September 2, 2020

expert reaction to a study looking at permanent hair dye and risk of cancers

A study, published in The BMJ, looked at permanent hair dye and potential risk of cancers.

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

“The Nurses’ Health Study from the USA is a very good prospective cohort study that has been carefully conducted and has produced many important findings.

“Data analysed was from more than 117,000 women with 36 years of follow-up where over 47,000 cases of cancer occurred and 4860 cancer related deaths.

“In relation to permanent hair dye use the authors concluded that: “No positive association was found between personal use of permanent hair dye and risk of most cancers and cancer related mortality”.  But there were some associations with basal cell carcinoma, some types of breast cancer, and ovarian cancer.

“However an association does not imply causation, and when examining many associations, as in this study, some may appear significant if only by chance.

“No one study should be considered in isolation, and further research is needed on the associations seen in this study, to see if they are replicated in other observational epidemiological studies in other populations and ethnic groups (the women in the Nurses’ Health Study are predominantly white), in men, in other countries where different chemicals may be used in permanent hair dyes, in populations with more recent hair dye use since manufacturers may change chemical ingredients over time, and in animal and biological studies.  However, the results are generally reassuring for women who have used personal permanent hair dye in the past.”

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

“This manuscripts reports a careful analysis of data from the Nurses Health Study, a well-established cohort from the USA that has been running for over thirty years.  The investigators have studied the association between self-reported personal use of hair dyes and cancer.  The headline result is that overall there is no difference in the rate of cancer in general in women who have used hair dyes and those that have not.

“The authors highlight an association of hair dye use for several specific cancers.  The sentence in the press release “But risk of some cancers, including basal cell carcinoma, some types of breast cancer and ovarian cancer is increased” is misleading as it could be interpreted to mean that permanent hair dye causes an increase in risk, but as this is an observational study then causality cannot be assumed – as is mentioned later in the press release. 

“For the cancers where an increase in risk is reported the results are not compelling.  The reported associations are very weak and given the number of associations reported in this manuscript they are very likely to be chance findings.  Even if they were real findings the associations may not be cause-and-effect and even if they were causal associations the magnitude of the effects are so small that any risk would be trivial.

“In short, none of the findings reported in this manuscript suggest that women who use hair dye are putting themselves at increased risk of cancer.”

‘Personal use of permanent hair dyes and cancer risk and mortality in US women: prospective cohort study’ by Zhang et al. was published in The BMJ at 23:30 UK time on Wednesday 2^nd September.

DOI: 10.1136/bmj.m2942

Declared interests

Dr Michael Jones: “Dr Michael Jones, and the team in which he works, is involved in ongoing collaborations with the Nurses’ Health Study.”

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