December 4, 2019

expert reaction to study looking at permanent hair dyes, chemical hair straighteners and risk of breast cancer

Research, published in the International Journal of Cancer, reports on the relationship between hair dye and chemical straightener use and risk of developing breast cancer. 

Dr Michael Jones, Senior Staff Scientist in Epidemiology, The Institute of Cancer Research, London, said:

“The new results are intriguing, but it is too early to make a firm recommendation on the basis of one study, and further research is needed.

“As with any single observational epidemiological study, these results will need wider confirmation – from other epidemiological studies in people, and from other types of research such as animal and biological studies.

“The Sisters Study is a good prospective cohort study – but women were recruited to the study because they had a sister with breast cancer, so the conclusions wouldn’t necessarily hold true for women in the wider population, hence the need for further confirmation.

“The study analysed data from more than 46,000 women and nearly 2,800 cases of breast cancer. But only just over 200 of the breast cancer patients were African American, limiting the power to draw conclusions about this group.

“It’s also important to note that the authors weren’t able to look at the exact ingredients in the hair dyes and chemical straighteners. The study was US based, so it’s also not clear if the products would be similar to those used in the UK.”

Dr Stephen Burgess, Group Leader at the Medical Research Council Biostatistics Unit, University of Cambridge, UK:

“This is a difficult study to comment on as the findings may be relatively serious, but they also may simply be a statistical fluke.

“Although the estimated associations between hair dye usage and breast cancer risk were strong for some analyses, the statistical evidence is not overwhelmingly convincing in any of the analyses. Confidence intervals, representing the range of likely values for the associations, were generally wide, and a zero association was always close to the range of the most likely values. There are also discrepancies in the evidence: while personal use of permanent dyes was associated with an increase in breast cancer risk both overall and in black women, applying permanent dyes to others was not associated with any increase in breast cancer risk either overall or in black women. For semi-permanent dyes, there was no evidence of association with an increase in breast cancer risk overall or in black women. However, there was some evidence for an association between applying semi-permanent dyes to others with increase in breast cancer overall, but not in black women. So, there was evidence of association in some analyses, but not others. Overall, while there is some evidence for an association between hair dye and breast cancer risk in this study, the statistical evidence is weak and inconsistent.

“There are several other limitations of the study. The study is observational, and so cannot answer causal questions. It is able to show some evidence that women who use hair dye products have higher risk of breast cancer. However, it cannot answer questions about what would happen if women who currently use hair dye products decided to stop using these products. There may be alternative reasons why hair dye usage and breast cancer risk are correlated without hair dye being a causal risk factor for breast cancer. It may be that women who are more susceptible to getting breast cancer are more likely to use hair dye. It may be that there are social or cultural factors that associate with patterns in both hair dye usage and breast cancer risk.

“Understanding relationships between potential risk factors and diseases is difficult. The authors are to be commended on gathering detailed data on hair product usage and breast cancer status for a large number of women. While the results demonstrated are concerning, further research is required on the long-term effect of hair dye usage and breast cancer before concluding that hair dye is a cause of breast cancer.”

Summarised findings with statistics:

“Overall, personal use of permanent dyes was associated with a 9% relative increase (95% confidence interval: 1% increase to 17% increase) in breast cancer risk, and personal use of semi-permanent dyes was associated with a 1% relative decrease (95% confidence interval: 15% decrease to 15% increase). Applying permanent dyes to others was associated with a 12% decrease (95% confidence interval: 51% decrease to 58% increase) in breast cancer risk, and applying semi-permanent dyes was associated with a 28% relative increase (95% confidence interval: 5% increase to 56% increase). In black women, the associations were 45% increase (95% confidence interval: 10% increase to 90% increase) for personal use of permanent dye, and 15% increase (95% confidence interval: 14% decrease to 53% increase) for personal use of semi-permanent dye; 12% decrease (95% confidence interval: 51% decrease to 58% increase) for use of permanent dye on others, and 35% increase (95% confidence interval: 21% decrease to 129% increase) for use of semi-permanent dye on others.”

Prof Paul Pharoah, Professor of Cancer Epidemiology, University of Cambridge, said:

“This is an interesting study that has been carefully conducted however the results need to be interpreted with caution.  The researchers have followed almost 50,000 women who had a sister with breast cancer for about 10 years.  At the start of the study the women were asked about their use of hair dyes and chemical hair straighteners.  The researchers then studied the association between use of these products and the women developing breast cancer during follow up.

“The researchers carried out many analyses and the results highlighted in the abstract have been rather cherry picked.

“It is always important to remember that association or correlation does not mean causation.

“The researchers report a weak association between the use of permanent hair dyes and breast cancer.  However, because this association is weak it is quite likely to be a chance finding (that is a statistical fluke).  Furthermore, the fact that the risk is not associated with either frequency of use or duration of use suggests that the association is not causal. 

“The researchers also report a stronger association for use of permanent hair dyes in black women compared to white women.  However, this association is restricted to personal use and is not found for women who apply permanent hair dyes to others and there is no association with duration of use.  This suggested that the specific findings in black women could simply have arisen by chance.

“The evidence for chemical hair straighteners is a little stronger, but again the findings lack coherence.  The association between frequency of use and risk is different in women with personal use of chemical straighteners compared to women who have applied chemical straighteners on others.

“In summary, while these results are intriguing, they do not provide good evidence that hair dyes or chemical straighteners are associated with a meaningful increase in risk of breast cancer or that any increased risk association is causal.  Women who have used such products in the past should not be concerned about their risks.”

‘Hair dye and chemical straightener use and breast cancer risk in a large US population of black and white women’ by Eberle et al. was published in the International Journal of Cancer at 05:01am UK time on Wednesday 4^th December. 

Declared interests

Dr Michael Jones: Dr Michael Jones, and the team in which he works, is closely collaborating with the authors of the study on other projects.

Dr Stephen Burgess: “No conflict of interest to declare.”

Prof Paul Pharoah: “I have no conflicts of interest to declare.”