What evidence links personal hair dye 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 inconsistent results. Because the small size of some studies may have limited their ability to detect associations, a pooled (combined) analysis of four case-control studies was carried out. 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 percent) 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. 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 risk 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.

This answer is based on source information from the National Cancer Institute.