Pulse oximetry is a standard non-invasive method of measuring blood oxygen saturation (SpO2). In developing countries, pulse oximeters are rare because of expense and electricity requirements. Our ECEM group has developed the Phone Oximeter, which uses a cell phone (which are widely available in developing countries) to compute and analyze information from a pulse oximeter sensor. To further reduce costs, we have developed an oximeter sensor (AudioOx) that plugs into the audio jack of a standard cell phone. This study aims to calibrate the AudioOx by exposing 30 healthy adult volunteers to various altitudes in UBC's hypoxia chamber.