Are typically based on electrochemical (galvanic) cells. The generated current in the sensor, which is produced from an oxidation reaction, is directly proportional to the rate of oxygen diffusion into the cell. Most meters are calibrated to measure oxygen concentrations between 0 and 25 percent by volume in air. Normal air contains about 20.9 percent oxygen. Meter alarms are usually set to indicate an oxygen deficient atmosphere at concentrations lower than 19.5 percent and an oxygen rich atmosphere at concentrations greater than 23.5 percent. Oxygen concentrations below 19.5 percent may result in difficulty breathing and impaired judgment. Oxygen concentrations below 16 percent result in rapid heartbeat and headache. Sudden physical exertion in an oxygen deficient environment may lead to loss of consciousness. Oxygen concentrations below 12 percent will bring about unconsciousness rapidly and without warning, and are considered IDLH. Oxygen enriched atmospheres present a fire and explosion hazard because ordinary combustible materials will burn more rapidly.