Complex sentence: In monoplace chambers that are compressed with pure oxygen a mask is available to provide the patient with "air breaks," periods of breathing normal air (21% oxygen), in order to reduce the risk of hyperoxic seizures.
Split into simpler sentences: In monoplace chambers that are compressed with pure oxygen, a mask is is needed to provide the patient with "air breaks", or periods of breathing normal air (21% oxygen). This is in order to reduce the risk of hyperoxic seizures.