"Other oils act as expectorants, stimulating the removal of heavy mucous in the lungs and bronchial tubes.  This is helpful in virtually all respiratory conditions, and the most helpful oils include eucalyptus, tea-tree, lemon, lavender, peppermint and sandalwood."

Reference:  Robert Tisserand, "To Heal and Tend the Body", Lotus Press, Wilmot, WI, 1988, p. 81.