So what else can hair dyes do to you? If you believe many of the scientific studies that have been done over the past 20 years, hair dyes may be carcinogenic. Why? Hair dyes are applied directly to the scalp, nearest the root. This is where the blood supply is the richest in the entire human body. It is also where the blood supply may carry any carcinogenic components right into the blood stream. Hair dyes make you more susceptible to bladder cancer.