Wiktionary
green lung
n. An area of natural parkland within an urban region, supposed to replenish the air with oxygen.
Usage examples of "green lung".
Paul's Square was a green lung in the heart of the city, an acre of grass and flower-beds and willow trees with a fountain in the centre surrounded on all four sides by Georgian terrace houses, most of which were used as offices by barristers, solicitors or doctors and beautifully preserved.