Hair dyes can trigger minor to severe allergic reactions in a small percentage of users. They can also cause dry, brittle hair and hair breakage with extended and frequent use. According to the American Cancer Society, the evidence linking personal hair dye use and cancer is limited and conflicting. As of 2014, personal use of semi-permanent and permanent hair dyes is not considered a known risk factor for cancer.
Professional hairstylists may have cause for concern, however. According to the American Cancer Society, numerous studies have suggested that workplace exposure to hair-dye chemicals can lead to a slightly elevated risk of bladder cancer. These findings have led the International Agency for Research on Cancer to identify occupational exposure to hair dyes as a probable risk factor for cancer.
Another potentially serious but rare side effect is an allergic reaction to the chemical paraphenylenediamine, a common ingredient in commercial hair dyes. This allergy affects about one in 250,000 people, and those with a history of allergies should be especially cautious of using a chemical dye. An allergic reaction to hair dyes may present itself as an itchy, red rash around the face and neck, and less commonly a rash on the scalp and extreme swelling of the face. The allergy can become more severe over time, resulting in blisters, hives and difficulty breathing, and continued use can be fatal.