is able to darken the thermal ink in the receipt just like heat would do. In fact, a thermal printer doesn’t actually print anything on the paper, it just heats up the the substance on the paper in which it breaks down the organic solvent part that interacts with the ink and changes its color.
Four different types of imaging chemicals are used in thermally sensitive papers: leuco dyes, developers, sensitizers and stabilizers. And if you don’t have acetone laying around heating the underside of the thermal paper receipt also darkens the ink. So does rubbing it with a coin. The friction between the metal and the paper is enough to result in color change.
Some thermal papers can contain BPA and BPS (Bisphenol A / S) which is a chemical considered to be an endocrine disruptor (messes with your hormones and shit) So this is an environmental and a health concern. So people who constantly handle thermal paper should be more careful.