We propose simple guidelines for defining a significant EIAH, which address two specific purposes: 1) to identify a significant threat to systemic O2 transport; and2) to quantify abnormalities in each of the two key determinants of PaO2PaO2 , namely, the ventilatory response and the efficiency of alveolar-to-arterial gas exchange. The choice of EIAH definition will depend on the research question one wishes to address. In this review, we consider EIAH broadly as reduced arterial oxygenation, which may result from a fall in PaO2PaO2 (and thus also in SaO2SaO2 ), from a rightward shift of the O2 dissociation curve without a fall in PaO2PaO2 or from a combination of these processes.