Hyperoxemia — more commonly spelled hyperoxaemia in British usage — is an unusually high concentration of oxygen in the blood and tissues, beyond normal arterial levels. It occurs in divers breathing oxygen-rich mixes, patients on excessive supplemental oxygen, and hyperbaric oxygen therapy. While oxygen sustains life, excess is toxic: it generates reactive oxygen species that damage lungs (absorption atelectasis, inflammation), constrict blood vessels paradoxically, and — at high pressures — trigger seizures. Neonatal intensive care learned this harshly, when liberal oxygen blinded premature babies with retinopathy. Modern practice titrates oxygen to targets, not maximums — the lesson that even the breath of life poisons in excess.