WARNING: The prolonged use of hair dye, especially permanent black, brown and red, may be hazardous to your health. That’s the conclusion reached by Yale researchers in a study published on January 15 in the American Journal of Epidemiology. The scientists found that long-term users of hair-coloring products have an increased risk of developing non-Hodgkin’s lymphoma, a cancer that attacks the lymphatic system, part of the body’s immune system. “We found that people who used permanent dark hair dye for more than 25 years and started before 1980 will have more than twice the risk compared to people who never used hair-dye products,” said Tongzhang Zheng, Sc.D., associate professor of epidemiology and environmental health. Zheng said the study was prompted by an unexplained jump in the number of non-Hodgkin’s lymphoma cases in the last 40 years. In the early 1970s, there were about 10 cases out of every 100,000 people in the United States. By 1990, that number had increased to 19 cases. Today it’s still increasing in the United States and around the world.

The health risks of hair dye have been explored for years, but Zheng says previous studies have been contradictory and inconclusive. He and his research team conducted a six-year, case-controlled study of 601 Connecticut women between the ages of 21 and 84 diagnosed with varying subtypes of non-Hodgkin’s lymphoma. The women were asked about the type of hair coloring they used, the length of time they used the products and their age when they stopped. The study included a control group of 717 healthy women matched by race, age and other factors with the case group.

Researchers found the highest risk among users of darker permanent dyes, rather than among those who used semipermanent or temporary dyes. Zheng says that’s because darker dyes may contain higher levels of chemicals, and permanent dyes use an oxidizing process that creates new, potentially harmful chemicals.

The good news is that researchers didn’t find any increased risk of non-Hodgkin’s lymphoma among women who started using hair-coloring products after 1980. This could be because the contents of hair-dye formulas may have changed and become safer, or it could simply mean that not enough time has passed to evaluate the effects on this group. Zheng said further studies would have to be conducted to determine whether post-1980 hair dyes are indeed safer.

Noting that hair color is directly related to image—“how people are perceived and how they perceive themselves,” Zheng said that the study results need to be duplicated in different populations. Meantime, users of hair dyes should consider