Cell-based hypoxia studies are reliant on close control of O2 concentration in the surrounding environment. Exposure to ambient O2 (21%) concentrations for short or sustained periods of time can result in confounding results due to oxygen toxicity. Reduced O2 cell culture incubators and workstations are examples of adopted measures designed to reduce this oxygen toxicity and to promote consistency of observation. We have identified a further layer of complexity to this scenario – culture media. With a starting O2 concentration of approximately 10% O2 , a high salt content, and high operating temperature (37 oC) it can take culture media approximately 3 hrs to reduce to 2% O2 in the presence of cells and essentially never in the absence of cells and in large volumes.