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Fact check: What is the safe partial pressure of oxygen for human inhalation?
Executive Summary
The three provided analyses converge on the idea that a single universal “safe” partial pressure of inspired oxygen is not specified; instead safety is discussed in percentage of oxygen in breathing gas and in clinical targets that avoid both hypoxia and hyperoxia. A commonly cited value for dry atmospheric O2 partial pressure at sea level is about 159 mmHg, but physiological oxygenation is managed by arterial partial pressure and peripheral saturation targets that vary by context [1] [2]. Older operational guidance offers percentage-based ranges for breathing mixtures (19–23% O2) and CO2 limits, reflecting different use-cases than modern clinical practice [3].
1. Why the headline number 159 mmHg keeps appearing — and what it really means
The statement that atmospheric oxygen partial pressure is “around 159 mmHg” refers to the partial pressure of oxygen in dry air at sea level and standard atmospheric pressure, not the oxygen pressure that tissues actually experience. That 159 mmHg is a physical property of inspired dry air (21% of 760 mmHg) and does not equal alveolar or arterial oxygen tensions, which are lower due to water vapor, CO2 exchange, and gas exchange inefficiencies [1]. The 2024 review highlights this distinction and emphasizes that physiologic PO2 fluctuates with altitude and ventilation; therefore, quoting 159 mmHg without context can mislead non-specialists about what is “safe” inside the body [1].
2. Operational safety ranges from an older perspective — percentages and CO2 limits
A 2007 operational document gives a different, pragmatic framing: it specifies safe breathing mixtures in percentage terms (19–23% O2) and limits on CO2 (not to exceed 0.5% normally, brief excursions up to 1.0%), aimed at preventing immediate debilitating or lethal effects in enclosed atmospheres or life-support systems [3]. This approach is oriented to engineering and environmental control rather than individualized clinical care. The emphasis on CO2 as well as O2 underscores that atmospheric composition, not O2 partial pressure alone, determines safety in closed environments such as spacecraft or submarines [3].
3. Clinical practice has shifted toward tailored oxygen targets — avoid both low and high
A 2022 clinical review argues against a one-size-fits-all number and recommends patient-tailored oxygen therapy informed by peripheral oxygen saturation, to prevent both hypoxemia and hyperoxemia [2]. This reflects evidence that excess oxygen can be harmful in many clinical scenarios, and that maintaining “normoxemia” by targeting SpO2 ranges often yields better outcomes than indiscriminately high FiO2. The clinical framing therefore treats inspired O2 percentage as a means to achieve appropriate arterial oxygenation, not an endpoint itself [2].
4. How these perspectives conflict and why both matter
The three sources reveal an important tension: engineering/operational guidance prioritizes atmospheric composition and immediate toxic thresholds, while clinical literature prioritizes physiological outcomes and tailored targets. The 2007 operational range may be conservative for closed environments, whereas the 2022 clinical guidance prioritizes avoiding hyperoxia in patients. The 2024 review serves as a bridge, reminding readers that physical partial pressures (e.g., 159 mmHg) differ from effective arterial PO2 and that context—altitude, device, patient condition—changes what “safe” means [3] [2] [1].
5. What important details are missing from the three analyses
None of the provided analyses fully quantifies safe arterial partial pressures (PaO2) across clinical conditions or specifies clear SpO2 targets for different patient groups; they also do not discuss duration-dependent harms of hyperoxia or the alveolar gas equation that links inspired and arterial tensions. Important omissions include explicit PaO2 ranges, conversion between FiO2 and PaO2, and time-dependence of oxygen toxicity, all of which matter for operational planning and medical care. The documents also do not address vulnerable populations (neonates, COPD patients) who have distinct safe thresholds [1] [3] [2].
6. Bottom line for readers seeking a practical answer right now
If you need a single operational rule, older guidance would keep inspired oxygen percentages roughly in the 19–23% range for general safety in enclosed atmospheres, including CO2 control [3]. For medical decisions, modern practice avoids fixed inspired partial pressures and instead uses SpO2 or PaO2 targets tailored to the patient to prevent both hypoxemia and hyperoxemia [2]. Remember that the commonly cited 159 mmHg is a physical property of dry air at sea level and is not equivalent to the arterial oxygen tension that determines tissue oxygenation [1].