Which tree's oil is inhaled to ease respiratory congestion?

The tree whose oil is inhaled to ease respiratory congestion is eucalyptus.

Eucalyptus is a large genus of trees and shrubs native mainly to Australia. Eucalyptus oil is distilled from the leaves of selected species, especially cineole-rich varieties. Its strong, camphor-like aroma comes largely from eucalyptol, also called 1,8-cineole.

The oil is widely used in decongestant formulations, vapor products, and traditional preparations for cold-related coughs and sinus discomfort. Inhaled vapour can create a sensation of easier breathing, while the oil’s aroma and cooling character help explain its popularity. This traditional use should not be confused with curing an infection: eucalyptus oil is mainly used for symptomatic relief.

Eucalyptus has a long medicinal history. Aboriginal Australians used leaf preparations for conditions including colds and sinus congestion, and in 1788 surgeons with the First Fleet distilled eucalyptus oil near Port Jackson. A common mix-up is tea tree oil, which also comes from an Australian plant but is chiefly associated with topical antiseptic use rather than inhaled decongestant preparations. Undiluted eucalyptus oil is toxic if swallowed, particularly by children.