Because it contains artificial pigment to dye hair.
Hair dyeing is generally artificial pigment or plant pigment. At present, the most widely used is artificial pigment. Among artificial pigments, phenylenediamine is the most common artificial pigment. Phenylenediamine itself is an industrial dye, which is highly carcinogenic. The proportion of phenylenediamine in these so-called one-wash black dyes basically exceeds 10%, which exceeds the national standard of 6%. So although the hair is washed black, it will do great harm to the human body.