Scheele and Priestly discovered oxygen independently of each other in 1772 and 1774, and it was quickly realized that this gas is not only lifegiving but might be poisonous as well. However, what man has known for only two centuries nature itself has known for some 700 million year. One of the biggest leaps forward in evolution occurred when blue green like algae developed enzymes that scavenge the superoxide radical enabling them to live in an oxygen rich atmosphere. Oxygen therapy for newborn infants was introduced in the USA in the 1930's and 1940's although the Finnish pediatrician Ylppo had already recommended intragastric administration of oxygen in 1917, a practice that continued a surprisingly long time, into the mid-1950's. Not before the discovery of its relation to retrolental fibroplasia (retinopathy of prematurity-ROP) were questions raised concerning the use of oxygen. The rest of the story is well known: in the 1950's and 60's the oxygen concentration was turned down in many incubators apparently resulting in a reduction in ROP but probably increased mortality.
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