- since oxygen toxicity only occurs when the po2 exceeds point 7 ata for prolonged periods - `` - nitrogen narcosis , in which nitrogen behaves like an anesthetic , will occur in mixed-gas atmospheres if partial pressures exceeds 1.5 ata in the dds - `` - demand open circuit is generally preferable , but you will be using semiclosed circuit with inspired fluctuations of 608 to 760 millimeters - `` he went back to sleep .