Breathing pure oxygen, especially for extended periods, can lead to a condition known as oxygen toxicity. This occurs because the high concentration of oxygen can cause damage to the central nervous system, lungs, and even the eyes. The body is not designed to handle such high levels of oxygen, and the excess can lead to a buildup of reactive oxygen species, which are molecules that can cause cellular damage. Symptoms of oxygen toxicity can include headache, dizziness, nausea, and visual disturbances such as tunnel vision or "snow" in the visual field. Prolonged exposure can lead to more severe effects, including seizures, respiratory problems, and even death. read more >>

If you breathe air with a much higher than normal O2 concentration, the oxygen in the lungs overwhelms the blood's ability to carry it away. The result is that free oxygen binds to the surface proteins of the lungs, interferes with the operation of the central nervous system and also attacks the retina.read more >>