Women who have been colouring their hair for 24 years or more are thought to have a higher risk of developing NHL cancer, which attacks the lymphatic system. The lymphatic system is part of the immune system, so people with immune weaknesses are at special risk - notably AIDS and organ transplant patients.
The study of 1,300 women claims that those who started dyeing their hair before 1980 were one-third more likely to develop NHL. Also women who used the darkest type of dyes for more than 25 years were thought to be twice as likely to develop the cancer.
According to the American Cancer Society, Non-Hodgkin lymphoma is expected to affect an estimated 54,000 Americans this year, killing 19,000 people. The incidence of NHL has doubled since the mid-1970s but no one knows why. Experts believe that exposure to chemicals could be a possible factor.
A European Commission watchdog criticised the hair dye industry in 2002 for failing to provide evidence of the safety of its products. Scientists over the last few years have been worried in particular about two chemical ingredients, para-phenylenediamine and tetrahydro-6-nitroquinoxaline. These chemicals have been shown to damage the body's genetic material, and to cause cancer in animals.
Yale School of Medicine professor, Tongzhang Zheng and colleagues studied 600 Connecticut women who had NHL in comparrison to 700 healthy women. The researchers did not find any larger risk of cancer in women who started using hair dye later than 1980.
"A person's absolute risk of developing lymphoma is very low, so doubling that risk still means a woman who dyes her hair is very unlikely to develop the cancer," said Zheng.
The researchers believe that the findings could reflect a change in hair dye formula contents over the past two decades, or that it could indicate that recent users are still in their induction and latent period. Since 1980, many carcinogens have been removed from some formulas, which vary depending on whether the dye is permanent, semi-permanent, darker or lighter.
A previous study by the University of Southern California found that women who used permanent hair dyes at least once a month were up to three times more likely to develop bladder cancer. Researchers found that long-serving hairdressers were also at increased risk.