Re: Personal use of permanent hair dyes and cancer risk and mortality in US women: prospective cohort study
The article presents a comprehensive investigation about the relationship between cancers in women and use of permanent hair dye (1); based on data from the Nurses’ Health Study, an invaluable large-scale cohort which has shed light on many significant correlations between risk factors and diseases. The analysis in this research is very detailed, confounders that have not been considered in previous works (2, 3) are envisaged, and main strengths and limitations are clearly discussed. However, three key objections are raised regarding the positive association found between breast cancer and cumulative doses of hair dyes:
First, breast cancer has some chief risk factors. These include a personal history of breast cancer, which has been considered in this study by excluding all women with a previous history of any cancer; a family history of breast or ovarian cancer, which has not been contemplated; and a positive relevant genetic mutation, which also has not been seen here; while data are present in the Nurses’ Health Study.
Second, breast cancer is strongly linked with age, and it can be presumed that hair dying is also age-dependent; since coloring is frequently used to cover whitening hair. However, the findings have not been adjusted for age in the present study.
Third, results show that the increased risk of breast cancer induced by hair coloring is stronger in women with naturally light hair. Nevertheless, the association of natural hair color with breast cancer regardless of hair coloring has not been investigated. Thus, the detected association might be because of the natural hair color per se and not for the hair dye. Obviously, natural hair color is related to many ethnic and genetic factors that would be important confounders for the positive findings.
These three issues seem important enough to affect the accuracy of results, unless the authors have rational explanations for each of them. While the association of hair dye and breast cancer does not seem unrealistic, such findings need even more detailed and cautious analysis and interpretation.
1. Zhang Y, Birmann BM, Han J, Giovannucci EL, Speizer FE, Stampfer MJ, Rosner BA, Schernhammer ES. Personal use of permanent hair dyes and cancer risk and mortality in US women: prospective cohort study. BMJ. 2020 Sep 2;370. doi: 10.1136/bmj.m2942
2. Eberle CE, Sandler DP, Taylor KW, White AJ. Hair dye and chemical straightener use and breast cancer risk in a large US population of black and white women. Int J Cancer 2020;147:383-91. doi:10.1002/ijc.32738
3. Alipour S. Comments on: Hair dye and chemical straightener use and breast cancer risk in a large US population of black and white women. Int J Cancer 2020;146:2651. doi:10.1002/ijc.32852
Competing interests: No competing interests