Many coal tar-based dyes—often used in darker hair colors—have been linked to cancer and other health problems, especially with repeated, long-term use.
Evidence: A 1997 Michigan State University study found that hairdressers were at a 2.7 times higher risk of developing salivary gland cancer as compared with the control population. Another study by scientists at the University of Southern California's School of Medicine showed that hairdressers and barbers who have been on the job for more than ten years face a fivefold increase in bladder cancer risk. The FDA also says "several coal-tar hair-dye ingredients have been found to cause cancer in laboratory animals."
Commonly used ingredients like p-phenylenediamine and resorcinol can cause severe allergic reactions, even if you've used them safely before. Other chemicals like triethanolamine can be bad for your lungs.
Safer choices: To reduce your exposure to these chemicals, dye your hair less frequently. Or try highlights, which are safer because less hair is colored and the dye isn't rubbed into your scalp. Also, lighter dyes are generally made from safer ingredients than darker dyes. And, of course, henna dyes, made from the flower, are a classic natural alternative to chemical products.
Checklist: You can compare at-home hair-dye brands by visiting cosmeticsdatabase.com, a database created by the Environmental Working Group, a nonprofit environmental research group in Washington, D.C. Products with a lower score have safer ingredients and tend to be the less permanent dyes. You can also check for p-phenylenediamine, resorcinol, and triethanolamine on the box yourself. When using any hair dye, remember to never leave it in your hair for longer than the directions recommend, and be sure to work in a well-ventilated area.
—Kristan Markey, chemist and analyst for the Environmental Working Group in Washington, D.C.