Most living things require oxygen to survive. The oxygen you breathe combines with nutrients from the food you eat to produce energy in a form usable by your body. If you are deprived of oxygen, your body very quickly shuts down, much like a fire dies when starved of oxygen (and for approximately the same reason). Ambient air is approximately 20.9% oxygen by volume, the majority of air (about 78% by volume) being nitrogen.
The oxygen content of air may be reduced by combustion (which combines oxygen with flammable substances to produce carbon dioxide and water vapor) or by displacement by a denser gas (such as propane) in a low-lying area or by any gas in sufficient quantity filling an enclosed area.
A modern oxygen sensor technology for safety applications is the micro fuel cell, generating a measurable electric current in the presence of oxygen by the oxidation of a self-contained fuel source. In many sensors, the fuel is pure lead (Pb), with the resulting chemical reaction producing lead oxide (PbO):
O2 + 2Pb → 2PbO
Fuel cell sensors are relatively rugged, accurate, and self-powering, enabling their use in portable oxygen analyzers. Due to their principle of operation, where an internal fuel is slowly oxidized over time, these sensors have a rather limited life and therefore must be periodically replaced.
An interesting and useful technique for testing the operation of an oxygen safety sensor is to exhale on the sensor, watching for a decrease in oxygen content to 15% or below. This testing technique makes use of the fact that your body extracts oxygen from the air, such that your exhaled breath contains less oxygen than it did when inhaled. Therefore, your own body acts as a crude “calibration gas” source for an oxygen safety analyzer.