Why is there concern that hair dyes may cause cancer?
Many people in the United States and Europe use hair dyes. It is estimated that more than one-third of women over age 18 and about 10% of men over age 40 use some type of hair dye.1
Modern hair dyes are classified as permanent (or oxidative), semipermanent, and temporary. Permanent hair dyes, which make up about 80% of currently marketed products, consist of colorless dye “intermediates” (chemicals called aromatic amines) and dye “couplers.” In the presence of hydrogen peroxide, the intermediates and couplers react with one another to form pigment molecules. Darker colors are formed by using higher concentrations of intermediates. Semipermanent and temporary hair dyes are nonoxidative and include colored compounds that stain hair directly.
Over 5,000 different chemicals are used in hair dye products, some of which are reported to be carcinogenic (cancer-causing) in animals.2–4 Because so many people use hair dyes, scientists have tried to determine whether exposure to the chemicals in hair coloring products is associated with an increased risk of cancer in people.
Early hair dye formulations contained chemicals, including aromatic amines that were found to cause cancer in animals. In the mid- to late 1970s, however, manufacturers changed the components in dye products to eliminate some of these chemicals.2,3,5 It is not known whether some of the chemicals still used in hair dyes can cause cancer. Given the widespread use of hair dye products, even a small increase in risk may have a considerable public health impact.3
Over the years, some epidemiologic (population) studies have found an increased risk of bladder cancer in hairdressers and barbers.2,6 A report from the International Agency for Research on Cancer (IARC) concluded that some of the chemicals these workers are exposed to occupationally are “probably carcinogenic to humans.”7
Although some studies have linked the personal use of hair dyes with increased risks of certain cancers of the blood and bone marrow, such as non-Hodgkin lymphoma (NHL) and leukemia, other studies have not shown such links. Studies of breast and bladder cancer have also produced conflicting results. Relatively few studies have been published about the association of hair dye use with the risk of other cancers.5 Based on its review of the evidence, the IARC Working Group concluded that personal use of hair dyes is “not classifiable as to its carcinogenicity to humans.”7
What is the evidence that personal hair dye use is associated with risk of NHL?
A number of studies have investigated the relationship between the personal use of hair dyes and the risk of NHL, with conflicting results. Because the small size of some studies may have limited their ability to detect an association between hair dye use and cancer, a pooled (combined) analysis of four case-control studies was carried out.8 All four studies had obtained detailed information on hair dye use, including dates and duration of use, and on NHL subtype. The pooled analysis included 4,461 women with NHL and 5,799 women who did not have NHL. The results of the study showed that women who began using hair dye before 1980 had a slightly (30%) increased risk of NHL compared with women who had never used hair dye, whereas no such increase in risk was seen for women who began using hair dye after 1980.
When the researchers analyzed the risks of several specific NHL subtypes, they found that hair dye users had increased risks of both follicular lymphoma and chronic lymphocytic leukemia/small lymphocytic lymphoma.8 For the most part, the increases were limited to women who began using hair dye before 1980, although an increased risk of follicular lymphoma was observed among women who began using dark-colored dyes after 1980. Although these results are consistent with the idea that earlier hair dyes were more carcinogenic, it is also possible that the absence of increased risks for hair dye users who began using dyes after 1980 reflects lower cumulative exposure levels or insufficient time since first exposure for any increase in risk to become apparent.
What is the evidence that personal hair dye use is associated with risk of leukemia?
Studies of the association between personal hair dye use and the risk of leukemia have had conflicting results. For example, one case-control study examined hair dye use among 769 patients with adult acute leukemia and 623 people without leukemia in the United States and Canada.9 It found that the risks of acute leukemia were higher among users of earlier formulations of both permanent and nonpermanent (i.e., semipermanent and temporary) dyes than among those who had not used dyes, although the increases were not statistically significant. No risk increases were seen among users of more recent dye formulations. Risk was greatest among those who had used permanent dyes for longer durations (15 or more years).
However, a case-control study in Italy found no association between use of permanent hair dye overall and risk of leukemia, although users of black permanent dyes, but not of other color dyes, did have an increased risk. This study, however, did not collect information on the timing or frequency of hair dye use.10