Myths and methodologies: Exposure and measurements under normobaric hypoxia

Usually, settings of normobaric hypoxia underestimate the environmental hypoxia they are supposed to reproduce. The likely steady state of water vapour pressure within the human airways keeps the partial pressure of inspired oxygen very close to the assumed value, but this does not apply to the calculation of environmental hypoxia or experimental settings with other species. Additionally, in confined spaces, the CO₂ can exceed permissible exposure limits unless sufficient ventilation is calculated and required. The conversion of parts per million (ppm) to milligrams per metre cubed in dry air is facilitated by the general formula: $\mathrm{Concentration}(\mathrm{mg}·m^{-3})\approx \mathrm{ppmv}·M_m·P\left(\mathrm{kPa}\right)/8314·T\left(K\right)$ , thus overcoming the common assumptions of dry air at 1 atm and 25°C. The use of spot values (i.e., single measurements taken at a specific point in time), especially those of oxyhaemoglobin saturation, should be avoided, given the significance of physiological fluctuations. By considering the assumptions of simulated altitude hypoxia, the calculations or estimations of the amount of carbon dioxide in the inspired air in confined spaces, and the avoidance of spot values, the physiological response will be more valid and informative.