It is a toxic chemical. It directly injures the lungs and other tissues (such as the retina's in premature infants) and is though to be bad for the cardiovascular system by causing increased oxidation (the thing people are trying to avoid when they take anti-oxidants).
On the other hand, you do need it to live. If giving 100% oxygen is the only way to get enough oxygen into the system then you just have to do it.
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