Is it safe to breathe pure oxygen?

Yes and No: Breathing 100% oxygen can be harmful over time. Symptoms may include disorientation, breathing problems, and vision changes. Prolonged high oxygen doses cause damage to cell membranes and can cause seizures. However, sometimes breathing pure oxygen is necessary for treatment of carbon monoxide poisoning or in adult respiratory distress syndrome (ards).

It is safe: Pure oxygen is only available in artificial conditions. It is used clinically for certain infections and inflammations. There is no danger to this although long term exposure has not been studied as that would require complex artificial conditions that no one is likely to attempt. But repeated exposures in clinical apparatus may treat the condition for which it is given without toxicity.