At what partial pressures can breathing 100% O2 become toxic?

Breathing 100% oxygen can become toxic at partial pressures ranging from approximately 1.3 to 1.6 atmospheres absolute (ata). This phenomenon is mainly due to the toxic effects of oxygen at elevated pressures, which can lead to oxygen toxicity affecting the central nervous system. When the partial pressure of oxygen exceeds this range, increased oxygen levels can lead to a variety of symptoms, including vision changes, seizures, and pulmonary effects.

Understanding the context of this answer involves recognizing that lower pressures, such as those cited in other options, do not typically trigger the toxic effects associated with oxygen exposure. At pressures below 1.3 ata, the body can generally handle higher concentrations of oxygen without adverse effects. Diving practices often consider these pressure ranges to avoid toxicity, ensuring diver safety during deep dives or prolonged exposure to high oxygen environments.