This study was a retrospective case-control study (it retroactively compared people who had breast cancer with those who did not). Researchers could not establish cause and effect.
Recall bias limited the strength of the results. The WCHS study relied on each participant to accurately remember and report her use of hair care products over the course of her lifetime.
The study did not ask women what brands of products they used, so researchers weren't able to identify links between cancer and specific ingredients.
There were only a small number of ER+ breast cancer cases. More studies are needed to confirm a link between hair product use and this type of breast cancer.