What is the harm caused by tea tree oil?

A study at the US National Institute of Environmental Health Sciences found some evidence that both tea tree and lavender oils cause endocrine disruptors.

The effect was investigated after three healthy boys were diagnosed with prepubertal breast enlargement. Boys used hair and skin products containing tea tree and lavender. The oils do not appear to alter hormone levels in the bloodstream, but they may affect the way cells respond to hormone signals. This has been partly confirmed by laboratory studies on human breast tissue cells.