A new study from a team at the National Institute of Environmental Health Sciences (NIEHS) is rekindling a debate over the safety of essential oils. The research suggests lavender and tea tree oils contain potential hormone disrupting chemicals and could be responsible for a condition called prepubertal gynecomastia, where young boys develop abnormal breast tissue.

Back in 2007 a study in the New England Journal of Medicine from NIEHS controversially suggested that repeated topical use of lavender or tea tree oil could be responsible for prepubertal gynecomastia. Based on three specific case studies and a small series of in vitro tests, the research was hotly debated and the essential oil industry hit back with a counter-study in 2013 suggesting there was no link between lavender oil and endocrine disruption.

Now a new report from NIEHS, including some of the researchers from the original 2007 study, is hitting back. The new research isolated eight specific chemical elements found in many essential oils including both lavender and tea tree oils. These chemicals were then applied to human cancer cells in vitro while observing any changes in estrogen receptor- and androgen receptor-target gene activity.

The researchers suggest that the varying estrogenic and/or anti-androgenic properties observed after applying these chemicals to the human cells were consistent with the observed endogenous conditions seen in cases of gynecomastia in prepubescent boys.

"Our society deems essential oils as safe," says lead investigator on the study J. Tyler Ramsey. "However, they possess a diverse amount of chemicals and should be used with caution because some of these chemicals are potential endocrine disruptors."

This new study, presented on March 19 at ENDO 2018, the Endocrine Society's 100th annual meeting in Chicago, is sure to spark new debate over the safety of essential oils, but this is far from definitive research. While the application of these specific chemicals to human cells could certainly display hormonal disruption, there is no clear dosage equivalency when we consider general human exposure.

A 2008 study on tea tree oil from the European Commission's Scientific Committee on Consumer Products notably found connections between topical use of the oil and gynecomastia to be "implausible." Importantly, the report also questioned whether any hormonally active chemicals potentially in the oil could effectively penetrate the skin.