They showed that regular exposure to lavender or tea tree oil was linked to abnormal breast growth in young boys - prepubertal gynecomastia - because the common plant-derived oils act as endocrine-disrupting chemicals.
Gynaecomastia is rare, and there is often no obvious cause. "The bottom line is that the public should be aware of these implications and risks when deciding to use essential oils", he said.
If you are pregnant you must seek the advice of your doctor, midwife or aromatherapist before using any essential oils. The team also cautioned against too much use of consumer products that contain lavender and tea tree oil such as soaps, lotions, shampoos, hair-styling products, cologne, and laundry detergents.
Researchers at National Institute of Environmental Health Sciences (NIEHS) found that the culprits for the incident are the tea tree and lavender oils.
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. "However, they possess a diverse amount of chemicals and should be used with caution because some of these chemicals are potential endocrine disruptors", he says.
The new study looked at eight key chemicals from the hundreds that make up the oils. They applied the chemicals to human cancer cells, and measured how estrogen and androgen hormone receptor genes within the cells were affected by the exposure.
The researchers found all eight demonstrated varying degrees of promoting oestrogen and/or inhibiting testosterone properties.
The chemicals found in tea tree and lavender are referred to as "endocrine-disrupting", which means they can interfere with hormones and their actions in the body. Four of the tested chemicals appear in both oils: eucalyptol, 4-terpineol, dipentene/limonene and alpha-terpineol.
Prof Ieuan Hughes, emeritus professor of paediatrics at the University of Cambridge said the findings "have confirmed why an individual using such oils containing these chemicals may develop breast tissue".
And he said: "We do not anticipate any long term effects on hormonal levels". Naturally, some individuals will be more sensitive to the effects than others, and the level of use/exposure is also an important factor - so individual mileage may vary.
Dr. Rod Mitchell, who was not part of the research, underlined that the study was conducted on cancer cells, instead of health breast tissue.
He added: "The concentration (dose) to which the cells are exposed may not be equivalent to exposure in humans". Mitchell insisted that the relationship between testosterone, estrogen and hormones in general can not be accurately replicate din laboratory settings.
He called for further larger studies.
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.