1. The science of measuring the concentration of smoke in the atmosphere, in a smoke plume, or in a flue gas (smoke that comes out of a stove or a furnace).
2. The measurement of carbon dioxide in a volume of gas.

The most common monitoring units are based on the selective absorption of infrared light by carbon dioxide and water vapor.

Capnometries usually work on the principle that carbon dioxide absorbs infra-red radiation. A beam of infra-red light is passed across the gas sample to fall on a sensor. The presence of carbon dioxide in the gas leads to a reduction in the amount of light falling on the sensor, which changes the voltage in a circuit.

Capnometry measures the concentration of carbon dioxide (which provides information on blood flow) through the lungs, which then goes on to other organs.