Cancer-causing compounds have been removed from hair dyes
Researchers have found more evidence that hair dyes may cause cancer.
Those who regularly dye their hair have a higher risk of developing lymphoma, a European study involving nearly 5,000 women found.
Experts stressed some agents were removed from dyes in the 1970s when they were found to be cancer causing.
However, they said people frequently exposed to dyes before this time, such as hair dressers, should be aware of the risk and check for abnormal lumps.
Lymphomas are cancers of the lymphatic system, a network of vessels which form part of the body's immune system, and carry other infection-fighting cells called "lymphocytes", as well as draining dead cells away from the tissues.
The authors from the International Agency for Research on Cancer, France, presented their findings at the International Conference on Malignant Lymphoma.
Among women who regularly used hair dye, their risk of developing lymphoma was increased by 20% compared to women who had never dyed their hair before 1980.
About one third of women in Europe and North America, along with 10% of men older than 40 years, use some type of hair dye, it is estimated.
Professor Paolo Boffetta said: "It is reassuring to notice that dyes used in the last 25 years do not seem to carry an increased risk.
"It might still be premature to conclude that older dyes are causally related to lymphoma, but this evidence is growing."
He said a study published in the Journal of the American Medical Association two weeks ago also identified a link between lymphoma and older hair dyes.
However, this study found no link with other cancers such as breast and bladder cancers, despite past concerns.
Professor Franco Cavalli, chairman of the conference, said: "In the last 20 years the frequency of lymphoma has doubled in the western world.
"While viruses and other infectious agents have possibly played the major role in this epidemic, scientists have since long suspected that other chemicals could be involved.
This study proving an increased frequency of lymphoma in women who regularly used older types of hair dye is therefore very important.
"We probably need many more studies looking at the environmental agents in the pathogenesis of lymphoma."
Professor Gordon McVie, senior consultant at the European Institute of Oncology, said: "It is a large enough study to raise concern. But regulatory action has been taken and the carcinogens have been removed from hair dyes, so people using them now should not be alarmed.
"Hairdressers and people who have used hair dyes a lot in the past might be advised to be alert and look out for any abnormal lumps and bumps."