More than half of all accidental deaths from poisoning are caused by carbon monoxide. This odourless, colourless gas can also cause long-term damage to health, short of death, when a person is exposed to high levels of carbon monoxide (CO, in chemical nomenclature) for a short period of time, or lower concentrations over an extended period. Given the deadly nature of CO, particularly in enclosed spaces, it is an important part of gas safety that detectors are installed in homes, offices and other inhabited areas.

The atomic structure of a carbon monoxide molecule is one atom of carbon bonded with one atom of oxygen. This molecule caused damage to human health by replacing oxygen in the normal carbon dioxide/oxygen exchange during breathing. The haemoglobin in red blood cells and myoglobin in muscle tissue more readily accept CO molecules than oxygen, which causes oxygen starvation to the body, including the brain and heart. As a result, even low concentrations of CO can cause headaches, nausea and memory loss; in higher levels, CO poisoning can result in convulsions, unconsciousness and, eventually, death from asphyxiation.

The primary source of CO is partial or inefficient combustion of fuel. This not only includes structure fires, but gas appliances (stoves and ovens, furnaces, hot water heaters, dryers and the like). Even when burning efficiently, gas appliances emit a small amount of CO (usually at the level of 40 parts per million, or ppm), which can build up over time if an area is not properly ventilated. For this reason, even structures with smoke detectors should also have CO detectors; it is an important part of gas safety that people occupying homes and work areas have protection against this invisible danger.

There are three basic types of CO detectors, each with its particular strengths and disadvantages. The different systems use different technologies to detect and warn occupants of a dangerous build-up of CO levels.

Biomimetic sensors use a capsule of synthetic haemoglobin, which is open to the air. If CO is present in the hair, the material in the sensor will turn darker – just as natural haemoglobin does when CO replaces oxygen. When the synthetic haemoglobin reaches a pre-determined shade, a sensor will trigger an alarm. The advantage of this system is that visual inspection can display the presence of CO in the air, even at levels lower than would cause an alarm. The disadvantage of the system is that it has a single threshold level; the system can only be set for one level of CO concentration.

Semi-conductor detectors are composed of two thin tin dioxide wires set into an insulating ceramic base. The wires, charged by an energy source, are part of a circuit which is closed when a pre-determined level of CO in the air reduces the resistance and sets off the alarm. Unlike the biomimetic system, visual inspection at lower CO levels will not provide a warning but, again, only one threshold level will trigger an alarm.

Electrochemical detectors account for the majority of CO alarms currently in use. The system is essentially designed as an incomplete fuel cell battery, consisting of an electrolyte (usually sulphuric acid) separated by two electrodes. One electrode oxidizes the carbon in any present CO molecules, while the other electrode consumes the oxygen atom. The greater the concentration of CO in the air, the higher the current created, until a threshold is reached to trigger an alarm. There are, however, several interesting differences between the electrochemical system and other designs: An independently powered processor can analyse the level, duration and rate of increase of the current created by the cell, which allows it to determine if either a dangerous condition of a long-term, low level concentration of CO or an acute "spike" increase in the gas is occurring. In either condition, the processor can be programmed to sound the alarm. Within the constraints of its memory, the system can also record and display CO levels, even at sub-alarm levels for future analysis.

All of these systems require careful placement, testing and replacement after their useful life (usually within five to six years). This requires consumer education and commitment to maintaining the systems, at a level beyond that required by smoke detectors; CO alarms are not "place and forget" tools. With sufficient understanding of their importance, however, CO sensors can greatly increase gas safety in the home and workplace.