Acute leukemia is a quickly progressing form of leukemia in which immature, non-functioning blood cells accumulate and crowd out normal cells. Hair dyes have long been studied as a potential risk factor for a number of cancers, but research has yielded conflicting findings.

Older formulations contained potentially cancer-causing chemicals, and there is evidence tying hair dyes to the risk of blood-related cancers such as leukemia and multiple myeloma. Not all studies, however, have come to this conclusion.

The new study compared 769 acute leukemia patients with 623 adults without the disease. It found that men and women who had used permanent dyes one to five times per year for 15 years or longer were more than twice as likely to develop leukemia as people who had never dyed their hair.

Temporary hair dyes that wash out with a few shampoos and hair dye use beginning in 1980 or later were not linked to the disease.

Together with past research, these findings suggest hair dye use is a "potential but not an especially strong risk factor" for leukemia and other blood-related cancers, according to lead study author Dr. Garth H. Rauscher of the University of Illinois in Chicago.

And it does appear that long-term use and use of older coloring products are key factors, Rauscher told Reuters Health.

He and his colleagues report the findings in the current issue of the American Journal of Epidemiology.

The findings are similar to those of a study earlier this year that linked long-term use of older permanent hair dyes to an increased risk of non-Hodgkin lymphoma in women. Again, women who used hair dyes after 1980 did not have an elevated cancer risk, and the researchers speculated that changes in product formulations made in response to cancer concerns could be the reason.

Rauscher said evidence so far suggests that while people who have colored their hair do not seem to face a greater risk of most cancers, the "possible exception" is cancer of the blood or lymph nodes-which includes leukemia and non-Hodgkin lymphoma.

The reason is unclear, but it may have to do with the fact that the blood is the "first point of contact" for cancer-promoting chemicals that are able to penetrate the scalp, Rauscher noted.

However, he also pointed out that while some studies like his - comparisons of leukemia or lymphoma patients with healthy adults - have linked hair dyes to a higher cancer risk, other studies that have followed hair dye users over time have failed to do so.