Risk of breast, endometrial and ovary cancer in Danish hair dressers

Introduction
Hair dressers are occupationally exposed to a variety of chemicals, including solvents and hormone disrupting chemicals. It has been suggested that such exposures may contribute to increased risk of hormon related cancers such as breast, endometrial and ovarian tumors. Only few studies have investigated such assocaitions, and existing studies have been small and with somewhat conflicting results, and most studies deal with a simple ever-versus-never exposure assessment.

Methods
We established three independent natiovide case-controls studies, nested within the entire group of female employees (born 1935-70), including 37 105, 5 109 and 3 867 cases of breast, ovary and endometrial cancers, respectively, diagnosed in the period 1970-2003. Primary cases were retrieved from the national Danish Cancer Registry. Randomly selected female population controls (1:1) were matched on age. Information on individual employment history, social determinants and reproductive history was retrieved from a comprehensive nationwide data linkage system by use of unique identifcation numbers applied to all residents in Denmark. Odds ratios (ORs), including 5 years of induction time and adjusted for socio-economic and reproductive status, were estimated by conditional logistic regression analysis, including 95% confidence intervals (CIs).

Results
The hairdressers have adjusted overall OR’s of 1.3 (CI: 1.1-1.5), 1.3 (0.8-2.1) and 1.6 (0.8-3.1) for breast, ovary and endometrial cancer, respectively, compared to Danish emploees. Further, a significant association between duration of work as a hairdresser and breast cancer was observed (p=0.007), including an OR of 1.7 (1.1-3.0) for those with over 10 years seniority. No similar clear tendencies were observed for the two other more rare cancers.

Discussion
Danish hair dressers exhibit an increased risk of three hormone sensitive cancers after adjustment for potential confounders. Aditionally, the risk seems to increase by increasing seniority which may indicate an association with occupational exposures. Further studies are needed to explore specific exposures witin and outside the occupation.