A US study on chemicals found in essential oils has shown that they may be linked to abnormal breast growth in young boys.
The growth of male breasts, called gynaecomastia, is rare and usually has no obvious cause, but a few cases have been linked to the use of essential oils like lavender and tea tree oils.
Many of the chemicals tested were also found to be present in 65 other essential oils, said lead researcher J Tyler Ramsey from the National Institute of Environmental Health Sciences (NIEHS), in North Carolina, as quoted by the BBC.
The increasing number of male gynaecomastia cases have been linked to the topical use of these essential oils and the symptoms only subsided after stopping the use of the product.
The study found that key chemicals found in the oil boost oestrogen and inhibit testosterone. Eight chemicals from the oils were tested on cancer cells and all eight had these properties, reported the BBC.
A previous study by the co-investigator of the latest study Dr Kenneth Korach also showed that lavender and tea tree oils had properties that "competed with or hindered the hormones that control male characteristics", reported the BBC. This, in turn, could affect puberty and growth.
These plant-derived oils can be found in everyday products like soaps and are sometimes even used for medical treatments. However, Mr Ramsey believes they should be "used with caution".
"Our society deems essential oils as safe. However, they possess a diverse amount of chemicals and should be used with caution because some of these chemicals are potential endocrine disruptors," said Mr Ramsey.
However, not everyone will experience the same effects, said another professor.
Professor Ieuan Hughes, emeritus professor of paediatrics at the University of Cambridge said that the findings show that essential oils may cause the development of breast tissue, but some individuals may be more sensitive to the effects of the chemicals than others.
"The anti-male hormone effects are rather unexpected and it is not possible to comment further without the data," Professor Hughes said according to the BBC.
It was also pointed out that the tests were carried out on cancer cells which may not represent the effects on normal breast tissue.
"The concentration (dose) to which the cells are exposed may not be equivalent to exposure in humans. There is a complex relationship between oestrogen, testosterone and other hormones in the body, that cannot be replicated in these experiments," said Dr Rod Mitchell, honorary consultant paediatric endocrinologist at the Queens Medical Research Institute in Edinburgh.
However, he called for larger studies as there is insufficient evidence currently to support the link between exposure to essential oils and gynaecomastia in children.