In this population-based case-control study, we observed no association between ever use of hair dyes and bladder cancer among either women or men. Among women overall, there was no association between color, product type, age at first use, year of first use, or number of applications. In subgroup analyses of women with a college degree, use of permanent hair dyes was associated with a significant three-fold risk of bladder cancer, whereas less educated women (no college degree) experienced no increase risk. Genetic analyses showed an increased risk of bladder cancer among women who were exclusive users of permanent dyes and had NAT2 slow acetylation phenotype, but again only among those with a college degree.
The reason for differences in the magnitude and direction of the hair dye associations among highly educated and less educated women is unclear. Media reports linking hair dye use to cancer may have led to differential reporting between cases and controls (i.e., recall bias). Alternatively, more educated women may report their hair dye use more accurately, and thus results in this subgroup might better reflect the true odds ratios associated with hair dye use.(31
) Another possibility is that more educated, and hence more affluent, women may go to salons for their hair dye applications and may therefore have been exposed to a different mixture of hair coloring products than women who personally applied over-the-counter products. Indeed, college educated women reported fewer lifetime applications of hair dyes than their less educated counterparts (Supplemental Table 1
), which could reflect greater use of salons. Hairdressers are a known high-risk occupation for bladder cancer risk and professional-strength hair dyes are thought to be the exposures responsible.(1
) Precise information on the potency of professional versus over–the- counter products to our knowledge has not been published. Further, although PPD is a main component of permanent hair dyes, we did not have information about what specific chemical might be contributing to the observed increased risks and thus whether exposure to specific chemical constituents of hair dyes differed by women’s educational status.
Our genetic results suggest that certain women may be particularly susceptible to the effects of hair dyes. N
-acetylation by NAT2 in the liver is a recognized detoxification pathway in aromatic amine metabolism (30
) and NAT2
slow acetylator phenotype increasing urinary bladder cancer risk following aromatic amine exposure from cigarette smoke has also been described.(32
) Although the interaction was not statistically significant, we observed an increased risk of bladder cancer primarily among exclusive users of permanent dyes who had NAT2 slow acetylation phenotypes compared to never users of dye with NAT2 rapid/intermediate acetylation phenotypes in females with a college degree. One study from Spain showed no modifying effect of NAT2
) while another, more comparable, California study showed a significant association between exclusive permanent hair dye use and bladder cancer in women with NAT2
slow acetylator phenotype.(24
) As in the California study, we did not observe a diminished effect of NAT2 slow phenotype after adjustment for NAT1, which is in linkage disequilibrium with NAT2 and thought to be the main detoxification pathway for hair dyes absorbed by the skin/scalp.(19
) In our study, we found genetic modifiers of risk only among a subgroup of more educated women. Even if we suspect recall bias, it is unlikely that women would know their genotype. Thus the expected increase in risk among permanent and exclusive permanent users with NAT2
slow acetylator phenotype supports a modifying role of NAT2
genotype on the hair dye-bladder cancer association.
NAT1, GSTM1, and GSTT1 genotypes did not appear to be important modifiers of the association between ever, permanent, or exclusive permanent use. Although there was an observed increased risk of bladder cancer associated with permanent hair dye use among college educated women with GSTT1-active genotypes compared to GTTT1 null genotypes, the lack of evidence for the presence of GSTT1-metabolized conjugated mutagenic intermediates in hair dyes and the low prevalence of GSTT1 null genotype indicates that this may be a chance association.
Several strengths of our study should be recognized. Our study is one of the largest case-control studies to evaluate the association between hair dyes and bladder cancer risk and includes a large number of exposed women in particular. It is also population-based and controlled for important confounders including smoking status. In an effort to minimize misclassification (28
), subjects in this study used a visual display card to aide in the identification of hair color, a tool that had not been used in previous bladder cancer studies of hair dye use. In addition, high quality genotype information in these subjects allowed for the evaluation of effect modification by genotype or phenotype status.
Some limitations of our study should also be acknowledged. Numbers of subjects in stratified analyses were often small, resulting in imprecise estimates, particularly in genotype/phenotype subgroups. In college educated women, we observed an association between permanent dye use and bladder cancer; however, small numbers precluded estimation of exposure-response for frequency and duration of permanent dyes within this putative high susceptibility group. We also observed an inverse association among less educated women. Thus, the observed qualitative interaction between permanent hair dye use and education may suggest that the increased risk observed among college educated women could be due to chance. Similarly, these results need to be replicated to rule out the possibility of a false positive result from the multiple tests of interaction. Lastly, we cannot rule out the possibility of recall bias in our observed association between various hair dye use characteristics and bladder cancer risk within educational strata.
In summary, we observed no increased risk for hair dye use and risk of bladder cancer overall in women or men. We observed an increased risk for permanent hair dye use and exclusive permanent hair dye use among college educated women that will require confirmation in other large studies. Genetic analyses of polymorphisms in enzymes known to influence aromatic amine-induced bladder cancer support the association between permanent hair dye use and bladder cancer risk in these women. Pooling data from studies with genetic information would provide greater statistical power to more definitively assess whether permanent hair dye use poses an increased risk of bladder cancer.