If you use permanent or semi-permanent hair colours, you are increasing your risk of developing cancer. Both animal and human studies show that the body rapidly absorbs chemicals in permanent and semi-permanent dyes through the skin during the more than 30 minutes that dyes remain on the scalp.
In the late 1970s, several studies found links between the use of hair dyes and breast cancers. A 1976 study reported that 87 of 100 breast cancer patients had been long term dye users (NY State J Med, 1976; 76: 394-6).In 1979, a US study found a significant relationship between frequency and duration of hair dye use and breast cancer (J Nat Cancer Inst, 1979; 62: 277-83). Those at greatest risk were 50 to 79 year olds, suggesting that the cancer takes years to develop. Women who started dying their hair at age 20 had twice the risk of those who'd started at 40.
Another study found that women who dye their hair to change its colour, rather than masking greyness, were at a threefold risk (J Nat Cancer Inst, 1980; 64: 23-8).
More recently, a jointly funded American Cancer Society and FDA study admitted a four fold increase in relatively uncommon cancers, including non-Hodgkin's lymphoma and multiple myeloma in hair dye users (J Nat Cancer Inst, 1994; 86: 215 310).
The darker the shades of permanent and semi-permanent dyes, this higher the risks of breast cancer; women who use black, dark brown or red dyes are at the greatest risk.