Originally native to Australia, eucalyptus trees are now grown all over the world and used for their medicinal properties to treat respiratory conditions. Eucalyptus oil has been used to relieve coughing and it can also help you get the mucus out of your chest.
Other respiratory conditions including asthma and sinusitis may be helped by inhaling steam with added eucalyptus oil. The oil reacts with mucous membranes, not only reducing mucus but helping loosen it so that you can cough it up.