A new study published in the American Journal of Epidemiology at Yale has shown that people who have used hair dye, particularly permanent dye in dark colors, for over 25 years are at a higher risk of developing Non-Hodgkin’s Lymphoma (NHL).

The six-year study examined approximately 600 Connecticut women between the ages of 21 and 84 diagnosed with NHL. The women identified the type of hair dyes applied and the amount of time they were used.

NHL attacks the lymphatic system which fights disease and infection in the body and is comprised of lymph nodes located in the abdomen, chest, groin and neck. The disease affects men and women equally, and its causes are still unknown.

NHL kills an estimated 300,000 people annually, and incidences have been increasing in the U.S., with a confirmed increase in Connecticut.

“[Lymphomas] are a serious group of diseases,” Yawei Zhang GRD ’08, who is the first author on the study, said. “Since the 1970s, [cases] have been increasing by almost 50 percent, and the time trend also shows that NHL is still increasing.”

While the connection between hair dye and NHL has been explored in the past, this study is the first to examine its effects in the long-term.

“We found for people who used permanent hair dye before 1980, for more than 25 years, the risk [of NHL] almost doubles,” Zhang said.

Hair dye formulations have changed drastically in past decades. Many studies identified carcinogenic factors in hair dyes in the 1980s, and consequently, the Food and Drug Administration requires that warning labels appear on hair dyes. Since then, many companies have altered hair dye formulas to remove these health dangers.

“Hair coloring products have undergone tremendous change over the last 20 years,” principal investigator Tongzhang Zheng, associate professor of epidemiology at Yale School of Medicine, said in a press release. “Since 1980, many carcinogens have been removed from some formulas, which vary depending on whether the dye is permanent, semi-permanent, darker or lighter.”

While trends show the greatest risks for users before 1980, not enough time has lapsed to guarantee that current hair dyes do not pose a health threat. Zhang said that current hair dyes may still cause NHL, which would not have been great enough to present symptoms in many cases.

“For the people who started using dye in 1980 and after, they haven’t reached the 25 years [of use] yet, so we don’t know [if more recent dyes cause NHL],” Zhang said. “The hair dye is safer now, but we still can’t tell. [The NHL] may be latent.”

Zhang said that while no immediate follow-up studies are planned, studies of cancer incidence in lifetime hair dye users five years from now should be better able to ascertain if contemporary hair dyes still pose a health threat.

Other researchers were professors of public health Theodore R. Holford and Brian Leaderer, professor of pathology Stuart Flynn, research associates Patricia Owens and Geovanni Tallini of Yale; Sheila Hoar Zahm of the National Cancer Institute; and Peter Boyle from the Europe Institute of Oncology in Milan, Italy.