Because the black color in carbon ink is produced by extremely fine carbon particles. As a nonmetallic element, carbon will not form ions, so it will not dissolve into blood and enter cells. Secondly, carbon itself is a very stable element. The more stable it is, the more it reacts with other substances. The process of human digestion and absorption is a chemical reaction, so carbon will not change into microstructure and enter human cells. Therefore, carbon ink is not only carcinogenic, but also harmless to human body.